Search

PREMISE Understanding the adaptive capacities of species over long timescales lies in examining the revived recent and millennia old resting spores buried in sediments. We show for the first time the revival, viability and germination rate of resting spores of the diatom Chaetoceros deposited in sub-seafloor sediments from three ages (recent: 0-80 years; ancient: ~1250 (Medieval Climate Anomaly) and ~6600 (Holocene Thermal Maximum) calendar year before present. METHODS Recent and ancient Chaetoceros spores were revived to examine their viability and germination rate. Light and scanning electron microscopy and Sanger sequencing was done to identify the species. KEY RESULTS We show that ~6600 cal. year BP old Chaetoceros resting spores are still viable and the vegetative reproduction in recent and ancient resting spores vary. The time taken to germinate is three hours to 2-3 days in both recent and ancient spores, but the germination rate of the spores decreased with increasing age. The germination rate of the recent spores was ~41% while that of the ancient spores were ~31% and ~12% for the ~1250 and ~6600 cal. year BP old resting spores. Based on the morphology of the germinated vegetative cells we identified the species as Chaetoceros muelleri var. subsalsum. Sanger sequences of nuclear and chloroplast markers identified the species as Chaetoceros muelleri. CONCLUSIONS We identify a unique model system, Chaetoceros muelleri var. subsalsum and show that recent and ancient resting spores of the species buried in sediments in the Baltic Sea can be revived and used for long-term evolutionary studies. This article is protected by copyright. All rights reserved.

The Laptev Sea and the East Siberian Sea are remote areas of the Arctic region where detailed data on phytoplankton composition and spatial distribution remain limited. In the context of the ongoing environmental changes (increasing warming and ice melting) and prospective exploration activities (oil and gas production) on the Arctic shelves, understanding of the seasonal and interannual phytoplankton community dynamics is of critical importance. Our study provides new specifying data on species composition of phytoplankton over the vast area of the Laptev Sea shelf and the East Siberian Sea shelf. We found that the outer shelf of the Laptev and East Siberian seas was characterized by typical late spring diatom species (Chaetoceros furcellatus, Chaetoceros diadema, Chaetoceros debilis, Chaetoceros constrictus). On the inner shelf of the Laptev Sea, which is strongly affected by the Lena River water masses, the phytoplankton were characterized by the transition from the summer to an autumn stage of development. Local algal assemblages were composed by mixo- and heterotrophic dinoflagellates (Dinophysis and Protoperidinium genera) together with marine and brackish water-marine diatoms (Thalassiosira hyperborea, Thalassiosira baltica, Thalassiosira gravida, Thalassiosira nordenskioeldii) accompanied by sporadically occurring freshwater riverine planktonic diatom species (Aulacoseira granulata, Aulacoseira italica, Asterionella formosa). These variations in species composition over the Laptev Sea shelf were attributed to differences in the hydrography, marine chemical conditions, and the sea-ice regime.

Diatoms have well-recognized roles in fixing and exporting carbon and supplying energy to marine ecosystems, but only recently have we begun to explore the diversity and importance of nano- and pico-diatoms. Here we describe a small (~5 {micro}m) diatom from the genus Chaetoceros isolated from a wintertime temperate estuary (2 {degrees}C, Narragansett Bay, RI), with a unique obligate specialization for low-light environments (< 120 {micro}mol photons m-2 sec-1). This diatom exhibits a striking interaction between irradiance and thermal responses whereby as temperatures increase, so does its susceptibility to light stress. Historical 18S rRNA amplicon data from our study site show this isolate was abundant throughout a six-year period, and its presence strongly correlates with winter and early spring months when light and temperature are low. Two ASVs matching this isolate had a circumpolar distribution in Tara Polar Ocean Circle samples, indicating its unusual light and temperature requirements are adaptations to life in a cold, dark environment. We expect this isolate's low light, psychrophilic niche to shrink as future warming-induced stratification increases both light and temperature levels experienced by high latitude marine phytoplankton.

The North Water region, between Greenland and Ellesmere Island, with high populations of marine birds and mammals, is an Arctic icon. Due to climate related changes, seasonal patterns in water column primary production are changing but the implications for the planktonic microbial eukaryote communities that support the ecosystem are unknown. Here we report microbial community phenology in samples collected over 12 years (2005–2018) from July to October and analysed using high throughput 18S rRNA V4 amplicon sequencing. Community composition was tied to seasonality with summer communities more variable than distinct October communities. In summer, sentinel pan-Arctic species, including a diatom in the Chaetoceros socialis-gelidus complex and the picochlorophyte Micromonas polaris dominated phytoplankton and were summer specialists. In autumn, uncultured undescribed open water dinoflagellates were favored, and their ubiquity suggests they are sentinels of arctic autumn conditions. Despite the input of nutrients into surface waters, autumn chlorophyll concentrations remained low, refuting projected scenarios that longer ice-free seasons are synonymous with high autumn production and a diatom dominated bloom. Overall, the summer sentinel microbial taxa are persisting, and a subset oceanic dinoflagellate should be monitored for possible ecosystem shifts as later autumn ice formation becomes prevalent elsewhere.

The impact of selective predation of weaker individuals on the general health of prey populations is well-established in animal ecology. Analogous processes have not been considered at microbial scales despite the ubiquity of microbe-microbe interactions, such as parasitism. Here we present insights into the biotic interactions between a widespread marine thraustochytrid and a diatom from the ecologically important genus Chaetoceros. Physiological experiments show the thraustochytrid targets senescent diatom cells in a similar way to selective animal predation on weaker prey individuals. This physiology-selective targeting of ‘unhealthy’ cells appears to improve the overall health (i.e., increased photosynthetic quantum yield) of the diatom population without impacting density, providing support for ‘healthy herd’ dynamics in a protist–protist interaction, a phenomenon typically associated with animal predators and their prey. Thus, our study suggests caution against the assumption that protist–protist parasitism is always detrimental to the host population and highlights the complexity of microbial interactions.

The formation of sinking particles in the ocean, which promote carbon sequestration into deeper water and sediments, involves algal polysaccharides acting as an adhesive, binding together molecules, cells and minerals. These as yet unidentified adhesive polysaccharides must resist degradation by bacterial enzymes or else they dissolve and particles disassemble before exporting carbon. Here, using monoclonal antibodies as analytical tools, we trace the abundance of 27 polysaccharide epitopes in dissolved and particulate organic matter during a series of diatom blooms in the North Sea, and discover a fucose-containing sulphated polysaccharide (FCSP) that resists enzymatic degradation, accumulates and aggregates. Previously only known as a macroalgal polysaccharide, we find FCSP to be secreted by several globally abundant diatom species including the genera Chaetoceros and Thalassiosira. These findings provide evidence for a novel polysaccharide candidate to contribute to carbon sequestration in the ocean., The fate of ocean carbon is determined by the balance between primary productivity and heterotrophic breakdown of that photosynthate. Here the authors show that diatoms produce a polysaccharide that resists bacterial degradation, accumulates, aggregates and stores carbon during spring blooms.

