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Iddingsitization is an aqueous alteration that is known to take place in meteorites and continental basalts providing a potential habitat for microbial life. However, little is known about the exact mode by which this reaction occurs in the hadal seafloor and its implication for the deep subsurface biosphere. A comprehensive investigation of hadal basalts from the southern Mariana Trench (SMT) conducted with microscopic examinations shows that iddingsite occurs as augite‐hosted veins, rims, and mineral grains formed completely in place of augite within the SMT basalts. Carbon geochemistry indicates that organic matter with homogenous δ13C values between –27.8‰ and –27.2‰ might be biogenically accumulated in the SMT basalts. Furthermore, the close spatial relationships between carbonaceous matter (CM) and goethite in iddingsite point to microbial attachment to iddingsite minerals. Thus, iddingsitization might have fueled H2‐utilizing microorganisms inhabiting the hadal oceanic crust, thereby leading to the formation of CM, as implied by oxygen isotopic compositions revealing low alteration temperatures (32–83°C) favorable for microbial growth. In all, microbial biosignatures associated with iddingsite in the SMT basalts are highlighted, and these results could pave the way for deciphering the deep subsurface biosphere at hadal zones. [ABSTRACT FROM AUTHOR]

The hadal zone environment, characterized by extreme hydrostatic pressure, low temperatures, and limited food availability, presents substantial survival challenges for deep-sea fish species. In this study, we captured five deepsea fish species (Bathysaurus mollis, Coryphaenoides rudis, Ilyophis sp., I. brunneus and Pseudoliparis swirei) from the Mariana Trench at depths ranging from 2027 to 7125 m, by employing China's “Exploration I and Exploration II”. By combining lipidomic and proteomic analyses, we aimed to explore the genetic basis of adaptive evolution to the hadal zone in fish. The results indicate several key findings: (1) P. swirei (Mariana hadal snailfish, MHS) may enhance energy storage and utilization during prolonged fasting periods by significantly increasing liver tissue levels of cholesterol ester (CE), ether-linked triacylglycerol (TG-O), coenzyme Q (CoQ), and ATPase content; (2) MHS could maintain membrane fluidity under high pressure by increasing the proportion of unsaturated fatty acids while reducing levels of cholesterol and phosphatidylethanolamine (PE) content. (3) The regulation of lipid types and ratios could increase the risk of lipid peroxidation. To counter oxidative stress, MHS likely elevates monounsaturated fatty acid content and enhances antioxidants such as transferrin and heat shock proteins. Overall, this study provides new insights into the adaptive mechanisms of MHS to deep-sea conditions through the lipidome and proteome analyses, thus broadening our understanding of its resilience in the hadal zone. [ABSTRACT FROM AUTHOR]

The hadal zone (water depths > 6000 m) are unlike the overlying shallower marine regions (bathyal and abyssal) as it does not follow a continuum from the continental shelves to abyssal plains, but rather exhibits a globally disjunct series of discrete deep-sea habitats confined within geomorphological features. From an ecological perspective, hadal communities are often endemic to individual or adjacent features and are partitioned and isolated by geomorphological structures. To examine the size, shape, depth and degree of isolation of features where hadal fauna inhabit, this study explores the broad seafloor geomorphology, and distinctly partitioned hadal areas, across the Southwest Pacific and East Indian oceans using global bathymetric datasets. This research revealed the area occupied by hadal depths to be 716,915 km2 of which 58% are accounted for by trenches, 37% in basins and troughs, and 5% fracture zones. The largest feature in terms of area > 6000 m depth is the Wharton Basin with 218,030 km2 spanning 376 discrete areas. The largest continuous hadal habitats were the Kermadec and Tonga trenches at 145,103 and 111,951 km2 respectively, whereas features such as the Java Trench comprise two hadal components partitioned by a bathymetric high. Conversely, no physical barrier exists between the New Britain and Bougainville trenches thus any literature pertaining to hadal species or habitats from these trenches can be merged. This study highlights that the hadal zone mainly comprises two main geomorphological features (trenches and basins) that differ in size, depth and complexity. Hadal basins cover vast, generally shallower areas, comparable to abyssal plains, whereas trenches, despite a lesser footprint, represent greater depth ranges and complexity. As such, sampling designs and interpretation of ecological data must differ and hadal basins likely play an increasingly important role in understanding ecological shifts from abyssal to hadal ecosystems. [ABSTRACT FROM AUTHOR]

Abyssal (3501–6500 m) and hadal (>6500 m) fauna evolve under harsh abiotic stresses, characterized by high hydrostatic pressure, darkness and food shortage, providing unique opportunities to investigate mechanisms underlying environmental adaptation. Genomes of several hadal species have recently been reported. However, the genetic adaptation of deep sea species across a broad spectrum of ocean depths has yet to be thoroughly investigated, due to the challenges imposed by collecting the deep sea species. To elucidate the correlation between genetic innovation and vertical distribution, we generated a chromosome‐level genome assembly of the macrourids Coryphaenoides yaquinae, which is widely distributed in the abyssal/hadal zone ranging from 3655 to 7259 m in depth. Genomic comparisons among shallow, abyssal and hadal‐living species identified idiosyncratic and convergent genetic alterations underlying the extraordinary adaptations of deep‐sea species including light perception, circadian regulation, hydrostatic pressure and hunger tolerance. The deep‐sea fishes (Coryphaenoides Sp. and Pseudoliparis swirei) venturing into various ocean depths independently have undergone convergent amino acid substitutions in multiple proteins such as rhodopsin 1, pancreatic and duodenal homeobox 1 and melanocortin 4 receptor which are known or verified in zebrafish to be related with vision adaptation and energy expenditure. Convergent evolution events were also identified in heat shock protein 90 beta family member 1 and valosin‐containing protein genes known to be related to hydrostatic pressure adaptation specifically in fishes found around the hadal range. The uncovering of the molecular convergence among the deep‐sea species shed new light on the common genetic innovations required for deep‐sea adaptation by the fishes. [ABSTRACT FROM AUTHOR]

2,4,6-Tripyridine-s-triazine (TPTZ) spectrophotometric method was applied to determine the concentrations of dissolved monosaccharides (MCHO), polysaccharides (PCHO), and total carbohydrate (TCHO) in seawater samples collected from sea surface to hadal zone and sediment-seawater interface of the Southern Yap Trench in the Western Pacific Ocean. Results show that the concentrations of MCHO, PCHO, and TCHO ranged from 6.3 to 22.3 µmol C/L, 1.1 to 25.4 µmol C/L, and 12.1 to 44.9 µmol C/L, respectively, from the euphotic layer to the hadal zone of the trench. At different sampling stations, the concentrations of MCHO, PCHO, and TCHO in the seawater showed complex vertical variation characteristics, but the overall variation trends were decreasing with water depth. In the Southern Yap Trench, the maximum concentration of MCHO in the seawater appeared in the euphotic layer, and the minimum in the hadal zone. The maximum concentration of PCHO appeared in the euphotic layer, and the minimum in the bathypelagic layer. The water layer where the maxima and minima of the average concentration of TCHO appeared was consistent with that of PCHO. PCHO was the major component of TCHO in the seawater of the Southern Yap Trench. In the seawater from the sediment-seawater interface, the concentrations of MCHO, PCHO, and TCHO ranged from 8.4 to 10.6 µmol C/L, 3.8 to 5.8 µmol C/L, and 12.2 to 15.2 µmol C/L, respectively, and MCHO was the major component of TCHO. The key factors affecting the concentration and existing forms of dissolved sugars in the seawater of the Southern Yap Trench included photosynthesis, respiration, polysaccharide hydrolysis, adsorption and desorption of particulate matter, trench "funnel effect", deep ocean currents, sediment resuspension, and etc. This study provided fundamental data about labile organic matter in abyss and hadal zone of marine environment, which is significant for further understanding of deep-sea organic carbon cycle. [ABSTRACT FROM AUTHOR]

