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The export of dissolved organic matter (DOM) from rivers is essential for linking terrestrial and marine carbon reservoirs in the global carbon cycle. However, there is limited knowledge regarding how the molecular composition of riverine DOM changes under different hydrological conditions, especially during extreme rainfall events. Moreover, the factors beyond hydrology that impact DOM composition have not been well defined. To address these gaps, samples were collected from a human‐impacted medium‐sized subtropical monsoonal river across various hydrological conditions throughout a complete hydrological cycle. Utilizing high‐resolution mass spectrometry, it was discovered that the solid‐phase extractable DOM (SPE‐DOM) during the high‐flow (1 < runoff (Q): annual mean runoff (Qm) < 3) and extreme‐rain (Q:Qm > 3) periods exhibited a higher number of molecular formulae, lower H/C, higher O/C, and a higher proportion of carboxylic‐rich alicyclic molecules compared to the low‐flow period (LFP) (Q:Qm < 1). These alterations were attributed to input from more diverse sources, particularly a greater input from soil organic matter with higher oxidation degrees. Additionally, the P‐containing formulae were more enriched during the extreme‐rain period, likely from agricultural lands and sediment release. Conversely, the fraction of S‐containing formulae was significantly higher during the LFP, possibly due to the amplified influence of anthropogenic input. Furthermore, the DOM aromaticity did not fluctuate with runoff but was significantly associated with temperature. In summary, the study indicated that the composition of DOM varied significantly under different hydrological conditions, with temperature and anthropogenic activities identified as crucial factors influencing riverine DOM export. Plain Language Summary: Rivers play a crucial role in the global carbon cycle by transporting dissolved organic matter (DOM) from land to the ocean. With the increasing frequency of extreme rainfall events attributed to climate change, there is a growing interest among researchers in understanding how hydrological conditions influence the seasonal variation of DOM. In this study, we investigated a subtropical river in China to explore how the molecular composition of DOM changes under different hydrological conditions and the various factors that contribute to shaping DOM composition. Our findings revealed that the composition of DOM during periods of high discharge and extreme discharge were similar. Still, extreme water discharge, including rainfall events, led to an increased export of heteroatom‐containing DOM. In addition to riverine discharge, factors such as temperature and human activities, including agriculture and sewage discharge, influenced DOM composition from different perspectives. For instance, sewage discharge predominantly introduced S‐containing compounds, while agricultural activities were associated with abundant P‐containing compounds. This study highlights the importance of considering not only changes in sources related to hydrology but also the influences of temperature and human activities when examining how riverine DOM responds to climate pressures. Key Points: The S‐containing formulae were more enriched during the low‐flow periodDissolved organic matter (DOM) exported during both high‐flow and extreme‐rain event periods were higher in O/C and lower in H/CDOM exported during the extreme‐rain event was more diverse, and enriched in P‐containing formulae [ABSTRACT FROM AUTHOR]

Diatoms as important phytoplankton components interact with and are colonized by heterotrophic bacteria. This colonization has been studied extensively in the past but a distinction between the bacterial colonization directly on diatom cells or on the aggregated organic material, exopolymeric substances (EPS), was little addressed. Here we show that the diatom Thalassiosira rotula and EPS were differently colonized by strains of Roseobacteraceae and Flavobacteriaceae in two and tree partner treatments and an enriched natural bacterial community as inoculum. In two partner treatments, the algae and EPS were generally less colonized than in the three partner treatments. Two strains benefitted greatly from the presence of another partner as the proportions of their subpopulations colonizing the diatom cell and the EPS were much enhanced relative to their two partner treatments. Highest proportions of bacteria colonizing the diatom and EPS occurred in the treatment inoculated with the enriched natural bacterial community. Dissolved organic carbon, amino acids and carbohydrates produced by T. rotula were differently used by the bacteria in the two and three partner treatments and most efficiently by the enriched natural bacterial community. Our approach is a valid model system to study physico‐chemical bacteria‐diatom interactions with increasing complexity. [ABSTRACT FROM AUTHOR]