Arsenic pollution is a widespread threat to marine life, but the ongoing rise pCO2 levels is predicted to decrease bio-toxicity of arsenic. However, the effects of arsenic toxicity on marine primary producers under elevated pCO2 are not well characterized. Here, we studied the effects of arsenic toxicity in three globally distributed diatom species (Phaeodactylum tricornutum, Thalassiosira pseudonana, and Chaetoceros mulleri) after short-term acclimation (ST, 30 days), medium-term exposure (MT, 750 days), and long-term (LT, 1460 days) selection under ambient (400 µatm) and elevated (1000 and 2000 µatm) pCO2. We found that elevated pCO2 alleviated arsenic toxicity even after short acclimation times but the magnitude of the response decreased after mid and long-term adaptation. When fed with these elevated pCO2 selected diatoms, the scallop Patinopecten yessoensis had significantly lower arsenic content (3.26–52.83%). Transcriptomic and biochemical analysis indicated that the diatoms rapidly developed arsenic detoxification strategies, which included upregulation of transporters associated with shuttling harmful compounds out of the cell to reduce arsenic accumulation, and upregulation of proteins involved in synthesizing glutathione (GSH) to chelate intracellular arsenic to reduce arsenic toxicity. Thus, our results will expand our knowledge to fully understand the ecological risk of trace metal pollution under increasing human activity induced ocean acidification.

In the Southern Ocean (SO), iron (Fe) limitation strongly inhibits phytoplankton growth and generally decreases their primary productivity. Diatoms are a key component in the carbon (C) cycle, by taking up large amounts of anthropogenic CO2 through the biological carbon pump. In this study, we investigated the effects of Fe availability (no Fe and 4 nM FeCl3 addition) on the physiology of Chaetoceros cf. simplex, an ecologically relevant SO diatom. Our results are the first combining oxygen evolution and uptake rates with particulate organic carbon (POC) build up, pigments, photophysiological parameters and intracellular trace metal (TM) quotas in an Fe-deficient Antarctic diatom. Decreases in both oxygen evolution (through photosynthesis, P) and uptake (respiration, R) coincided with a lowered growth rate of Fe-deficient cells. In addition, cells displayed reduced electron transport rates (ETR) and chlorophyll a (Chla) content, resulting in reduced cellular POC formation. Interestingly, no differences were observed in non-photochemical quenching (NPQ) or in the ratio of gross photosynthesis to respiration (GP:R). Furthermore, TM quotas were measured, which represent an important and rarely quantified parameter in previous studies. Cellular quotas of manganese, zinc, cobalt and copper remained unchanged while Fe quotas of Fe-deficient cells were reduced by 60% compared with High Fe cells. Based on our data, Fe-deficient Chaetoceros cf. simplex cells were able to efficiently acclimate to low Fe conditions, reducing their intracellular Fe concentrations, the number of functional reaction centers of photosystem II (RCII) and photosynthetic rates, thus avoiding light absorption rather than dissipating the energy through NPQ. Our results demonstrate how Chaetoceros cf. simplex can adapt their physiology to lowered assimilatory metabolism by decreasing respiratory losses.

Two sediment cores were collected from two sites in Nagatsura-Ura Lagoon, the mouth of which was destroyed in the 2011 Tohoku-oki tsunami. Although sediment conditions differed widely between the central and inner parts of the lagoon, we could identify traces of the 2011 Tohoku-oki tsunami at both, and those of earlier tsunamis in the inner part of the lagoon. Chlorophyll a maxima were observed at core depths between 62 and 72 cm in the central part of the lagoon. We inferred that this horizon correlated with the 2011 Tohoku-oki tsunami deposit. In the inner part of the lagoon, the horizon at core depths of 116—142 cm could be aged to the 1400s, and is thus thought to have been formed as a consequence of the Kyotoku earthquake of 1454. The dinoflagellate Alexandrium spp. accounted for ~ 22.6% of the total read numbers of phytoplankton from next generation sequencing. Diatoms made up the second largest group of phytoplankton, with Chaetoceros spp. representing ~ 10.5% of the total. The vertical profiles of dinoflagellate and diatom compositions showed little change. Although traces of tsunamis were observed in cores from the lagoon, and in nearby closed bays, we believe that the microbial composition has changed only slightly in the lagoon due to poor seawater exchange through the closed lagoon mouth.

The Fram Strait is an area with a relatively low and irregular distribution of diatom microfossils in surface sediments, and thus microfossil records are scarce, rarely exceed the Holocene, and contain sparse information about past richness and taxonomic composition. These attributes make the Fram Strait an ideal study site to test the utility of sedimentary ancient DNA (sedaDNA) metabarcoding. Amplifying a short, partial rbcL marker from samples of sediment core MSM05/5-712-2 resulted in 95.7 % of our sequences being assigned to diatoms across 18 different families, with 38.6 % of them being resolved to species and 25.8 % to genus level. Independent replicates show a high similarity of PCR products, especially in the oldest samples. Diatom sedaDNA richness is highest in the Late Weichselian and lowest in Mid- and Late Holocene samples. Taxonomic composition is dominated by cold-water and sea-ice-associated diatoms and suggests several reorganisations – after the Last Glacial Maximum, after the Younger Dryas, and after the Early and after the Mid-Holocene. Different sequences assigned to, amongst others, Chaetoceros socialis indicate the detectability of intra-specific diversity using sedaDNA. We detect no clear pattern between our diatom sedaDNA record and the previously published IP25 record of this core, although proportions of pennate diatoms increase with higher IP25 concentrations and proportions of Nitzschia cf. frigida exceeding 2 % of the assemblage point towards past sea-ice presence.