Methane (CH4) and dimethylsulphoniopropionate (DMSP) are major carbon and sulfur sources for bacterioplankton in the ocean. We investigated the characteristics of CH4 and DMSP in the southern Yap Trench from sea surface to hadal zone in June 2017. We found that concentrations of CH4 varied from 1.5 to 4.5 nmol/L with saturation between 94% and 204% in the euphotic layer. Concentrations of dissolved DMSP (DMSPd) ranged from 0.5 to 3.7 nmol/L with higher values in surface water and decreased with depth. Concentrations of particulate DMSP (DMSPp) varied from 0 to 13.6 nmol/L. Concentrations of total DMSP (DMSPt) ranged 2.0–15.2 nmol/L. Their concentrations decreased slightly and reached consistent levels in 200–3 000-m depth due probably to heterotrophic bacterial production in marine aphotic and high-pressure environments. An exception occurred around 4 000-m depth where their concentrations increased considerably and then decreased in deeper water. This previously unrecognized phenomenon sheds light on the elevated concentrations of DMSP in the abyssal layer that might be affected by the Lower Circumpolar Deep Water (LCPW). Concentrations of CH4 in seawater of the Benthic Boundary Layer of the southern Yap Trench were slightly higher than those in the water column at approximate depth, and concentrations of DMSP in seawater of the Benthic Boundary Layer of the southern Yap Trench were not much higher than those in the water column at the approximate depth, indicating that sediment was a weak source of CH4 but was not a source of DMSP for seawater in the study area. This study presented clear correlations between CH4 and DMSP from sea surface to sea bottom, proving that DMSP might be a potential substrate for CH4 not only in oxic surface seawater but also in deep water. [ABSTRACT FROM AUTHOR]

The observation of singleton or rare species in the deep sea is extremely valuable for gaining a census of biodiversity. At hadal depths (> 6000 m), these records provide a more complete picture of the vertical distribution of fauna. In this study, we present new in situ video records for Trachymedusae (Hydrozoa), Ascidiacea (Tunicata), and Tentaculata (Ctenophora) taken by submersible and supporting landers in the western Pacific Ocean. Together, these three taxonomic groups are present at depths far deeper than previously known. Observations of the rhopalonematid trachymedusa (Pectis cf. profundicola) from both lander and submersible dives at 10,063 and 10,040 m in the Philippine Trench (NW Pacific Ocean) extend the maximum depth of the Hydrozoa by a further 997 m and is the first record of Hydrozoa > 10,000 m. The predatory tunicate Octacnemidae sp. was observed nine times at 7799 m in the Mariana Trench and once at 8077 m in the Izu-Ogasawara Trench (NW Pacific Ocean), therefore extending its maximum depth by 1726 m, and 1002 m to historical accounts of what might also be in the Phlebobranchia order of Ascidiacea. Several observations of large, globular ctenophores with long filamentous tentacles and lacking oral lobes from 10,040 m in the Kermadec Trench (SW Pacific Ocean) increase the maximum depth of the Ctenophora by 2823 m and is the first record of Ctenophora > 10,000 m. Benthic ctenophores were also recorded from the Japan Trench at 8001 m, extending their known depth range by a further 2750 m. [ABSTRACT FROM AUTHOR]

• First insights into NE Pacific deep-sea macrofauna from the Bering Sea and Aleutian Trench (AT) and slope. • The Bering Sea differs from Aleutian slope and trench. • Depth is the most important driver of the assemblage composition in the North Pacific. • Adding 235 times more faunistic information to known data from GBIF and OBIS. • Hadal zones in the eastern AT show variable taxa richness. The deep sea, Earth's largest biome, harbors numerous unknown species. Prior to the AleutBio (Aleutian Trench Biodiversity Studies) expedition from July to September 2022, the Northeast (NE) Pacific at abyssal and hadal depths was virtually unexplored. Our study presents new findings from the AleutBio project on the macrofaunal composition of the Bering Sea (BS) and Aleutian Trench (AT) collected by means of an epibenthic sledge (EBS), comparing these results with data from the Kuril-Kamchatka Trench (KKT) and the Northwest (NW) Pacific. Additionally, we examine variations in macrofaunal composition and abundance across different regions and depths. A biogeographic gap analysis using data from the Ocean Biodiversity Information System (OBIS) and the Global Biodiversity Information Facility (GBIF) found that, out of 170,627 occurrence records from the North Pacific and Bering Sea, only 153 were from depths below 3,500 m. The AleutBio project addressed this gap by significantly expanding the dataset with 36,499 new records collected during the expedition using an EBS. Nearly 98% of the specimens were from five phyla: Arthropoda, Annelida, Mollusca, Echinodermata, and Nematoda, with Polychaeta, Copepoda, and Nematoda being the most abundant taxa. Although the number of individuals varied between stations, there was no significant decrease in abundance with increasing depth, and some hadal stations had similar numbers of invertebrates as abyssal stations. Regional differences were observed, with Polychaeta and Nematoda being dominant in the BS, and Copepoda more prevalent at western abyssal stations. Depth emerged as the key factor influencing macrofaunal distribution, with distinct patterns across bathyal, abyssal, and hadal depths. Comparisons with other NW Pacific regions, like the Sea of Japan and the Sea of Okhotsk, show that depth and water body isolation play crucial roles in shaping faunal communities. AleutBio's extensive sampling below 3,500 m has vastly increased available data, aiding in the understanding and conservation of deep-sea biodiversity. While certain taxa showed patchy distributions, no significant differences in faunal composition were found between geographic areas or depth zones. These findings underscore the dynamic nature of deep-sea ecosystems and highlight the importance of depth in shaping macrofaunal communities, emphasizing the need for continued research in these fascinating environments. [ABSTRACT FROM AUTHOR]

Arsenic compounds are accumulating in deep ocean, but their ecological impacts on deep-sea ecosystem remain elusive. We studied 32 sediment cores (101 layers for metagenomes, along with 41 global reference sediment metagenomes) collected from the South China Sea and the Mariana Trench (MT), characterized with high arsenic accumulation in MT. In these metagenomes we revealed a significantly positive correlation between relative abundance of arsenite methyltransferase gene (arsM) and sampling depth, which suggests that arsenic methylation is the most prevalent arsenic biotransformation process in the deep sea. Lower relative abundance of arsenic efflux gene, compared with arsM , indicates that microbes in deep-sea sediments were prone to methylate arsenite and retain it rather than efflux it. Phylogenetic analysis identified seven clades of ArsM proteins, including two new clades derived primarily from deep-sea microorganisms. Five metagenome-assembled genomes containing aioA for arsenite oxidation also harbor carbon fixation genes in the deep-sea sediment layers, suggesting previously unnoticed contribution of arsenite-oxidizing autotrophic bacteria to the carbon cycle. Therefore, deep-sea microorganisms adopt different detoxification and transformation strategies in response to arsenic compounds, which renews our understanding of arsenic in their ecological impacts and potential contribution in deep ocean. [Display omitted] • Microbial biotransformation impacts deep-sea arsenic cycle. • Deep-sea microbes might use arsenic as osmolytes under high pressure. • Arsenite oxidizers contribute to the carbon cycle in the deep sea. • Novel ArsM clades found in deep-sea sediments. [ABSTRACT FROM AUTHOR]

The composition and concentration of dissolved free amino acid (DFAA) of seawater samples collected in May 2016 from the surface to the hadal zone of the northern region of the Yap Trench were analyzed by pre-column derivatization of o-phthalaldehyde. Results show that the average concentration of DFAA in the study area was 0.47±0.36 µmol/L. In different sampling stations, the concentrations of DFAA with water depth showed complex variation patterns. At the sediment-seawater interface, the concentrations of DFAA in the western side of the trench were obviously higher than that in its eastern side. In the study area, there were no significant correlations between the concentrations of DFAA and the environmental parameters such as concentrations of chlorophyll a (Chl a), dissolved oxygen (DO), pH, and dissolved inorganic nitrogen (DIN), indicating that the concentrations of DFAA in seawater of the trench are affected by many factors, such as photosynthesis, respiration, temperature, pressure, illumination, and circulation. The dominant DFAA are similar in different water layers of sampling stations, including aspartic acid (Asp), glutamic acid (Glu), glycine (Gly), and serine (Ser). The composition of different amino acids, and the relative abundance of acidic, basic, and neutral amino acids might be related to the sources and consumption of various amino acids. Nine pairs of amino acids in the DFAA showed significantly positive relationship by correlation matrix analysis, suggesting that they might share similar biogeochemical processes. The degradation index (DI) of the DFAA in seawater of the Yap Trench could reflect the degradation, source, and freshness of DFAA in the trench to some extents. This is a preliminary study of amino acids from sea surface to hadal zone in the ocean, more works shall be done in different trenches to reveal their biogeochemical characteristics in extreme marine environments. [ABSTRACT FROM AUTHOR]

We describe a new tryphosid amphipod, Paronesimoides calceolus sp. nov., from the Shinkai Seep Field, Mariana Trench, at a depth of 5689–5683 m. This is the first species of Paronesimoides to be described in the Mariana Trench and the deepest record of the genus. Paronesimoides calceolus sp. nov. differs from its congeners in characteristics of its head, antennae, mandible, coxae, gnathopods and telson. A key to the Paronesimoides species is provided. [ABSTRACT FROM AUTHOR]