Marine dissolved organic matter (DOM) is a major global carbon pool, consisting of thousands of compounds with distinct lifetimes. While marine DOM persists for millennia, its molecular and isotopic composition imply that it is dynamic on shorter timescales. To determine the extent to which DOM deviates from conservative water mass mixing, we conducted a two‐endmember mixing analysis on dissolved organic carbon (DOC) concentration and DOM molecular composition in the Atlantic and Pacific. Endmembers were the deep water masses near their formation sites. For DOM composition, we considered 6118 molecular formulae (MF) identified via Fourier‐transform ion cyclotron resonance mass spectrometry in solid‐phase extracts (SPE) of 837 samples. Bulk DOC and SPE‐DOC concentrations behaved conservatively in both basins and ≥70% of the MF (14–20 μM SPE‐DOC) mixed conservatively. However, a small fraction (10%–20%) of the MF (<3 μM SPE‐DOC) were added or removed during mixing. These MF were more reduced and oxidized, respectively, than the conservative fraction. There were also MF absent from the endmembers; these accounted for ≤1 μM of SPE‐DOC and positively correlated with DOM lability. Based on their distribution across the two basins, we conclude that the conserved MF are formed in the surface subtropical ocean and modified in overturning areas. In the deep ocean, however, these MF are solely controlled by mixing. This finding contrasts with the current paradigm of slow, continuous degradation of recalcitrant DOM in the deep ocean. Our analysis illustrates the importance of the surface ocean in controlling DOM cycling in the deep. Key Points: 70% of the molecular composition of dissolved organic matter (DOM) follows two‐endmember water mass mixing in the deep Atlantic and PacificThere are small fractions of DOM with distinct chemical compositions that are added and removed relative to mixingMolecular formulae that behave conservatively in the deep are formed in the surface ocean and modified in overturning zones [ABSTRACT FROM AUTHOR]

Dissolved black carbon (DBC) is the largest known slow‐cycling organic carbon pool in the ocean; yet, its sources and processing in the coastal seas remain poorly understood. To address this knowledge gap, we conducted a study in the East China Sea and South Yellow Sea to quantify and molecularly characterize DBC using benzene polycarboxylic acids (DBCBPCA) and ultra‐high resolution mass spectrometry (DBCFT), respectively. The concentration of DBCBPCA was 0.70–1.9 μmol C L−1 and exhibited a significant negative correlation with salinity. Significant correlations were also observed between salinity and molecular characters (oxygen‐to‐carbon ratio; double bond equivalent; molecular weight) of DBCFT. These findings collectively suggest that mixing processes primarily control the concentration and composition of DBC. A two‐end‐member mixing model revealed that riverine input and offshore water contributed ~ 22% and 63%, respectively, to the DBCBPCA in the study region. Additionally, atmospheric deposition and potentially other unidentified sources contributed at least ~ 15% DBCBPCA, assuming no removal. Furthermore, ~ 70% of DBCFT in the coastal seawater could be found in the river water, and aerosols, which was in line with the information from DBCBPCA. Our study provides the first comprehensive assessment of both the sources and influential processes of DBC in coastal seas, highlighting the importance of additional sources, such as atmospheric deposition, and emphasizing the need to identify previously unknown sources of DBC. [ABSTRACT FROM AUTHOR]

Zooplankton grazing onphytoplankton promotes the release of particulate and dissolved organic matter (DOM) into the water column and therefore plays a key role in organic matter cycling in aquatic systems. Prokaryotes are the main DOM consumers in the ocean by actively remineralizing and transforming it, contributing to its molecular diversification. To explore the molecular composition of zooplankton‐derived DOM and its bioavailability to natural prokaryotic communities, the DOM generated by a mixed zooplankton community in the coastal Atlantic off Spain was used as substrate for a natural prokaryotic community and monitored over a ~ 5‐d incubation experiment. The molecular composition of solid‐phase extracted DOM was characterized via Fourier‐transform ion cyclotron resonance mass spectrometry. After ~ 4 d in the zooplankton‐derived DOM amended incubation, the prokaryotic community demonstrated a 17‐fold exponential increase in cell number. The prokaryotic growth resulted in a reduction in bulk dissolved organic carbon concentration and the zooplankton‐derived DOM was considerably transformed at molecular and bulk elemental levels over the incubation period. The C : N ratio (calculated from the obtained molecular formulae) increased while the functional diversity decreased over the incubation time. In addition, molecular indices pointed to a reduced bioavailability of DOM at the end of the experiment. These findings show that zooplankton excreta are a source of labile organic matter that is quickly metabolized by the prokaryotic community. Therefore, a fraction of carbon is shunted from transfer to secondary consumers similarly to the viral shunt, suggesting that the zooplankton–prokaryotic interactions play an important role in the ocean's carbon cycle. [ABSTRACT FROM AUTHOR]