Ozone depletion and climate change are causing the Southern Annular Mode (SAM) to become increasingly positive, driving stronger winds southward in the Southern Ocean (SO), with likely effects on phytoplankton habitat due to possible changes in ocean mixing, nutrient upwelling, and sea ice characteristics. This study examined the effect of the SAM and 12 other environmental variables on the abundance of siliceous and calcareous phytoplankton in the seasonal ice zone (SIZ) of the SO. A total of 52 surface-water samples were collected during repeat resupply voyages between Hobart, Australia, and Dumont d'Urville, Antarctica, centred around longitude 142∘ E, over 11 consecutive austral spring–summer seasons (2002–2012), and spanning 131 d in the spring–summer from 20 October to 28 February. A total of 22 taxa groups, comprised of individual species, groups of species, genera, or higher taxonomic groups, were analysed using CAP analysis (constrained analysis of principal coordinates), cluster analysis, and correlation. Overall, satellite-derived estimates of total chlorophyll and measured depletion of macronutrients both indicated a more positive SAM was associated with greater productivity in the SIZ. The greatest effect of the SAM on phytoplankton communities was the average value of the SAM across 57 d in the previous austral autumn centred around 11 March, which explained 13.3 % of the variance in community composition in the following spring–summer. This autumn SAM index was significantly correlated pair-wise (p) with the relative abundance of 12 of the 22 taxa groups resolved. A more positive SAM favoured increases in the relative abundance of large Chaetoceros spp. that predominated later in the spring–summer and reductions in small diatom taxa and siliceous and calcareous flagellates that predominated earlier in the spring–summer. Individual species belonging to the abundant Fragilariopsis genera responded differently to the SAM, indicating the importance of species-level observation in detecting SAM-induced changes in phytoplankton communities. The day through the spring–summer on which a sample was collected explained a significant and larger proportion (15.4 %) of the variance in the phytoplankton community composition than the SAM, yet this covariate was a proxy for such environmental factors as ice cover and sea surface temperature, factors that are regarded as drivers of the extreme seasonal variability in phytoplankton communities in Antarctic waters. The impacts of SAM on phytoplankton, which are the pasture of the SO and principal energy source for Antarctic life, would have ramifications for both carbon export and food availability for higher trophic levels in the SIZ of the SO.

Resting stages are reported for several unicellular eukaryotes including diatoms that can produce spores or resting cells. These stages represent a repository of diversity in sediments but the factors that induce their formation are elusive. We investigated spore formation in the marine diatom Chaetoceros socialis. Our results confirm that nitrogen starvation is an effective experimental trigger for spore formation. However, we observed that spores are also produced when the external pool of nutrients is not limiting, and by using a semicontinuous setup, we could show that they appear only when cell density is high. The presence of a chemical cue mediating spore formation was supported by experiments carried out incubating cells with culture media conditioned by both healthy and lysed cells. These findings shed new light on the mechanisms that regulate the transition between life cycle stages and represent an experimental baseline for the identification of the signaling molecules.

In the coastal waters of the Black Sea near Sevastopol, a gradual temperature elevation and an increase in anthropogenic pressure since the early 2000s have caused significant structural and functional changes in phytoplankton. Currently, there is a significant decrease in the contribution of small diatom species (Skeletonema sp. and Chaetoceros socialis H.S.Lauder as well as coccolithophorids Emiliania huxleyi (Lohmann) W.W.Hay and H.P.Mohler) to the total phytoplankton biomass in these waters. Previously these species caused regular weak blooms. In the warm periods (from May to October), during which the main phytoplankton biomass is formed, large diatom species Pseudosolenia calcar-avis (Schultze) B.G.Sundström, 1986, Proboscia alata (Brightwell) Sundström and dinoflagellates predominate. Therefore, the maximum values of the phytoplankton community’s specific growth rate are about two times lower than in the preceding periods and do not exceed 1.10–1.40 day−1. There was also a decrease observed in the microzooplankton grazing rate, which, during the year, was no higher than 0.70–1.20 day−1. This is primarily conditioned by the increased role of large algae in phytoplankton, which means a decline in nutrition quality for microzooplankton. As a result of the joint influence of nutrition quality and water pollution, the relative share of net primary production consumed by microzooplankton in the warm periods of the year averaged only 32%, which is two times lower than the average values generally accepted for marine ecosystems. This means that the transfer of matter and energy from phytoplankton to higher trophic levels is significantly decreased.

Sedimentary ancient DNA (sedaDNA) analyses are increasingly used to reconstruct marine ecosystems. The majority of marine sedaDNA studies use a metabarcoding approach (extraction and analysis of specific DNA fragments of a defined length), targeting short taxonomic marker genes. Promising examples are 18S-V9 rRNA (~121–130 base pairs, bp) and diat-rbcL (76 bp), targeting eukaryotes and diatoms, respectively. However, it remains unknown how 18S-V9 and diat-rbcL derived compositional profiles compare to metagenomic shotgun data, the preferred method for ancient DNA analyses as amplification biases are minimised. We extracted DNA from five Santa Barbara Basin sediment samples (up to ~11 000 years old) and applied both a metabarcoding (18S-V9 rRNA, diat-rbcL) and a metagenomic shotgun approach to (i) compare eukaryote, especially diatom, composition, and (ii) assess sequence length and database related biases. Eukaryote composition differed considerably between shotgun and metabarcoding data, which was related to differences in read lengths (~112 and ~161 bp, respectively), and overamplification of short reads in metabarcoding data. Diatom composition was influenced by reference bias that was exacerbated in metabarcoding data and characterised by increased representation of Chaetoceros, Thalassiosira and Pseudo-nitzschia. Our results are relevant to sedaDNA studies aiming to accurately characterise paleo-ecosystems from either metabarcoding or metagenomic data.

Chaetoceros is a species-rich diatom genus with broad distribution and plays an important role in global carbon cycle and aquatic ecosystems. However, genomic information of Chaetoceros species is limited, hindering advanced researches on Chaetoceros biodiversity and their differential impact on ecology. In this study, we constructed full-length chloroplast genomes (cpDNAs) for seven Chaetoceros species, including C. costatus, C. curvisetus, C. laevisporus, C. muelleri, C. pseudo-curvisetus, C. socialis, and C. tenuissimus. All of these cpDNAs displayed a typical quadripartite structure with conserved genome arrangement and specific divergence. The sizes of these cpDNAs were similar, ranging from 116,421 to 119,034 bp in size, and these cpDNAs also displayed similar GC content, ranging from 30.26 to 32.10%. Despite extensive synteny conservation, discrete regions showed high variations. Divergence time estimation revealed that the common ancestor of Chaetoceros species, which formed a monophyletic clade at approximately 58 million years ago (Mya), split from Acanthoceras zachariasii at about 70 Mya. The availability of cpDNAs of multiple Chaetoceros species provided valuable reference sequences for studying evolutionary relationship among Chaetoceros species, as well as between Chaetoceros species and other diatom species.