During a manned submersible dive in the Philippine Trench, a solitary oegopsid squid of the monogeneric family Magnapinnidae was observed swimming close to the seafloor at 6212 m. The estimated mantle length of the squid was ca. 10 cm. The long slender terminal arm and tentacle filaments characteristic of adult Magnapinna were not obvious in the video. The filaments may have been contracted or may not yet have developed. This observation is the first record of squid at hadal depths and extends the known bathymetric range for any squid by 1477 m; an increase of ca. 30%. We also observed four cirrate octopods between 6212 and 6224 m. Although the video quality was poor, these octopods did not appear to be the same species as those reported previously in the Java Trench. These observations extend the known hadal occurrence of cirrates, and cephalopods in general, from the Indian Ocean to the equatorial North Pacific Ocean, suggesting that their global presence in depths > 6000 m may be more extensive than previously recognized. [ABSTRACT FROM AUTHOR]

Late Pleistocene thick laminated mats of Ethmodiscus rex (E. rex) have been found on the seafloor of the western tropical Pacific. Exploring the causes of E. rex mats formation can improve our understanding of the role of diatom mats in the global carbon-silicon cycle. Nutrients, such as silicate, are a major factor contributing to the growth of diatoms. However, the origin of the nutrients necessary for E. rex to bloom remains unclear. Here we analysed the 14C ages and silicon isotope compositions (δ30Si) of E. rex mats from deep-sea sediment cores collected in the western Pacific to reveal the nutrient source of the diatom bloom. The results indicate that the age of the cores ranges from 11 ka to 19 ka. This is the first discovery of a last deglacial thick layer of E. rex mats in the western Pacific Hadal zone. Furthermore, the silicon isotope results show that, similar to previous studies in the western Pacific, all δ30Si of E. rex frustules are negative. This is in contrast to the North Atlantic upwelling region where all δ30Si are positive. According to the binary mixing model calculations, eolian dust were responsible for supplying 80%–90% of the silica during the last deglaciation, while upwelling waters accounted for only about 10%–20% of the silica. During Heinrich Stadial 1 (HS1) period, the strengthened East Asian Winter Monsoon (EAWM) transported a lot of eolian dust to the western Pacific, providing nutrients for diatom blooms. In addition, the enhanced stratification in the western Pacific since the late HS1 period was a significant factor that contributes to the blooms of E. rex when the supply of eolian dust material decreased. • First thick mats of Ethmodiscus rex were discovered in the western Pacific during the last deglaciation. • Ethmodiscus rex 's silicon isotope (δ30Si) suggested silicate source from Asian eolian. • Stratification in western Pacific promoted Ethmodiscus rex blooms since late HS1. [ABSTRACT FROM AUTHOR]

Natural whale falls and falls of smaller-sized food have been reported for more than 30 years and are known to be important sources of nutrients and organic matter for the seafloor community. However, the associated species composition and ecological processes during such events in the hadal zone were unknown. Therefore, we aimed to determine the impact of depth and predation on the hadal community during the early stages of a food-fall event. This is the first study to investigate the preliminary stages of the two deepest artificial dolphin-fall events in the Philippine Basin (PB) (∼7729 m) and Mariana Trench (MT) (∼8200 m). A total of nine dives were conducted over a period of 86 days (PB) and 50 days (MT) using the deep-sea manned submersible "Fendouzhe." Our observations in the PB indicated that the first stage (mobile-scavenger) was controlled by the feeding ecology of hadal amphipods and snailfish. In contrast, the absence of predatory snailfish in the MT enhanced the degradation rate of the carcass compared to that in the PB. Most soft tissues were entirely consumed by scavenging amphipods within days of the event in the MT, whereas in the PB, amphipods were observed taking a feeding hiatus on Day 10 to escape snailfish predation. The second stage (enrichment-opportunist), which hosted few grazing faunas, began at different times in each location and overlapped with the mobile-scavenger stage. Dolphin carcasses, being smaller than those of full-grown whales, can only sustain a large community of scavenging amphipods, and indirectly, predatory snailfish. After the first stage, the dispersed organic matter and limited lipid content in dolphin bones were likely insufficient to sustain an active grazing community or the chemosynthetic community that typically follows. We concluded that water depth influences the successional stages and decomposition rate of food falls in the hadal zone by controlling the dynamic relationship between prey and predators. Our study elucidates the ecology of food-fall events in the hadal zone and highlights the key differences in food-fall events at different depths. • First observation of food fall communities in the hadal zone. • Dolphin carcasses can sustain an active scavenging community and snailfish at hadal depths. • Presence of predator at hadal depths results in better distribution of organic matter during food falls, as observed in the Philippine Basin. • In spite of the presence of limited species in the hadal zone, decomposition of the carcass can be surprisingly fast, as observed in the Mariana Trench dolphin fall. [ABSTRACT FROM AUTHOR]

Ocean sediments are the largest sink for mercury (Hg) sequestration and hence an important part of the global Hg cycle1. Yet accepted global average Hg flux data for deep-ocean sediments (> 200 m depth) are not based on measurements on sediments but are inferred from sinking particulates2. Mercury fluxes have never been reported from the deepest zone, the hadal (> 6 km depth). Here we report the first measurements of Hg fluxes from two hadal trenches (Atacama and Kermadec) and adjacent abyssal areas (2–6 km). Mercury concentrations of up to 400 ng g−1 were the highest recorded in marine sediments remote from anthropogenic or hydrothermal sources. The two trench systems differed significantly in Hg concentrations and fluxes, but hadal and abyssal areas within each system did not. The relatively low recent mean flux at Kermadec was 6–15 times higher than the inferred deep-ocean average1,3, while the median flux across all cores was 22–56 times higher. Thus, some hadal and abyssal sediments are Hg accumulation hot-spots. The hadal zone comprises only ~ 1% of the deep-ocean area, yet a preliminary estimate based on sediment Hg and particulate organic carbon (POC) fluxes suggests total hadal Hg accumulation may be 12–30% of the estimate for the entire deep-ocean. The few abyssal data show equally high Hg fluxes near trench systems. These results highlight a need for further research into deep-ocean Hg fluxes to better constrain global Hg models. [ABSTRACT FROM AUTHOR]

The elpidiid holothurian Peniagone leander Pawson and Foell, 1986 is recorded for the first time from the Mariana Trench area at a depth of 5 571 m. The type description of this species was based on only in-situ photographs. P. leander is re-described and illustrated based on a preserved material and in-situ photographs taken on the seabed. The 16S rRNA was also obtained from the specimen and submitted to GenBank. This is the fourth discovery of this species, extending its bathymetric range in the western Pacific Ocean. [ABSTRACT FROM AUTHOR]

The concentrations of dissolved monosaccharides (MCHO), polysaccharides (PCHO), and total carbohydrates (TCHO) in the seawater samples collected from the north of the Yap Trench in the western Pacific Ocean were measured by 2,4,6-tripyridine-s-triazine (TPTZ) spectrophotometry method. The results show that the concentrations of MCHO, PCHO, and TCHO ranged from 4.6 to 22.1 μmol C/L, 3.5 to 27.3 μmol C/L, and 13.8 to 36.3 μmol C/L, respectively. In different sampling stations, the concentrations of MCHO, PCHO, and TCHO with water depth showed complex variation patterns. In the study area, the maximum concentrations of MCHO, PCHO, and TCHO occurred in the euphotic layer and the minimum concentrations occurred in mesopelagic seawater layer. Generally, the concentrations of MCHO, PCHO, and TCHO decreased with water depth from the euphotic layer to the hadal zone. The average value of PCHO/TCHO was higher than the average value of MCHO/TCHO, indicating that PCHO was the main component of TCHO in north of the Yap Trench. [ABSTRACT FROM AUTHOR]