Bacteria play a key role in sustaining the chemodiversity of marine dissolved organic matter (DOM), yet there is limited direct evidence of a major contribution of bacterial exometabolites to the DOM pool. This study tests whether molecular formulae of intact exometabolites can be detected in natural DOM via untargeted Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). We analyzed a series of quantitative mixtures of solid‐phase extracted DOM from the deep ocean, of a natural microbial community and selected model strains of marine bacteria. Under standard instrument settings (200 broadband scans, mass range 92–1000 Da), 77% of molecular formulae were shared between the mesocosm and marine DOM. However, there was < 10% overlap between pure bacterial exometabolome with marine DOM, and in mixing ratios closest to mimicking natural environments (1% bacterial DOM, 99% marine DOM), only 4% of the unique bacterial exometabolites remained detectable. Further experiments with the bacterial exometabolome DOM mixtures using enhanced instrument settings resulted in increased detection of the exometabolites at low concentrations. At 1000 and 10,000 accumulated scans, 23% and 29% of the unique molecular formulae were detectable at low concentrations, respectively. Moreover, windowing a specific mass range encompassing a representative fraction of exometabolites tripled the number of unique detected formulae at low concentrations. Routine FT‐ICR‐MS settings are thus not always sufficient to distinguish bacterial exometabolome patterns from a seawater DOM background. To observe these patterns at higher sensitivity, we recommend a high scan number coupled with windowing a characteristic region of the molecular fingerprint. [ABSTRACT FROM AUTHOR]

The temporal dynamics of dissolved organic matter (DOM) are inherently linked with the functioning of aquatic ecosystems. Because DOM represents a complex mixture of millions of different compounds, the statistical analysis of DOM dynamics poses a huge challenge. Here, we present a statistical approach based on hierarchical clustering of time series that groups DOM compounds with synchronous dynamics. We applied this approach to time series of Fourier‐transform ion cyclotron resonance mass spectrometry data of DOM sampled over a period of 26 months near Helgoland, an island in the Southern North Sea. We identified three DOM clusters, which represented a total of 1392 different molecular formulae and showed distinct chemical properties and noticeably compound matches within the PubChem database. Correlations of the three DOM clusters with abundance data of prokaryote and phytoplankton species and with environmental parameters provided consistent indications on the potential origin of the clustered compounds. The first cluster integrated terrestrial DOM originating from riverine discharge reaching Helgoland waters. The second cluster was attributed to DOM related to phytoplankton and microbial activity, whereas the third cluster was interpreted as representing the marine refractory DOM background. Accordingly, while further partitioning divided each of the first two clusters into five sub‐clusters with distinct temporal dynamics and molecular characteristics, the third cluster persisted as a stable feature. Applying a purely mathematical approach, we thus confirmed the differential dynamics of individual DOM compounds and compound groups and showed that temporal dynamics of dissolved molecules are linked to their origin and transformation history. [ABSTRACT FROM AUTHOR]

Microalgae produce copious amounts of structurally diverse polysaccharides, some are bound within cells and cell walls, while others are secreted into the surrounding seawater. A fraction of the secreted polysaccharides assembles into particles promoting aggregation and in turn formation of aggregates increases the export of carbon into the deep ocean via sinking. However, specific polysaccharides contributing to particle formation and carbon export remain unknown. Here, we studied microalgae polysaccharide composition in a system of reduced complexity consisting of lab grown monospecific cultures of the centric diatom species Thalassiosira weissflogii and Chaetoceros socialis. We followed the abundance and dynamics of five specific polysaccharide types in the dissolved and particulate organic matter (DOM and POM) for two weeks. Polysaccharides were detected using monoclonal antibodies (mAbs) specific for β‐1,4‐mannan, β‐1,4‐xylan, arabinogalactan, and two fucose‐containing sulfated polysaccharide (FCSP) epitopes. Additionally, glycan composition of all samples was analyzed by monosaccharide analysis. The time series revealed polysaccharides partition differently between the dissolved and particulate carbon pools. β‐1,4‐xylan and β‐1,4‐mannan were mainly present in POM, possibly as cell wall polymers, while FCSPs were found in both DOM and POM. The data showed that the main glycan component secreted by diatoms was fucose‐containing polysaccharide, which accumulated in DOM over time. Roller tank experiments were used to induce aggregate formation finding FCSP transitioned from DOM to POM under aggregating conditions. These results suggest that diatom‐secreted FCSPs are involved in the formation of aggregates, which promote the formation of particles, and potentially carbon export. [ABSTRACT FROM AUTHOR]