Data on the elemental composition of the diatom Chaetoceros spp. from natural phytoplankton communities of Arctic marine ecosystems are presented for the first time. Samples were collected during the 69th cruise (22 August–26 September 2017) of the R/V Akademik Mstislav Keldysh in the Kara, Laptev, and East Siberian Seas. The multi-element composition of the diatom microalgae was studied by ICP-AES and ICP-MS methods. The contents of major (Na, Mg, Al, Si, P, S, K and Ca), trace (Li, Be, B, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb, Bi, Th and U) and rare earth (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) elements varied greatly, which was probably associated with the peculiarities of the functional state and mineral nutrition of phytoplankton in the autumn period. Biogenic silicon was the dominant component of the chemical composition of Chaetoceros spp., averaging 19.10 ± 0.58% of dry weight (DW). Other significant macronutrients were alkaline (Na and K) and alkaline earth (Ca and Mg) metals as well as biogenic (S and P) and essential (Al and Fe) elements. Their total contents varied from 1.26 to 2.72% DW, averaging 2.07 ± 0.43% DW. The Al:Si ratio for natural assemblages of Chaetoceros spp. of the shelf seas of the Arctic Ocean was 5.8 × 10−3. The total concentrations of trace and rare earth elements on average were 654.42 ± 120.07 and 4.14 ± 1.37 μg g−1 DW, respectively. We summarize the scarce data on the average chemical composition of marine and oceanic phytoplankton and discuss the limitations and approaches of such studies. We conclude on the lack of data and the need for further targeted studies on this issue.

Diatom aggregates constitute a significant fraction of the particle flux from the euphotic zone into the mesopelagic ocean as part of the ocean's biological carbon pump. Modeling studies of their exchange processes with the surrounding water usually assume spherical shape and that aggregates are impermeable to flow. Using particle image velocimetry, we examined flow distributions around individual aggregates of various irregular shapes formed from two different diatom species: (1) Skeletonema marinoi, known for its cell–cell stickiness, and (2) Chaetoceros affinis, exhibiting cell‐TEP (transparent exopolymeric particles) stickiness. Chaetoceros aggregates formed porous, highly irregularly shaped aggregates as compared to the more compact and near‐spherical Skeletonema aggregates, yet flow distributions around both types of aggregates were relatively similar at a millimeter scale. At a micrometer scale, the irregular shape of diatom aggregates caused velocity gradients and vorticity close to the surface to locally vary more than for spherical model aggregates (agar‐yeast spheres). Water was deflected from the surface of all aggregate types and we found no direct evidence that flow occurred within aggregates. Digital holographic imaging and Alcian blue staining revealed a substantial presence of TEP likely clogging the interstitial pore spaces in Chaetoceros aggregates. Radial oxygen concentration distributions measured by O2 microsensors within the aggregates were similar to those modeled for aggregates and spheres impermeable to flow. Thus, transport of gases, nutrients, and solutes likely occurs by diffusion, even within large, irregularly shaped diatom aggregates during sinking.

Algicidal bacteria can lyse phytoplankton cells, thereby contributing to algal bloom dynamics in the oceans. The target specificity of these bacteria determines their ecological impact. While species specificity of algicidal bacteria is documented, little is known about intra-species variability of their activity against phytoplankton. We describe variability in the Chaetoceros genus (Bacillariophyceae) to resist the lytic activity of the algicidal Flavobacterium Kordia algicida. This variability is evident between different Chaetoceros species, but even intra-specific variability of resistance is observed within one phytoplankton sample. This proves an ecological role of the individuality of diatom cells within a bloom.

Planktonic diatoms thrive in the water column, and several species can transform into resting stages – spores or resting cells – that sink to the bottom of the sea. Resting stages are generally pro...

Eco-commensal association between the chain forming planktonic diatom Chaetoceros coarctatus and the peritrich aloricate ciliate Vorticella oceanica is well known in tropical waters. This epibiosis provides several advantages to the epibiont V. oceanica, and includes protection, buoyancy, drift, supply of resources and possible assistance in reproduction. While it is a well-established fact that the host usually remains neutrally affected, without any gain or loss due to the attached epibiont, in the present study, the host showed signs of negative impact due to overgrowth of the epibiont. The present study noted for the first time a gregarious stage of V. oceanica ‘true colony’ from the coastal waters off the central west coast of India. The true-colonies of V. oceanica impacted the C. coarctatus chains as V. oceanica swarmed with high density and disintegrated the host setae. This study discusses all the stages of attachment of this species- specific consortium, along an estuarine gradient, and its possible ecological implications.

International audience; Incubation under controlled laboratory conditions were performed to assess the toxic effects of two plastic derived chemicals, bisphenol A (BPA) and di-(2-ethylhexyl) phthalate (DEHP), on the growth, photosynthetic efficiency and photosynthetic activity of the cosmopolitan diatom Chaetoceros decipiens-lorenzianus. Non-axenic diatom cells were exposed to concentrations of BPA and DEHP (separately and in mixture), mimicking concentrations observed in contaminated marine ecosystems, for seven days. Upon short-term exposure (i.e., during the first 48 h), BPA and DEHP induced a slight but significant stimulation of biomass and photosynthetic activity relative to the control, whereas, no significant impact was observed on the photosynthetic efficiency. Nevertheless, this pattern was transient. The stimulation was followed by a return to control conditions for all treatments at the end of incubation. These results showed that the cosmopolitan diatom Chaetoceros was not impacted by representative in situ concentrations of plastic derivatives, thus confirming its ability to thrive in coastal anthropogenic environments.

Calothrix rhizosoleniae and Richelia intracellularis are heterocystous cyanobacteria found in the tropical oceans. C. rhizosoleniae commonly live epiphytically on diatom genera Chaetoceros (C-C) and Bacteriastrum (B-C) while R. intracellularis live endosymbiotically within Rhizosolenia (R-R), Guinardia (G-R), and Hemiaulus (H-R); although, they occasionally live freely (FL-C and FL-R). Both species have much shorter trichomes than the other marine filamentous cyanobacteria such as Trichodesmium spp. and Anabaena gerdii. We investigated the trichome lengths of C. rhizosoleniae and R. intracellularis in the South China Sea (SCS) and the Philippine Sea (PS) between 2006 and 2014. On average, H-R had the shortest trichome lengths (3.5 cells/trichome), followed by B-C and C-C (4.9–5.2 cells/trichome) and FL-C (5.9 cells/trichome), and R-R, G-R, and FL-R had the longest trichome lengths (7.4–8.3 cells/trichome). Field results showed the trichome lengths of C-C and B-C did not vary seasonally or regionally. However, FL-C and H-R from the SCS and during the cool season had longer trichomes, where/when the ambient nutrient concentrations were higher. R-R, G-R, and FL-R also showed regional and seasonal variations in trichome length. Ultrastructural analysis found no gas vesicles within the C. rhizosoleniae cells to assist in buoyancy regulation. Results suggest that the trichome lengths of C. rhizosoleniae and R. intracellularis might be regulated by their diatom hosts’ symbiotic styles and by ambient nutrients. Short trichome length might help C. rhizosoleniae and R. intracellularis to stay in the euphotic zone regardless as to whether they are free-living or symbiotic.