Two sediment cores were retrieved in the hadal zone of the Yap Trench, and their concentrations of six major elements Mg, Al, Ca, Ti, Mn and Fe and nine trace elements Sr, Ba, Pb, V, Cr, Co, Ni, Cu and Zn were determined in inductively coupled plasma atomic emission spectrometry (ICP-AES). According to the vertical distribution profiles of the 15 elements, the correlation of their concentrations, the ratios of Ni/Co, V/Cr, Fe/Al, and Ti/Al, and morphological characteristics of the sediment samples, the implications of the depositional environment and the sediment provenance were analyzed. The results show that the ratio of Ni/Co in all depths of the two sediment cores were below 5, and the ratio of V/Cr were lower than 2, indicating that the depositional environment of the hadal zone in the trench was oxidative and might have inflow of the Antarctic bottom oxygen-rich water. The sediment samples on the eastern side of the trench were siliceous mud mainly composed of diatoms, radiolarian, and sponge needles from surface to deep layer. The vertical profile of the elements, the concentration of TOC and the fossil record indicated that the sediment sample from station Dive113 was well mixed from surface to bottom layer. Based on the correlation of concentrations of the elements, the morphological characteristics of the sediment, and the ratios of Fe/Al and Ti/Al, we inferred that the sediment in the hadal zone of the trench had terrestrial, volcanic, biological, and authigenic sources. Major source of the sediment in the eastern side of the trench were terrestrial; whereas the sediment in the western side of the trench received more volcaniclastic input. [ABSTRACT FROM AUTHOR]

In this study, cored sediment samples collected by the Jiaolong Submersible at 6 779 m depth from the hadal zone of the Yap Trench in May 2016 were sliced in 1-cm interval from top to bottom, and lipids in each sediment layer were quantitatively and qualitatively analyzed. The vertical distribution profiles of the lipids in the sediment sample, their main existing forms, and their possible sources were investigated. The results show that the concentration of lipid in the surface sediment was the highest with the carbon number from 12 to 27, dominated by medium and short-chain lipids. The total concentration of fatty acids in surface sediment was much higher than those in the offshore and deep-sea areas, being up to 325.77 μg/g due to the funnel effect caused by the "V" terrain of the trench. Fatty acid 18.0 was the most abundant lipids in the sediment sample. Abnormal high concentrations of fatty acid 18.ω7 and alkanes indicated the existence of hydrothermal fluids in the study area. In addition, saturated fatty acids and monounsaturated fatty acids existed mainly in free form, and polyunsaturated fatty acids existed mainly in bound form. Most of the alkanes were in bound form, and their major source was autochthounous input. The carbon number of alcohols in the sediment sample ranged from 12 to 20, mainly existed in bound form. The source of fatty acids was mainly autochthonous input, and the neutral lipids had both marine and terrestrial origin. This is the first study of lipids in hadal sediment of the Yap Trench. The results will promote deeper understanding of organic carbon cycle in marine environment. [ABSTRACT FROM AUTHOR]

Knowledge about organic carbon loadings (ratio of sedimentary organic carbon (SOC) content to specific surface area (SSA)) and the fate of organic carbon (OC) is critical to understand the marine carbon cycle. We investigated the variations in the patterns of OC loadings and the preservation capacities of sedimentary OC in the Yap Trench and other marine environments. The average OC loading in sediment cores from various marine environments decreases with increasing water depth at a rate of ~0.06 mg OC/(m2·km) (R2 =0.23, P<0.01). Distinct low OC loadings (0.09±0.04 mg OC/m2) were observed in the Yap Trench, with the lowest values as ~0.02 mg OC/m2. A further comparative analysis indicated that OC/SSA=0.2 mg OC/m2 is a good indicator to distinguish between oxic deep-sea regions and suboxic energetic deltaic areas. Regression analysis between OC loading and bulk carbon isotope compositions indicates that marine OC (δ13C ~−20.4‰ to −18.6‰) dominates the lost OC within the Yap Trench and does not differ from that of the abyssal zone. In contrast, terrestrial OC with δ13C values of approximately −27.4‰ to −20.5‰ was the major source of remineralized OC in the sublittoral zone. The ratios of OC loadings in the bottom layer relative to those in the top layers of sediment cores indicate that the preservation capacities of hadal trenches are much lower than those of other environments, and only approximately 30% of the SOC deposited in hadal trenches is finally buried. The value is equivalent to 0.066% of the primary production-derived OC and much lower than the global ocean average (~0.3%). Overall, the hadal zone exhibits the lowest OC loading and preservation capacity of SOC of the different marine environments investigated, despite the occurrence of a notable funneling effect. [ABSTRACT FROM AUTHOR]

Amphipods of the superfamily Lysianassoidea that inhabit the hadal zone (> 6000 m) have large bathymetric ranges and play a key role in deep ocean ecosystems. The endemism of these amphipod species makes them a good model for investigating potent natural selection and restricted dispersal in deep ocean trenches. Here, we describe genetic diversity and intraspecific population differentiation among three amphipod species from four Pacific trenches based on a mtDNA concatenated dataset (CO Ι and 16S rRNA genes) from 150 amphipod individuals. All amphipod populations had low genetic diversity, as indicated by haplotype and nucleotide diversity values. Population geographic relationship analysis of two Alicella gigantea populations revealed no genetic differentiation between these two localities (pairwise genetic differentiation coefficient = 0.00032, gene flow = 784.58), and the major variation (99.97%) was derived from variation within the populations. Historical demographic events were investigated using Tajima's D and Fu's F neutrality tests and analysis of mismatch distribution. Consistent results provided strong evidence to support the premise that demographic expansion occurred only for the Mariana population of Hirondellea gigas, possibly within the last 2.1–3.4 million years. These findings suggest that the formation of amphipod population structure might be the result of multiple factors including high hydrostatic pressure, food distribution, trench topographic forcing and potential ecological interactions. [ABSTRACT FROM AUTHOR]

• Hadal trenches show alarming pollutants, plastics, antibiotic resistance genes, and black carbon. • Anthropogenic and natural sources contribute to organic pollutants in hadal trenches. • Organic pollutants reach hadal trenches via biological pumping, sedimentation, and transverse transport. • Isotopes, biomarkers, and chiral analysis trace hadal pollutants effectively. The "V"-shaped structure of hadal trenches acts as a natural collector of organic pollutants, drawing attention to the need for extensive research in these areas. Our review identifies significant concentrations of organic pollutants, including persistent organic pollutants, black carbon, antibiotic-resistant genes, and plastics, which often match those in industrialized regions. They may trace back to both human activities and natural sources, underscoring the trenches' critical role in ocean biogeochemical cycles. We highlight the complex lateral and vertical transport mechanisms within these zones. Advanced methodologies, including stable isotope analysis, biomarker identification, and chiral analysis within isotope-based mixing models, are crucial for discerning the origins and pathways of these pollutants. In forthcoming studies, we aim to explore advanced methods for precise pollutant tracing, develop predictive models to forecast the future distribution and impacts of pollutants in hadal zones and on the Earth's larger ecological systems. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract The deepest point of the oceans is over 10 km depth and is characterized with extremely high hydrostatic pressure. Water sampling from the hadal zone for microbiological studies is therefore difficult. In this study, we demonstrate the application of an in situ microbial filtration and fixation (ISMIFF) apparatus in sampling work from the Challenger Deep. A total of 2300 L hadal waters were filtered by the ISMIFF equipped on a hadal lander during seven deployments. The microbial communities in the hadal samples (>6000 m) collected by the ISMIFF were composed of more heterotrophic Marinimicrobia and less photoautotrophic Prochlorococcus and SAR11 bacteria, compared with those by Niskin bottles. Such a difference in the community structure was not observed in the samples collected by the two methods from ~5000-m depth, indicating an effect of greater depths on sample quality. Our results indicate that the ISMIFF or similar filtration apparatuses are indispensable for hadal water sampling since using Niskin bottles is probably difficult to obtain pristine samples from the hadal zone. Application of the ISMIFF will pave the way to meta-omics studies on the hadal microbes. Highlights • The ISMIFF could obtain pristine microbial samples from the Challenger Deep. • The ISMIFF sampler could filter a large volume of water sample from the hadal zone. • In situ filtration and preservation of microbes at hadal depths may provide high-quality samples for future omics work. [ABSTRACT FROM AUTHOR]

Abyssal and hadal sediments represent two of the most type ecosystems on Earth and have the potential interactions with geochemistry. However, little is known about the prokaryotic community assembly and the response of prokaryotic communities to metal(loid)s in trench sediments due to the lack of adequate and appropriate samples. In this study, a systematic investigation combined the assembly mechanisms and co-occurrence patterns of prokaryotic communities between the hadal and abyssal sediments across the Yap Trench. The results revealed that the hadal prokaryotes had less species diversity, but more abundant function than the abyssal prokaryotes. The prokaryotic communities in the abyssal sediments had more core taxa than the hadal sediments. Twenty-one biomarkers mostly affiliated with Nitrosopumilaceae were detected using Random-Forests machine learning algorithm. Furthermore, stochasticity was dominant in the prokaryotic community assembly processes of the Yap Trench sediments. Meanwhile, homogeneous selection (32.6%–52.9%) belonging to deterministic processes governed the prokaryotic community assembly in hadal sediments with increasing of sediment depth. In addition to total nitrogen and total organic carbon, more metal(loid)s were significantly correlated with the prokaryotic community in the hadal sediments than that in the abyssal sediments. The hadal prokaryotic communities was most positively related to bismuth (r = 0.31, p < 0.01), followed by calcium, chromium, cerium, potassium, plumbum, scandium, titanium, and vanadium. Finally, co-occurrence networks revealed two potential dominant prokaryotic modules in Yap Trench sediments covaried across oceanographic zonation. By contrast, the hadal network had relatively more complexity, more bacterial taxa, and more associations among prokaryotic taxa, relative to the abyssal network. This study reveals potentially metal variables and community assembly mechanisms of the prokaryotic community in abyssal and hadal sediments and provides a better understanding on the prokaryotic diversity and ecology in trench sediment ecosystems. [Display omitted] • Most of biomarkers in trench sediments were affiliated with Nitrosopumilaceae. • Stochastic processes shaped the prokaryotic community assembly of trench sediments. • More metal(loid)s were significantly related with the hadal prokaryotic community. • The hadal network had relatively more complex interaction than the abyssal network. • The hadal prokaryotes had less species diversity, but more abundant function. [ABSTRACT FROM AUTHOR]

Abstract The hadal zone remains one of the least studied environments because of its inaccessibility, in part because of hydrostatic pressures extending to 110 MPa. Few instruments are capable of sampling from such great depths. We have developed a full-ocean-depth-capable lander that can be fit with sampling packages for the collection of still images, video, motile megafauna, and hadal seawater. One payload includes a pressure-retaining sampler (PRS) able to maintain seawater samples under in situ pressure during recovery. We describe the technical specifications of the lander and the PRS and preliminary results from three deployments at depths in excess of 10,700 m in the Mariana Trench. Seawater from full-ocean depth was recovered at 81% of the in situ pressure. This facilitated the collection of microbial genomes affiliated with the family Flavobacteriaceae within the Bacteroidetes and the phylum Marinimicrobia. We show that these microbes are specifically enriched in hadal zones, representing novel trench lineages, and describe their adaptations for living in hadal environments. These findings highlight the utility of this lander system, which facilitates scientific inquiry at depths greater than 6000 m. Highlights • Description of a hadal lander capable of collecting video, megafauna, and water. • Water obtained from the Mariana Trench with 81% of the in situ pressure retained. • Genomes related to the Flavobacteriaceae and Marinimicrobia show hadal adaptation. [ABSTRACT FROM AUTHOR]

Hadal environments sustain diverse microorganisms. A few studies have investigated hadal microbial communities consisting of free-living or particle-associated bacteria and archaea. However, animal-associated microbial communities in hadal environments remain largely unexplored, and comparative analyses of animal gut microbiota between two isolated hadal environments have never been done so far. In the present study, 228 Gb of gut metagenomes of the giant amphipod Hirondellea gigas from two hadal trenches, the Mariana Trench and Japan Trench, were sequenced and analyzed. Taxonomic analysis identified 49 microbial genera commonly shared by the gut microbiota of the two H. gigas populations. However, the results of statistical analysis, in congruency with the alpha and beta diversity analyses, revealed significant differences in gut microbial composition across the two trenches. Abundance variation of Psychromonas, Propionibacterium, and Pseudoalteromonas species was observed. Microbial cooccurrence was demonstrated for microbes that were overrepresented in the Mariana trench. Comparison of functional potential showed that the percentage of carbohydrate metabolic genes among the total microbial genes was significantly higher in the guts of H. gigas specimens from the Mariana Trench. Integrating carbon input information and geological characters of the two hadal trenches, we propose that the differences in the community structure might be due to several selective factors, such as environmental variations and microbial interactions.IMPORTANCE The taxonomic composition and functional potential of animal gut microbiota in deep-sea environments remain largely unknown. Here, by performing comparative metagenomics, we suggest that the gut microbial compositions of two Hirondellea gigas populations from the Mariana Trench and the Japan Trench have undergone significant divergence. Through analyses of functional potentials and microbe-microbe correlations, our findings shed light on the contributions of animal gut microbiota to host adaptation to hadal environments. [ABSTRACT FROM AUTHOR]

Starfish (phylum Echinodermata) are ecologically important and diverse members of marine ecosystems in all of the world's oceans, from the shallow water to the hadal zone. The deep sea is recognized as an extremely harsh environment on earth. In this study, we present the mitochondrial genome sequence of Mariana Trench starfish Freyastera benthophila, and this study is the first to explore in detail the mitochondrial genome of a deep‐sea member of the order Brisingida. Similar to other starfish, it contained 13 protein‐coding genes, two ribosomal RNA genes, and 22 transfer RNA genes (duplication of two tRNAs: trnL and trnS). Twenty‐two of these genes are encoded on the positive strand, while the other 15 are encoded on the negative strand. The gene arrangement was identical to those of sequenced starfish. Phylogenetic analysis showed the deep‐sea Brisingida as a sister taxon to the traditional members of the Asteriidae. Positive selection analysis indicated that five residues (8 N and 16 I in atp8, 47 D and 196 V in nad2, 599 N in nad5) were positively selected sites with high posterior probabilities. Compared these features with shallow sea starfish, we predict that variation specifically in atp8, nad2, and nad5 may play an important role in F. benthophila's adaptation to deep‐sea environment. This study is the first determination of the mitogenome of a deep‐sea member of the order Brisingida and may shed light on the adaptive evolution of Brisingida species to the deep‐sea environment. [ABSTRACT FROM AUTHOR]

Abstract Trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle, but the inherent dynamics are not fully understood. One interesting case is the migration of the Izu-Bonin-Marianas trench (IBM) that accommodates the subduction of the Pacific beneath the Philippine Sea Plate (PSP), which is in turn subducting beneath the Eurasian plate along the Ryukyu trench. The history of the IBM trench is dominated by fast, episodic retreat from 40 to 15 Ma. However, around 10–5 Ma, the IBM trench reversed its motion from retreat to advance. The switch in trench motion occurred soon after the breakoff of the PSP slab along the Ryukyu trench and the onset of new subduction, and represents a fundamental change in the dynamics of the western Pacific subduction zones. Here, Following the modelling study of Čížková and Bina (2015), which suggested a link between IBM trench advance and Ryukyu subduction, we run 2-D numerical experiments to test the influence of a newly formed Ryukyu slab on the established IBM subduction zone, we run two-dimensional numerical experiments to test the influence of this newly formed Ryukyu slab on the IBM subduction zone. The results from our geodynamic model compare favourably with the reconstructed trend, indicating that the switch in trench motion along the IBM trench may indeed be related to the onset of a new subduction zone along the Ryukyu trench. Our analysis substantiates the idea that advancing trench motions in the western Pacific are due to the establishment of a double subduction system. Further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission. [ABSTRACT FROM AUTHOR]

Abstract The purpose of this study is to explore the conditions in which trenches form beneath oscillating cylinders – such as pipelines, cables or idealised chains – close to the seabed. Experiments are conducted by oscillating a circular cylinder in a direction normal to an initially flat sandy bed. Across a relatively wide parameter space, the transport patterns and trench geometries reveal three transport regimes that are linked to vortex dynamics and depend primarily on the ratio of oscillation amplitude to cylinder diameter (K C number). For K C ≲ 4 sediment motion results in bedload transport that is symmetric about the cylinder centreline. This leads to the formation of two parallel trenches with a prominent ridge forming directly beneath the cylinder. For 4 ≲ K C ≲ 9 sediment motion occurs via localised transport events, which are associated with the motion of vortices shed from the cylinder. These transport events are irregular but occur on both sides of the cylinder and lead to the formation of a symmetric trench geometry. For 9 ≲ K C ≲ 12 the sediment motion is characterised by localised transport events and asymmetric bedload transport driven by overall vortex dynamics. In terms of trench size, the maximum (equilibrium) depth is found to increase with K C and a mobility number (ψ) defined in terms of the maximum cylinder velocity. The initial rate of trench development also increases with K C number and ψ , with an additional dependency on the cylinder β number. The cylinder motions required to initiate trenching are predicted well using continuity arguments and an oscillatory boundary layer assumption, provided the K C number and minimum gap between the cylinder and the bed are relatively small. The findings in this study provide insight into the mechanisms and prediction of trench formation. In particular, this study reveals that significant trenches can form in sandy seabeds solely due to fluid flow induced by pipeline/cable/chain motion without direct seabed contact, which has implications for structural fatigue. Highlight • Experiments investigate sediment transport and trench formation beneath a cylinder oscillating normal to a sandy bed. • Transport mechanisms and trench geometry linked to pumping of fluid beneath the cylinder and to vortex dynamics. • Control volume arguments combined with an oscillatory boundary layer assumption predict initiation of trenching for small oscillation amplitudes close to the bed. • Rate of trench development and maximum trench depth increase with oscillation amplitude and maximum cylinder velocity. [ABSTRACT FROM AUTHOR]