Key Points: • diatom sedaDNA composition is concordant with reconstructed sea-ice dynamics, SSTs and subsurface salinities over the past millenia • loss of diatom richness at ~11.1 cal kyr BP is possibly a consequence of increased freshwater input from Kamchatka • shifts of potential ecotypes of species from the genus Chaetoceros with changing environmental conditions We traced diatom composition and diversity through time using diatom derived sedimentary ancient DNA (sedaDNA) from eastern continental slope sediments off Kamchatka (North Pacific) by applying a short, diatom‐specific marker on 63 samples in a DNA metabarcoding approach. The sequences were assigned to diatoms that are common in the area and characteristic of cold water. SedaDNA allowed us to observe shifts of potential lineages from species of the genus Chaetoceros that can be related to different climatic phases, suggesting that pre‐adapted ecotypes might have played a role in the long‐term success of species in areas of changing environmental conditions. These sedaDNA results complement our understanding of the long‐term history of diatom assemblages and their general relationship to environmental conditions of the past. Sea‐ice diatoms (Pauliella taeniata (Grunow) Round & Basson, Attheya septentrionalis (Østrup) R.M.Crawford and Nitzschia frigida (Grunow)) detected during the late glacial and Younger Dryas are in agreement with previous sea‐ice reconstructions. A positive correlation between pennate diatom richness and the sea‐ice proxy IP25 suggests that sea ice fosters pennate diatom richness, whereas a negative correlation with June insolation and temperature points to unfavorable conditions during the Holocene. A sharp increase in proportions of freshwater diatoms at ∼11.1 cal kyr BP implies the influence of terrestrial runoff and coincides with the loss of 42% of diatom sequence variants. We assume that reduced salinity at this time stabilized vertical stratification which limited the replenishment of nutrients in the euphotic zone.

Phytoplankton blooms in the Ross Sea generally occur during late spring-early summer, especially in the marginal ice zone, polynya areas and continental shelves. These blooms are generally dominated by many species of diatoms and the haptophyte Phaeocystis antarctica, and are driven by water column dynamics. However, most of the information on diatoms in the Ross Sea regards their ecological and biogeochemical role in Antarctic waters, with few and fragmented data describing the species composition. In order to contribute to the information on diatom diversity in the Ross Sea, data from two oceanographic cruises carried out in austral summers 2014 and 2017, have been analyzed also with respect to their spatial variability. Up to four genera, such as the pennates Fragilariopsis and Pseudo-nitzschia and the centrics Dactyliosolen and Chaetoceros, were mainly observed during the cruises. In both years, pennates dominated at coastal stations in early summer, while centric diatoms dominated in offshore waters particularly at 0 m in mid-summer. The most abundant species was Pseudo-nitzschia subcurvata contrarily to that reported by previous studies where Fragilariopsis curta and F. cylindrus were the most abundant species. Regarding the role of diatoms in the Antarctic food web, changes in species composition should be considered in light of recent studies on the effect of the ongoing climate change in the Ross Sea.

Microplastic contamination has been linked to a range of impacts on aquatic environments. One important area that is only beginning to be addressed is the effect of microplastics on marine carbon cycling and how these compare to the effects related to inorganic particles typically present in ocean waters. The present study explores these impacts on dissolved organic matter dynamics by comparing three scenarios: a particle-free environment, a particle-enriched system with polystyrene microplastics, and a particle-enriched system with inorganic particles (water insoluble SiO2). Natural marine organic matter was obtained by culturing a non-axenic strain of Chaetoceros socialis in 2L flasks under each of three scenarios. Following the diatom growth phase, filtered samples from the three flasks containing dissolved organic matter and bacteria were incubated separately in the dark for 5 days to monitor changes in dissolved organic matter. Chromophoric dissolved organic matter (CDOM), a bulk optical property, was monitored daily to examine changes in its quality and quantity and to compare degradation dynamics in the three systems. CDOM absorbance (quantity) remained higher in the control with respect to particle-enriched systems, suggesting that the presence of particles led to different rates of CDOM production and degradation. Using indicators for CDOM that could be related to microbial activity, results showed a higher CDOM alteration in the particle-enriched systems. These results indicate that microplastics have a potential role in modifying marine organic matter dynamics, on a similar magnitude to that of biogenic inorganic particles. Given their increasing concentrations of marine ecosystems, their role in marine microbial processing of organic matter needs to be better understood.

Coastal eutrophication is one of the pivotal factors driving occurrence of harmful algal blooms (HABs), whose underlying mechanism remained unclear. To better understand the nutrient regime triggering HABs and their formation process, the phytoplankton composition and its response to varying nitrogen (N) and phosphorus (P), physio-chemical parameters in water and sediment in Johor Strait in March 2019 were analyzed. Surface and sub-surface HABs were observed with the main causative species of Skeletonema, Chaetoceros and Karlodinium. The ecophysiological responses of Skeletonema to the low ambient N/P ratio such as secreting alkaline phosphatase, regulating cell morphology (volume; surface area/volume ratio) might play an important role in dominating the community. Anaerobic sediment iron-bound P release and simultaneous N removal by denitrification and anammox, shaped the stoichiometry of N and P in water column. The decrease of N/P ratio might shift the phytoplankton community into the dominance of HABs causative diatoms and dinoflagellates.

The western subarctic Pacific (WSP) is known as one of the most productive regions among the world’s oceans in spring. However, its oceanic waters are also known as a High Nutrient, Low Chlorophyll (HNLC) region during summer due to low iron (Fe) availability in seawater. Indeed, recent studies have demonstrated that the distribution of Fe in the WSP is complex and heterogeneous. This study thus investigated the effects of Fe availability on the community composition and photophysiology of surface phytoplankton from coastal to offshore waters in the WSP in the summer of 2014. Although relatively high concentrations (>2 mg m(−3)) of chlorophyll (chl) a were found in the Sea of Okhotsk and some coastal waters, low chl a concentrations (

The nature of the plankton symbioses between ciliates and diatoms has been investigated from the tropical South Atlantic Ocean, and Mediterranean and Caribbean Seas. The obligate symbioses of the diatoms Chaetoceros dadayi or C. tetrastichon with the tintinnid Eutintinnus spp., and Chaetoceros coarctatus with the peritrich ciliate Vorticella oceanica are the most widespread, and the consortium of Chaetoceros densus and Vorticella sp. have been rediscovered. Facultative symbioses between Eutintinnus lususundae and Chaetoceros peruvianus, Hemiaulus spp., and Thalassionema sp. are less frequent, often containing three or four partners because Hemiaulus can also harbor the diazotrophic cyanobacteria Richelia intracellularis. Another three-partner consortium is the peritrich ciliate Zoothamnium pelagicum, ectobiont bacteria, and the diatom Licmophora sp. The predominantly oligotrophic conditions of tropical seas do not favor the survival of large diatoms, but large species of Coscinodiscus and Palmerina in facultative symbiosis with Pseudovorticella coscinodisci have a competitive advantage over other diatoms (i.e., reduction of sinking speed and diffusive boundary layer). Symbioses allow sessile peritric ciliates to extend their distribution in the pelagic environment, permit boreal-polar related diatoms such as C. coarctatus or Fragilariopsis doliolus to inhabit tropical seas, and help large diatoms to extend their survival under unfavorable conditions.