Abstract Human curiosity for the natural world is the leading factor for the progress in deep ocean exploration. However, only in the last 60 years has the technological capability satisfied the human desire to reach the deepest point of the oceans. In this paper, the history of human exploration to the deep oceans is reviewed with a focus on the lessons learned from these pioneers. From this review, it is shown that deep-sea trenches remain one of the least explored ocean ecosystems due to a wide range of issues such as lack of investigation platforms and cruise funding. Although humans have twice reached the deepest point of the oceans, up to now there were no operational platforms for ocean scientists to freely conduct scientific research in hadal trenches. Therefore, a new strategy is being introduced in China to overcome the technical and funding issues to develop a comprehensive investigation system for hadal trenches. Through more than four years' practice, the new strategy has proved to be effective and the first phase of the comprehensive investigation system has been constructed. In this paper, this new strategy and its progress are presented and the underlying philosophy is explained. [ABSTRACT FROM AUTHOR]

Abstract The hadal zone (6000–~ 11,000 m deep) arguably represents the last great frontier in marine science. Although scientific endeavour in these deepest ecosystems has been slow relative to other more accessible environments, progress is steadily being made, particularly in the last 10 years. This paper details the latest developments in technology and sampling effort at full ocean depth, scientific literature, representation in international conferences and symposia, the recent acquisition of large ecological data sets, conservation, the potential for biodiscovery and describes some key strategic sampling approaches to ensure recent progress is sustained effectively. The timing of this article is indeed to reflect on recent sampling efforts and resulting publications to provide perspectives on where the scientific community is with regards to hadal science and where it might lead in the immediate future. Highlights • Reviews contemporary technology and sampling effort at full ocean depth. • Provides an overview of the recent acquisition of large ecological data sets. • Details recent conservation initiatives and potential for biodiscovery. • Provides key strategic sampling approaches to ensure continued progress in the field. [ABSTRACT FROM AUTHOR]

Abstract The hadal zone is the deep ocean with depths exceeding 6000 m. With an area approximately equal to the size of Australia, but constituting the deepest 45% of the vertical depth of the global ocean, it should come as no surprise that the hadal zone is the last great frontier of the ocean science. Historically, the hadal trenches are believed to have limited activities of life, microscopic or macroscopic, due to the extreme environmental conditions such as high hydrostatic pressure, lacking of sunlight and food supply, as well as frequent subduction-zone earthquakes. However, seminal discoveries have been made recently by scientists working at hadal trenches on microbial and faunal community structure, abundance, diversity, and mechanisms for endemism, speciation and adaptation, likely regulated by a myriad of specific geological, physical, and chemical factors of the hadal zone, forming the so-called "hadal biosphere". These discoveries ignited a new wave of hypotheses and theories on organismal metabolism, energy acquisition, and the origins of life on Earth. Here, we review the recent development in understanding the hadal biosphere and discuss future directions on hadal biosphere research. [ABSTRACT FROM AUTHOR]

Abstract Deep-sea animals inhabiting the hadal zone (>6000 m deep) are frequently reported to occupy large bathymetric ranges, but details of intra-specific population structure and the underlying drivers remain poorly understood. Here we describe the population structure of the dominant SW Pacific hadal amphipod, Bathycallisoma schellenbergi, across its bathymetric range in the Kermadec (10,047 m) and New Hebrides (7156 m) trenches and explore the factors likely responsible for the observed patterns. These two trenches are relatively close geographically but underlie different water masses and are topographically isolated by the Kermadec fore-arc. The minimum depth of B. schellenbergi was ~1000 m shallower in the New Hebrides Trench than in the Kermadec Trench although similar patterns of ontogenetic stratification were present; juveniles were restricted to depths <~7000 m, larger adults were deeper toward the trench axes, regardless of absolute depth. We suggest that the increased juvenile presence around the abyssal-hadal transition zone is driven by reduced competition and a relaxation of pressure effects on metabolism. Evidence is also presented for morphological heterogeneity indicating geographic isolation between the two trenches. Interestingly, the absence of another major scavenging amphipod, Hirondellea dubia, at mid-trench depths corresponded to maximum densities of B. schellenbergi , a trend that was reversed towards the trench axes regardless of actual depth. This suggests that the population structure and amphipod assemblages are more affected by how trench topography influences the distribution of food and potential ecological interactions, rather than simply by the effects of hydrostatic pressure. Highlights • The hadal amphipod, Bathycallisoma schellenbergi, was sampled across its full depth range within two hadal trenches. • Their minimum depth range was 1000 m shallower in the shallower trench (New Hebrides) and their maximum abundance occurred 1000 m deeper in the deeper trench (Kermadec). • Relative trends in population structure were the same in both trenches but occurred over different depths. • The lower depth limit for juveniles was the same in both trenches. • Trench morphology and ecological interactions influence population structure rather than hydrostatic pressure alone. [ABSTRACT FROM AUTHOR]

Abstract Hadal trenches with water depth ranging from 6000 to 11,000 m are the deepest biogeochemical province in the world's oceans. Due to technological challenges of sampling and observation at such extreme depths, these trenches are one of the least exploited habitats on our planet. Recent findings show high sedimentation rates, high biomass, intense microbial activity and chemosynthetic communities in hadal trenches, suggesting that they may play a more important role in global ocean biogeochemical cycles than previously recognized. Here we review the hadal biogeochemistry in four aspects: food supply and its effect on hadal life; hadal carbon cycle; microbial-mediated stable carbon isotope fractionation in biosynthesis of lipids under high hydrostatic pressures; and pollution in the hadal trenches. Our current knowledge of hadal biogeochemistry is rudimentary and many fundamental questions remain to be answered. However, with the rapid development of the full-ocean-depth exploration and sampling technologies, it is very likely hadal scientists will make significant breakthroughs in hadal biogeochemistry research in the coming decades. [ABSTRACT FROM AUTHOR]

Abstract Heat-shock proteins (HSPs) are a prominent family of cellular chaperones that are involved in the folding, assembly and degradation of cellular proteins, cell-cycling and signal transduction. HSPs are high conserved across taxa and form a key component of the stress response with signatures of molecular adaptation in some species exposed to extreme environmental stressors such as dehydration, heavy metal pollutants and arctic temperatures. Here we characterise two key heat-shock protein genes (hsp70 and hsp90) in deep-sea Lysianassoidea amphipods, with a focus on copy number variation and signatures of selection on the DNA sequences. Four phylogenetically distinct isoforms were resolved for both hsp70 and hsp90, with one isoform in each gene being exclusive to the hadal genus Hirondellea. Signatures of purifying selection were shown across hsp70 and hsp90 from dN:dS ratios. The GC content of each gene was lower, and the number of codons used was higher, than in shallow water amphipods suggesting a relaxation in codon usage bias. Such observations suggest that increased hydrostatic pressure is an important environmental stress that shapes the adaptation of heat-shock protein genes in deep-sea amphipods. [ABSTRACT FROM AUTHOR]

Deep-sea organisms must cope with hydrostatic pressure, up to ~ 110 MPa (~ 1100 atm) at 11 km in the Mariana Trench. Pressure perturbs protein conformations, ligand binding, and can promote damaging protein aggregates. Salinity is another stressor: seawater, averaging ~ 1000 mOsm/kg, is dehydrating towards the ~ 250–400 mOsm/kg of universal cellular solutes. Most marine organisms prevent dehydration by becoming isosmotic with cellular organic osmolytes, which, unlike NaCl, do not perturb proteins and can have chemical chaperone properties, stabilizing proteins against various physicochemical stressors. In shallow-dwelling invertebrates, these osmolytes are typically taurine, glycine, and betaine. Previous work on crustaceans down to 3 km found that “shallow” osmolytes decreased with depth, replaced with trimethylamine N-oxide (TMAO) and other apparent chemical chaperones. In bony fishes, TMAO increases with depth down to 8 km, and is probably a piezolyte (pressure counteractant) as it reverses pressure's inhibitory effects. Here we analyzed amphipods over the full ocean depth range of 0–11 km, from coastal habitats and numerous depths in the Kermadec and Mariana Trenches. We found that “shallow” osmolytes decreased while potential piezolytes, in different combinations in different species, increased with depth. These are TMAO, glycerophosphorylcholine and proline betaine (all methylamine chaperones), glycerophosphorylethanolamine, and scyllo -inositol, a polyol chaperone (being tested for Alzheimer's Disease) that works differently than methylamines by preventing damaging protein aggregations. Though only TMAO has been tested for piezolyte properties, these other solutes may explain how some hadal amphipods live deeper than fish, which appear to be limited to a maximum of 8.4 km. [ABSTRACT FROM AUTHOR]

Hirondellea species are common inhabitants in the hadal region deeper than 7,000 m. We found that Hirondellea gigas thrived in the Challenger Deep possessed polysaccharide hydrolases as digestive enzymes. To obtain various enzymes of other H. gigas, we captured amphipods from the Japan Trench, and Izu-Ogasawara (Bonin) Trench. A phylogenetic analysis based on the cytochrome oxidase I gene showed close relationships among amphipods, despite the geographic distance between the localities. However, several differences in enzymatic properties were observed in these H. gigas specimens. We also carried out RNA sequencing of H. gigas from the Izu-Ogasawara Trench. The cellulase gene of H. gigas was highly homologous to cellobiohydrolase of Glucosyl Hydrolase family 7 (GH7). On the other hand, enzymatic properties of H. gigas’s cellulase were different from those of typical GH7 cellobiohydrolase. Thus, these results indicate that hadal-zone amphipod can be good candidates as the new enzyme resource. [ABSTRACT FROM AUTHOR]

The initial, anthropocentric view of the deep ocean was that of a hostile environment inhabited by organisms rendered lethargic by constant high pressure, low temperature and sparse food supply, hence evolving slowly. This conceptual framework of a spatially and temporally homogeneous, connected, strongly bottom-up controlled habitat implied a strong constraint on, or poor incentive for, speciation. Hence, the discovery in the late 1960s of high species diversity of abyssal benthic invertebrates came as a surprise. Since then, the slow-motion view of deep-sea ecology and evolution has speeded up and diversified in the light of increasing evidence accumulating from in situ visual observations complemented by molecular and other tools. The emerging picture is that of a much livelier, highly diversified and more complex deep-sea fauna than previously assumed. In this review we examine the consequences of the incoming information for developing a broader view of evolutionary ecology in the deep sea, and for scavenging amphipods in particular. We revisit the food supply to the deep-sea floor and hypothesize that the dead bodies of animals, ranging from zooplankton to large fish are likely to be a more important source of food than their friable faeces. Camera observations of baited traps indicate that amphipod carrion-feeders arrive within hours at the bait which continues to draw new individuals for days to months later, presumably by scent trails in tidal currents. We explore the different stages of food acquisition upon which natural selection may have acted, from detection to ingestion, and discuss the possibility of a broader range of food acquisition strategies, including predation and specializations. Although currently neglected in deep-sea ecology, top-down factors are likely to play a more important role in the evolution of deep-sea organisms. Predation on amphipods at baits by bathyal and abyssal fishes, and large predatory crustaceans in the hadal zone, is often observed. Finally, we develop hypotheses regarding the effects of past, present and imminent anthropogenic activities on scavenger biomass and how these can be tested with the most modern tools. [ABSTRACT FROM AUTHOR]

Abstract: Hadal trenches have been proposed as depocenters of organic material and hot spots for organic matter mineralization. In this study, we for the first time quantified the total benthic O2 uptake in hadal trenches using in situ chamber incubations. Three trenches in the tropical Pacific were targeted and exhibited relatively high diagenetic activity given the great water depths, that is, the Mariana Trench (2.0 × 102 μmol O2 m−2 d−1, 10,853 m), the Mussau Trench (2.7 ± 0.1 × 102 μmol O2 m−2 d−1, 7,011 m), and the New Britain Trench (6.0 ± 0.1 × 102 μmol O2 m−2 d−1, 8,225 m). Combined with the analyses of total organic carbon and δ13C of total organic carbon in the sediments and previously published in situ O2 microprofiles from hadal settings, we suggest that hadal benthic carbon mineralization partly is governed by the surface production and also is linked to the distance from land. Therefore, we highlight that terrestrial organic matter can be of importance in sustaining benthic communities in some hadal settings. [ABSTRACT FROM AUTHOR]

The observed maximum magnitude of subduction megathrust earthquakes is highly variable worldwide. One key question is which conditions, if any, favor the occurrence of giant earthquakes (M w  ≥ 8.5). Here we carry out a multivariate statistical study in order to investigate the factors affecting the maximum magnitude of subduction megathrust earthquakes. We find that the trench-parallel extent of subduction zones and the thickness of trench sediments provide the largest discriminating capability between subduction zones that have experienced giant earthquakes and those having significantly lower maximum magnitude. Monte Carlo simulations show that the observed spatial distribution of giant earthquakes cannot be explained by pure chance to a statistically significant level. We suggest that the combination of a long subduction zone with thick trench sediments likely promotes a great lateral rupture propagation, characteristic of almost all giant earthquakes. [ABSTRACT FROM AUTHOR]

The hadal zone largely comprises a series of subduction trenches that do not form part of the continental shelf-slope rise to abyssal plain continuum. Instead they form geographically isolated clusters of deep-sea (6000–11,000 m water depth) environments. There is a growing realization in hadal science that ecological patterns and processes are not driven solely by responses to hydrostatic pressure, with comparable levels of habitat heterogeneity as observed in other marine biozones. Furthermore, this heterogeneity can be expressed at multiple scales from inter-trench levels (degrees of geographical isolation, and biochemical province), to intra-trench levels (variation between trench flanks and axis), topographical features within the trench interior (sedimentary basins, ridges, escarpments, ‘deeps’, seamounts) to the substrate of the trench floor (seabed-sediment composition, mass movement deposits, bedrock outcrop). Using best available bathymetry data combined with the largest lander-derived imaging dataset that spans the full depth range of three hadal trenches (including adjacent slopes); the Mariana, Kermadec and New Hebrides trenches, the topographic variability, fine-scale habitat heterogeneity and distribution of seabed sediments of these three trenches have been assessed for the first time. As well as serving as the first descriptive study of habitat heterogeneity at hadal depths, this study also provides guidance for future hadal sampling campaigns taking into account geographic isolation, total trench particulate organic matter flux, maximum water depth and area. [ABSTRACT FROM AUTHOR]

Hadal trenches are isolated habitats that cover the greatest ocean depths (6,500–11,000 m) and are believed to host high levels of endemism across multiple taxa. A group of apparent hadal endemics is within the snailfishes (Liparidae), found in at least five geographically separated trenches. Little is known about their biology, let alone the reasons for their success at hadal depths around the world. This study investigated the life history of hadal liparids using sagittal otoliths of two species from the Kermadec ( Notoliparis kermadecensis ) and Mariana ( Pseudoliparis swirei ) trenches in comparison to successful abyssal macrourids found at the abyssal-hadal transition zone. Otoliths for each species revealed alternating opaque and translucent growth zones that could be quantified in medial sections. Assuming these annuli represent annual growth, ages were estimated for the two hadal liparid species to be from five to 16 years old. These estimates were compared to the shallower-living snailfish Careproctus melanurus , which were older than described in previous studies, expanding the potential maximum age for the liparid family to near 25 years. Age estimates for abyssal macrourids ranged from eight to 29 years for Coryphaenoides armatus and six to 16 years for C. yaquinae . In addition, 18 O/ 16 O ratios (δ 18 O) were measured across the otolith using secondary ion mass spectrometry (SIMS) to investigate the thermal history of the three liparids, and two macrourids. Changes in δ 18 O values were observed across the otoliths of C. melanurus , C. armatus , and both hadal liparids, the latter of which may represent a change of >5 °C in habitat temperature through ontogeny. The results would indicate there is a pelagic larval stage for the hadal liparids that rises to a depth above 1000 m, followed by a return to the hadal environment as these liparids grow. This result was unexpected for the hadal liparids given their isolated environment and large eggs, and the biological implications and plausibility of interpretations of these data are discussed. This study presents a first look at the life history of some of the deepest-living fishes through otolith analyses. [ABSTRACT FROM AUTHOR]