Diatoms have well-recognized roles in fixing and exporting carbon and supplying energy to marine ecosystems, but only recently have we begun to explore the diversity and importance of nano- and pico-diatoms. Here we describe a small (~5 μm) diatom from the genus Chaetoceros Isolated from a wintertime temperate estuary (2° C, Narragansett Bay, RI), with a unique obligate specialization for low-light environments (< 120 μmol photons m-2 sec-1). This diatom exhibits a striking interaction between irradiance and thermal responses whereby as temperatures increase, so does its susceptibility to light stress. Historical 18S rRNA amplicon data from our study site show this isolate was abundant throughout a six-year period, and its presence strongly correlates with winter and early spring months when light and temperature are low. Two ASVs matching this isolate had a circumpolar distribution in Tara Polar Ocean Circle samples, indicating its unusual light and temperature requirements are adaptations to life in a cold, dark environment. We expect this isolate’s low light, psychrophilic niche to shrink as future warming-induced stratification increases both light and temperature levels experienced by high latitude marine phytoplankton.

Sulfonamides are frequently detected in the environment, where these compounds adsorb to soil particles and are retained in the environment. However, adsorption of sulfonamides to planktonic particles in the sea is not known. Here we demonstrate that sulfonamides adsorb to a diatom (Chaetoceros) and an arthropod (Artemia), albeit at low levels, under laboratory conditions. In both plankton, sulfamethazine (SMT) was more readily adsorbed than was sulfamethoxazole (SMX). The adsorption occurred quickly and the concentration on the plankton was stable for at least 24 h (Chaetoceros) or 5 h (Artemia). These data suggest that marine plankton may retain sulfonamides, although the adsorbed concentration per cell or individual is not high.

Neurotoxin β-N-methylamino-L-alanine (BMAA) produced by cyanobacteria and microalgae has been concerned by scientists due to its toxicity to human motoneurons. The source and environmental behaviors of BMAA in marine ecosystem are important to understand its risk to human health. Here a diatoms-dominated marine ecosystem in Jiaozhou Bay, China, was investigated for BMAA contamination in phytoplankton, zooplankton, and marine animals, during four seasons in 2019. Results showed that BMAA was migrated and biomagnified along the food chains from phytoplankton to higher trophic organisms, in which the trophic magnification factors (TMF) for zooplankton, bivalve mollusks, carnivorous crustacea and saprophytic gastropod mollusks were approximately 4.58, 30.1, 42.5, and 74.4, respectively. An isomer of BMAA, β-aminomethyl-L-alanine (BAMA), was ubiquitous in phytoplankton samples and its content ratios to BMAA looks gradually decreased with increasing trophic levels. A total of 56 diatom strains (Pseudo-nitzschia spp., Thalassiosira spp., Chaetoceros spp., Planktoniella spp., Minidiscus spp.) were isolated from the Chinese coast and cultured in the laboratory, among which 21 strains produced BMAA mainly presented in the precipitated bound fraction ranging from 0.11 to 3.95 µg g-1 dry weight. BMAA was firstly detected in both Pseudo-nitzschia and Planktoniella genera in this study. Only 2,4-diaminobutyric acid (DAB) but not BMAA or BAMA was detected in seven symbiotic bacteria isolated from the gut of Neverita didyma, which further proves that the benthic vector of BMAA, N. didyma, accumulated BMAA through food chains. These findings demonstrated that the coastal residents have a risk to accumulate BMAA by consumption of seafood products.

The primary ecological effects of the eutrophication of upwelling in the nearshore waters are the triggering and sustenance of phytoplankton (diatom) blooms. Here, we present the changes in the morphological characteristics and body size of microautotrophs associated with the nutrient enrichment of coastal upwelling along the southwest coast of India. The FlowCAM data of microautotrophs representing two time series locations in the coastal environment along the southwest coast of India at 18 weekly/biweekly frequencies showed the following features. The most significant response of the microautotrophs to the eutrophication associated with coastal upwelling during the Southwest Monsoon (June-September) was an increase in their mean bio-volume, either by increasing the individual cell size or by forming large colonies. During the Southwest Monsoon, large colonies of Thalassiosira (~ 8 cells/colony), Thalassionema (> 16 cells/colony), and Asterionellopsis (> 8 cells/colony) become very abundant. Streptotheca formed large mat-like colonies (av. 192350 ± 348 μm3/individual), and Fragilariopsis grew as very long ribbons (av. 272997 ± 5071 μm3/individual). Large-sized Chaetoceros (av. 45715 ± 1273 μm3/individual) with longer setae were also plentiful during the Southwest Monsoon. All these changes in the morphology and life forms of microautotrophs would help them to rapidly assimilate nitrate under turbulent conditions during the Southwest Monsoon.

International audience; Over the last decades, the western Arctic Ocean has undergone unprecedented environmental changes. However, long-term in situ observations of marine phytoplankton are still rare and therefore insufficient to fully characterize their evolutionary trends. This study investigated the diatom fluxes and composition in sediment trap material collected from the Northwind Ridge, western Arctic Ocean, from August 2008 to September 2009. Our data showed that Chaetoceros resting spores were the predominant species, accounting for >40% of the diatom composition. The sea ice diatom group, which included Fossula arctica, Fragilariopsis cylindrus and F. oceanica, dominated the rest of the assemblage throughout the observation period. While diatom fluxes in winter were extremely low, higher flux values were found in summer, and summer 2009 flux values were twice as high as those in 2008. The high total mass and diatom fluxes in summer 2009 were attributed to the combined effect of a weakened Beaufort Gyre, a strengthened Pacific water inflow (PWI) and the distribution pattern of the sea ice. The higher levels of coastal diatoms and terrigenous proxies in summer 2009 were consistent with the intensified PWI. Sea ice diatoms and sea ice biomarker IP25 fluxes were both high during the sea ice melting season and were significantly correlated with each other (r2 = 0.64, p < 0.01). Our data also suggest that sea ice diatoms are prone to selective dissolution in the water column and sediments, implying the existence of biases in diatom assemblage data and subsequently in paleoceanographic reconstructions.