The habitability of the hadal zone is not well understood and how microbes survive and adapt to this extreme environment is still largely unknown. Here we report the discovery of microbial biofilm that experience extensive Fe-mineralization on the surface of rocks from the Challenger Deep in the southern Mariana Trench. We found microbial biofilms of abundant globules internally permineralized by acicular goethite. The EDS analyses, combined with SEM and TEM, show that the globules are enriched in carbon and phosphorus, potentially indicating their biological origin. Cross sections of globules reveal a bilayer structure consisting of an acicular nano-crystalline core and a colloidal envelope. Vibrational spectroscopy reveals that this colloidal envelope is composed of EPS (extracellular polymeric substances). This mineralization is completely different from Fe-mineralization induced by all other known Fe-oxidizing bacteria, suggesting previously unknown interactions between Fe and microorganisms in hadal settings. Our findings highlight the significance of Fe-mineralization and its contributions to the ecosystem in hadal environments. • Discovery of microbial biofilm that experience extensive Fe-mineralization on basalts from the Mariana Trench. • Microbial biofilms of abundant globules internally permineralized by acicular goethite. • Study highlights the significance of Fe-mineralization and its contributions to the ecosystem in the deepest part of the world's ocean. [ABSTRACT FROM AUTHOR]

A new maximum depth record for bony fish (Teleostei) has been set at 8336 m deep in the Izu- Ogasawara Trench (NW Pacific Ocean) represented by video imagery of a snailfish probably Pseupodoliapris belyaevi or a new, endemic species since no physical specimen was captured. Sampling by trap and video was undertaken between 4534 and 9773 m and snailfish (Liparidae) were observed between 6824 and 8336 m, the deepest being a solitary juvenile. In the neighbouring Japan Trench, 31 deployments were made between 4913 and 8022 m and P. belyaevi was observed in 22 of the 24 deployments >6579 m. Two specimens, identified as P. belyaevi, were collected from 8022 m and represent the deepest fish ever caught. In the Japan and Izu-Ogasawara trenches at 6579–7001 m and 6824–7050 m, respectively, two other snailfish species were observed (Liparid sp. indet. 4-JPT and Liparid sp. indet. 4-IOT). A species of snailfish belonging to the putative 'ethereal' morphotype was also observed in the Japan Trench at 7621 and 8022 m (Liparid sp. indet. 3-JPT), but its identification beyond family is still obscure. This study also documents the first hadal fish from the Ryukyu Trench (Liparid sp. indet. 1-RKT) at 7079 and 7339 m. • New maximum depth record for bony fish (Teleostei) has been set. • Snailfish, Pseupodoliapris belyaevi, observed at 8336 m deep in the Izu- Ogasawara. • P. belyaevi, collected from Japan Trench (8022 m) represent the deepest fish ever caught. • Two other snailfish species were observed in Japan and Izu-Ogasawara trenches. • First hadal fish from the Ryukyu Trench (Liparid sp.) observed at 7079 and 7339 m. [ABSTRACT FROM AUTHOR]

Hadal zones are immensely difficult to explore due to their incredibly harsh environments. In this study, an autonomous underwater glider called Petrel-XPLUS was developed for monitoring hadal zones at large spatiotemporal scales. This glider is equipped with a bionic gradient functional housing, the multi-material pressure housing, to cope with the coupled effects of extremely high pressure and density variations, endowing it with light weight, near-neutral buoyancy, and sufficient onboard energy. Furthermore, an innovative dual-eccentric attitude-regulating mechanism is proposed to increase the pitch angle range from [−45°, 45°] to [−90°, 90°] and enrich four types of glider motion modes. This mechanism and a depth-averaged flow-based path planning method enable the glider to handle the superimposed effects of unknown currents and complex topography for effective access to the hadal zone floor. The developed glider has an operating range of 5000 km or 80 dives to a depth of 11,000 m in a single mission lasting 200 d. Petrel-XPLUS successfully completed three dives exceeding 10,000 m with a maximum depth of 10,619 m and an average station-keeping accuracy of 2.018 km during a sea trial in the Mariana Trench. Our study provides a novel technical reference for facilitating hadal exploration. • A gradient functional pressure housing with near-neutral buoyancy, high load-bearing capacity, and impact resistance is proposed. • A compact mechanism is designed to enrich the motion modes of the underwater glider by extending the pitch angle range to −90°∼90°. • A new method of accessing the bottom region of the hadal zones has been implemented. • Petrel-XPLUS completed three consecutive profiles over 10,000 m in the Challenger Deep, with a maximum depth of 10,619 m and an average station-keeping accuracy of 2.018 km. [ABSTRACT FROM AUTHOR]

Metabolic enzyme activities of muscle tissue have been useful and widely-applied indicators of whole animal metabolic capacity, particularly in inaccessible systems such as the deep sea. Previous studies have been conducted at atmospheric pressure, regardless of organism habitat depth. However, maximum reaction rates of some of these enzymes are pressure dependent, complicating the use of metabolic enzyme activities as proxies of metabolic rates. Here, we show pressure-related rate changes in lactate and malate dehydrogenase (LDH, MDH) and pyruvate kinase (PK) in six fish species (2 hadal, 2 abyssal, 2 shallow). LDH maximal reaction rates decreased with pressure for the two shallow species, but, in contrast to previous findings, it increased for the four deep species, suggesting evolutionary changes in LDH reaction volumes. MDH maximal reaction rates increased with pressure in all species (up to 51±10% at 60 MPa), including the tide pool snailfish, Liparis florae (activity increase at 60 MPa 44±9%), suggesting an inherent negative volume change of the reaction. PK was inhibited by pressure in all species tested, including the hadal liparids (up to 34±3% at 60 MPa), suggesting a positive volume change during the reaction. The addition of 400 mM TMAO counteracted this inhibition at both 0.5 and 2.0 mM ADP concentrations for the hadal liparid, Notoliparis kermadecensis . We revisit depth-related trends in metabolic enzyme activities according to these pressure-related rate changes and new data from seven abyssal and hadal species from the Kermadec and Mariana trenches. Results show that, with abyssal and hadal species, pressure-related rate changes are another variable to be considered in the use of enzyme activities as proxies for metabolic rate, in addition to factors such as temperature and body mass. Intraspecific increases in tricarboxylic acid cycle enzymes with depth of capture, independent of body mass, in two hadal snailfishes suggest improved nutritional condition for individuals deeper in the hadal zone, likely related to food availability. These new data inform the discussion of factors controlling metabolism in the deep sea, including the visual interactions hypothesis and extend published trends to the planet's deepest-living fishes. [ABSTRACT FROM AUTHOR]

Appropriate drainfield design in septic systems can be an effective tool to optimize nitrogen (N) removal and protect public and environmental health. Our objective was to investigate how the conventional drainfield designs affect N transformations in septic systems. We investigated the N treatment efficiency of two in-situ drainfield designs (6.1 m long and 0.61 m wide), which were constructed by (1) adding 30.5 cm sand on top of natural soil (referred to as drip dispersal) and (2) adding 30.5 cm gravel layer on top of 30.5 cm of sand and natural soil (referred to as gravel trench). A drip line was placed on top of the sand layer (in drip dispersal) or gravel layer (in gravel trench) to disperse 120 L of effluent before adding another 15.3 cm of sand layer and planting St. Augustine grass. Suction cup lysimeters, installed at 30.5, 61.0, and 106.7 cm depth below the drip lines of both drainfields, were used to collect soil-water samples over 64 events (May 2012–December 2013). Mean total N concentration was 66 mg L −1 in the effluent, which significantly (p < 0.05) decreased by 49% in the drip dispersal and 21% in the gravel trench systems. Most of the effluent N was nitrified within 30.5–106.7 cm depth of drainfields. Significantly (p < 0.05) greater NO x –N in the gravel trench (45.6 ± 1.3 mg L −1 ) than drip dispersal (28 ± 3.1 mg L −1 ) was attributed to the presence of additional 30.5 cm gravel layer that caused greater oxygen diffusion resulting in lower gaseous N loss via coupled nitrification-denitrification. Total organic N was not significantly different between the effluent (7.3 mg L −1 ; n = 61) and drainfields (5.5–6.4 mg L −1 ; n = 192) suggesting that organic N forms remained mobile in the drainfields. We conclude that the drip dispersal was more effective at attenuating NO x –N than gravel trench due to the greater gaseous N loss and should be preferred in areas with shallow groundwater (< 1 m deep) to protect groundwater N contamination. [ABSTRACT FROM AUTHOR]