Rachman A. 2020. Checklist and estimation of total number of phytoplankton species in Pari, Tidung, and Payung Islands, Indonesia. Biodiversitas 21: 2446-2454. A checklist of phytoplankton species from an anthropogenically pressured ecosystem has become a necessity to study the marine ecosystem changes and to detect the presence of introduced species in the area. This study aimed to list and describe the phytoplankton community in the Tidung, Pari, and Payung Islands and to estimate the total number of phytoplankton species in those islands. Sampling was conducted in May 2019 in 10 sites, consisted of 5 sites around Pari Island, 1 site next to Payung Island, and 4 sites around Tidung Island. As many as 150 species of phytoplankton, consisted of 109 species of diatoms, 38 species of dinoflagellates, 3 species of cyanobacteria, were recorded in this study. However, up to 47 phytoplankton species were estimated to be missing from the current sampling effort. Three diatom species, Chaetoceros curvisetus, Chaetoceros distans, and Chaetoceros affinis were the most abundant and most common species in the studied areas. Dinoflagellates were found in low density and have limited distribution compared to diatoms. Tripos trichoceros, Tripos fusus, and Tripos macroceros were the three most common dinoflagellates in the studied islands. Several notable harmful species, such as Pseudo-nitzschia spp., Alexandrium sp., Tripos furca, Cochlodinium sp., Dinophysis miles, Prorocentrum lima, and Noctiluca scintillans, were present albeit with low cell density. However, concern on future blooms of those harmful species in the greater water regions (i.e. Seribu Island or Jakarta Bay) should be noted, as it might cause environmental damages, including a shift in species dominance and disappearance of some native phytoplankton species from the ecosystem in the future.

Weekly field observations were conducted in Harima-Nada in the eastern part of the Seto Inland Sea, Japan from 21 August to 26 October 2015 to investigate the role of heterotrophic dinoflagellate Noctiluca scintillans on a coastal ecosystem. We observed a rapid N. scintillans growth just after increasing Chl. a, of which the dominant species were Chaetoceros spp., Thalassionema spp., Thalassiothrix spp., Coscinodiscus spp., Thalassiosira spp., and Stephanopyxis spp. We also observed under a microscope that N. scintillans cells grazed on these species of diatoms. The biomass of N. scintillans was estimated to increase from 4.50 to 470 mgC m−2 in 7 days. On the other hand, phytoplankton biomass drastically decreased from 2110 to 725 mgC m−2. The increase in N. scintillans biomass could account for 33.6% of the decrease in phytoplankton biomass. If we assume all the decrease in phytoplankton biomass was due to the N. scintillans grazing of diatoms, the 33.6% indicates the apparent gross growth efficiency of N. scintillans under a sufficient source of feed. Our results indicated the active feeding of N. scintillans on diatoms ends a diatom bloom in coastal water.

Chaetoceros is one of the most species-rich diatom genera, but about one-third to one-half of the taxa are considered synonyms, and a modern revision is needed on the basis of a combination of morphological and molecular data. Chaetoceros subtilis Cleve and C. subtilis var. abnormis (Proschkina-Lavrenko) Proschkina-Lavrenko were revisited in the present study. Monoclonal strains were established from the southeast Chinese coast. Morphology of vegetative cells and resting spores was observed using light microscopy, as well as scanning and transmission electron microscopy. The hypervariable D1–D3 region of the nuclear large-subunit ribosomal RNA-encoding gene (LSU ribosomal DNA) was sequenced for information on phylogenetic relationships. Species delineations were evaluated for the two taxa. Morphologically, C. subtilis var. abnormis was quite similar to C. subtilis, but differences were found in the number of thick terminal posterior setae, detailed structure and morphometric data of intercalary a...

Epipont peritrich ciliates are one of the most sessilie protists that attached to the substrate of zooplankton communities especially copepods and crustaceans. Peritrich ciliates can be solitary or colonial form found from the coastal zones and embayment around the world. Present research report is the first snapshot study of the peritrich ciliates and the horizontal distribution in open waters from surface to 200 m depth in the eastern Indian Ocean. Recently, five peritrich ciliates, e.g., Vorticella oceanica, Zoothamnium alternans, Z. alrasheidi, Z. pelagicum, and Z. marinum were collected from plankton net tow samples during the cruise from April 10 to May 13, 2014. The characteristics of the peritrich ciliate were determined according to the shape of the zooids, the ciliary structure and the stalks. The morphometric shape, sizes and characters also explained by examination under light/scanning electron microscopy. Vorticella oceanica and Z. pelagicum showed their association with host such as diatom (i.e, Chaetoceros coarctatus) and copepod (Oithona brevicornis) including some individuals around the dinoflagellate species (Ceraitum tripose). The distribution of these sessilid ciliates was dominated by the V. oceanica, Z. pelagicum and Z. marinum at the southeast zone while the large colonies of Z. alrasheidi observed at the Bay of Bengal. This distribution can be influenced by substrate availability like diatom (Ch. coarctatus) and copepods (O. brevicornis).

In line with the ASEAN-India project “Extent of Transfer of Alien Invasive Organisms in South/Southeast Asia via Shipping”, phytoplankton and bacterial communities in the waters off South Harbour, Manila Bay were investigated. Sampling was done in July and August 2012 and in April and May 2013. A total of 67 phytoplankton species including 29 diatoms and 38 dinoflagellates were identified. Potentially toxic Pseudo-nitzschia spp. were among the diatoms found as well as dinoflagellates Alexandrium spp., and Gymnodinium spp. The diatom Skeletonema costatum appeared to be the dominant species in July and August 2012, whereas Chaetoceros spp. constituted over 85% of the total phytoplankton assemblage in April and May 2013. Mean bacterialabundance ranged from 9.53 x 102–3.18 x 105 cells/mL in July 2012. In addition, 93 bacterial isolates were identified using 16S rDNA, several of which belonged to the following phyla: Actinobacteria, Bacteriodetes, Firmicutes, and Proteobacteria; whereas, others were determined as uncultured bacterial clones. These results will serve as a valuable baseline for future studies on phytoplankton and bacterial community structure in Manila Bay.

Phaeocystis antarctica is an important phytoplankter of the Ross Sea where it dominates the early season bloom after sea ice retreat and is a major contributor to carbon export. The factors that influence Phaeocystis colony formation and the resultant Ross Sea bloom initiation have been of great scientific interest, yet there is little known about the underlying mechanisms responsible for these phenomena. Here, we present laboratory and field studies on Phaeocystis antarctica grown under multiple iron conditions using a coupled proteomic and transcriptomic approach. P. antarctica had a lower iron limitation threshold than a Ross Sea diatom Chaetoceros sp., and at increased iron nutrition (> 120 pM Fe') a shift from flagellate cells to a majority of colonial cells in P. antarctica was observed, implying a role for iron as a trigger for colony formation. Proteome analysis revealed an extensive and coordinated shift in proteome structure linked to iron availability and life cycle transitions with 327 and 436 proteins measured as significantly different between low and high iron in strains 1871 and 1374, respectively. The enzymes flavodoxin and plastocyanin that can functionally replace iron metalloenzymes were observed at low iron treatments consistent with cellular iron-sparing strategies, with plastocyanin having a larger dynamic range. The numerous isoforms of the putative iron-starvation-induced protein (ISIP) group (ISIP2A and ISIP3) had abundance patterns coinciding with that of either low or high iron (and coincident flagellate or the colonial cell types in strain 1871), implying that there may be specific iron acquisition systems for each life cycle type. The proteome analysis also revealed numerous structural proteins associated with each cell type: within flagellate cells actin and tubulin from flagella and haptonema structures as well as a suite of calcium-binding proteins with EF domains were observed. In the colony-dominated samples a variety of structural proteins were observed that are also often found in multicellular organisms including spondins, lectins, fibrillins, and glycoproteins with von Willebrand domains. A large number of proteins of unknown function were identified that became abundant at either high or low iron availability. These results were compared to the first metaproteomic analysis of a Ross Sea Phaeocystis bloom to connect the mechanistic information to the in situ ecology and biogeochemistry. Proteins associated with both flagellate and colonial cells were observed in the bloom sample consistent with the need for both cell types within a growing bloom. Bacterial iron storage and B12 biosynthesis proteins were also observed consistent with chemical synergies within the colony microbiome to cope with the biogeochemical conditions. Together these responses reveal a complex, highly coordinated effort by P. antarctica to regulate its phenotype at the molecular level in response to iron and provide a window into the biology, ecology, and biogeochemistry of this group.

Frequent blooms of phytoplankton occur in coastal upwelling zones creating hotspots of biological productivity in the ocean. As cold, nutrient-rich water is brought up to sunlit layers from depth, phytoplankton are also transported upwards to seed surface blooms that are often dominated by diatoms. The physiological response of phytoplankton to this process, commonly referred to as shift-up, is characterized by increases in nitrate assimilation and rapid growth rates. To examine the molecular underpinnings behind this phenomenon, metatranscriptomics was applied to a simulated upwelling experiment using natural phytoplankton communities from the California Upwelling Zone. An increase in diatom growth following 5 days of incubation was attributed to the genera Chaetoceros and Pseudo-nitzschia. Here, we show that certain bloom-forming diatoms exhibit a distinct transcriptional response that coordinates shift-up where diatoms exhibited the greatest transcriptional change following upwelling; however, comparison of co-expressed genes exposed overrepresentation of distinct sets within each of the dominant phytoplankton groups. The analysis revealed that diatoms frontload genes involved in nitrogen assimilation likely in order to outcompete other groups for available nitrogen during upwelling events. We speculate that the evolutionary success of diatoms may be due, in part, to this proactive response to frequently encountered changes in their environment.

The distribution of resting stage cells of diatoms in the surface sediments and water samples collected in the Chukchi Sea was investigated using the most probable number (MPN) analysis to understand the mechanism underlying diatom blooms in the Arctic region. High densities (3.5 × 105 to 6.8 × 106 MPN cells cm−3 wet sediment) of viable resting stage cells of typical Arctic diatom species were found in the surface sediments of the Chukchi Sea. Resting stage cells of the sea-ice–related diatom genus Fragilariopsis were more abundant (2.6 × 106 MPN cells cm−3 wet sediment) than Chaetoceros (2.18 × 106 MPN cells cm−3 wet sediment) or Thalassiosira (1.76 × 106 MPN cells cm−3 wet sediment) in the southern Chukchi Sea. This reflected a previous bloom of Fragilariopsis ahead of typical planktonic blooms of Chaetoceros and Thalassiosira and suggests that sea-ice–related blooms are a substantial factor for primary production in this area. During the sampling period, diatom assemblages in the in situ water...

Consortia of the peritrich ciliate Vorticella oceanica and the planktonic diatom Chaetoceros coarctactus are reported from the South Atlantic Ocean. The morphologies of the constituent species were similar to their counterparts from other geographical regions, revealing a widespread distribution in tropical waters. Besides, we describe Pseudovorticella coscinodisci sp. nov. that lives on Coscinodiscus wailesii and other pelagic diatoms off Brazil. The zooids were 34-45μm long, and 19-30μm wide with a stalk too short to coil. The cell surface showed a reticulate pellicle with 18-19 and 11-12 rows above and below the aboral ciliary wreath, respectively. In the SSU rDNA phylogeny V. oceanica clustered between the clades of Vorticella and Pseudovorticella/Epicarchesium, and Pseudovorticella coscinodisci sp. nov. clustered as a sister group of Pseudovorticella paracratera and P. sinensis. The V. oceanica-C. coarctactus consortium likely represents an obligate species-specific mutualistic symbiosis as the constituents are not known as free-living forms. In contrast, the diatom hosts are known as free-living forms in the consortia of Pseudovorticella coscinodisci. The new molecular data and species description will be valuable dataset for future research on the diversity and ecological significance of ciliate-diatom epibiotic consortia.

A 24-meter jumbo piston core (NBP0101 JPC41) collected from an inner shelf basin in Iceberg Alley reveals an approximately 2000-year history of unusually high primary productivity. Iceberg Alley, an ~ 85 km long and 10–20 km wide cross-shelf trough on the Mac.Robertson Shelf, East Antarctica, reaches depths of 850 m and is bounded on either side by shallow banks that are lined with grounded icebergs. The sediments are laminated on a mm- to cm-scale throughout and are highly biosiliceous. Microscopic examination of smear slides, quantitative diatom slides, and sediment thin sections reveals that the sediments are visually dominated by the diatom Corethron pennatum, a large and lightly silicified species notable for its long and narrow shape; the valves, girdle bands and spines are all exceptionally well-preserved, suggesting rapid sedimentation. Other common species include sea ice-related Fragilariopsis, such as F. curta and F. cylindrus, with lesser contribution from other large diatoms, including Rhizosolenia spp. and Chaetoceros Ehrenberg subg. Chaetoceros. Chaetoceros Ehrenberg subg. Hyalochaete Gran resting spores, typically associated with large early-season blooms and common in many laminated sedimentary sections around the Antarctic margin, are surprisingly rare. Laminae with any significant terrigenous component are also very rare. Individual laminations appear to represent blooms, and in some cases sub-seasonal events are likely preserved. We suggest that this productive system is associated with the continuous presence of low-salinity meltwater derived from a combination of sea ice melt and grounded icebergs, which may be a source for a steady supply of micronutrients such as iron to the surface mixed layer.