The Thomas Fire began on December 4, 2017 and burned 281,893 acres over a 40‐day period in Ventura and Santa Barbara Counties, making it one of California's most destructive wildfires to date. A major rainstorm then caused a flash flood event, which led to the containment of the fire. Both airborne ash from the fire and the runoff from the flash flood entered into the Santa Barbara Basin (SBB). Here, we present the results from aerosol, river, and seawater studies of black carbon and metal delivery to the SBB associated with the fire and subsequent flash flood. On day 11 of the Thomas Fire, aerosols sampled under the smoke plume were associated with high levels of PM2.5, levoglucosan, and black carbon (average: 49 μg/m3, 1.05 μg/m3, and 14.93 μg/m3, respectively) and aerosol metal concentrations were consistent with a forest fire signature. Metal concentrations in SBB surface seawater were higher closer to the coastal perimeter of the fire (including 2.22 nM Fe) than further off the coast, suggesting a dependence on continental proximity rather than fire inputs. On days 37–40 of the fire, before, during, and after the flash flood in the Ventura River, dissolved organic carbon, dissolved black carbon, and dissolved metal concentrations were positively correlated with discharge allowing us to estimate the input of fire products into the coastal ocean. We estimated rapid aerosol delivery during the fire event to be the larger share of fire‐derived metal transport compared to runoff from the Ventura River during the flood event. Plain Language Summary: In December 2017, a wildfire on the southern California coast (the Thomas Fire) burned an area almost as large as the city of Los Angeles making it one of California's most destructive wildfires to date. A major rainstorm then began in January 2018, which helped put out the fire and caused a flash flood event. Both ash in the air from the fire and in the water from the flood entered into the coastal ocean. The goal of this study was to observe how the ash in the air and flood waters affected the water chemistry and phytoplankton biomass in the coastal ocean. We sampled the air and seawater in the coastal ocean during the fire and sampled water from the Ventura River, which drains into the ocean, during and after the flood. While both the airborne particulate matter released by the fire and the floodwaters contained high amounts of pollutants, such as metals and soot, we found that these had no major impact on coastal seawater chemistry or phytoplankton biomass. With the increase of wildfires on the western coast of the United States, the importance of studies on how wildfires affect nearby environments, such as the coastal ocean, grows. Key Points: The fire produced large amounts of aerosol metals, including 65, 35, and 5 tons of iron, zinc, and copper, respectivelyWildfire aerosols transported a larger amount of metals to the coastal ocean versus several days of runoff from the Ventura RiverRiver water metal concentrations increased on the 37th day of the fire, including iron and lead increases of 61 and 27 times, respectively [ABSTRACT FROM AUTHOR]

Summary: Ammonia‐oxidizing archaea (AOA) constitute a considerable fraction of microbial biomass in the global ocean, comprising 20%–40% of the ocean's prokaryotic plankton. However, it remains enigmatic to what extent these chemolithoautotrophic archaea release dissolved organic carbon (DOC). A combination of targeted and untargeted metabolomics was used to characterize the exometabolomes of three model AOA strains of the Nitrosopumilus genus. Our results indicate that marine AOA exude a suite of organic compounds with potentially varying reactivities, dominated by nitrogen‐containing compounds. A significant fraction of the released dissolved organic matter (DOM) consists of labile compounds, which typically limit prokaryotic heterotrophic activity in open ocean waters, including amino acids, thymidine and B vitamins. Amino acid release rates corresponded with ammonia oxidation activity and the three Nitrosopumilus strains predominantly released hydrophobic amino acids, potentially as a result of passive diffusion. Despite the low contribution of DOC released by AOA (~0.08%–1.05%) to the heterotrophic prokaryotic carbon demand, the release of physiologically relevant metabolites could be crucial for microbes that are auxotrophic for some of these compounds, including members of the globally abundant and ubiquitous SAR11 clade. [ABSTRACT FROM AUTHOR]

Each year, tropical rivers export a dissolved organic carbon (DOC) flux to the global oceans that is equivalent to ~4% of the global land sink for atmospheric CO2. Among the most refractory fractions of terrigenous DOC is dissolved black carbon (DBC), which constitutes ~10% of the total DOC flux and derives from the charcoal and soot (aerosol) produced during biomass burning and fossil fuel combustion. Black carbon (BC) has disproportionate storage potential in oceanic pools and so its export has implications for the fate and residence time of terrigenous organic carbon (OC). In contrast to bulk DOC, there is limited knowledge of the environmental factors that control riverine fluxes of DBC. We thus completed a comprehensive assessment of the factors controlling DBC export in tropical rivers with catchments distributed across environmental gradients of hydrology, topography, climate, and soil properties. Generalized linear models explained 70 and 64% of the observed variance in DOC and DBC concentrations, respectively. DOC and DBC concentrations displayed coupled responses to the dominant factors controlling their riverine export (soil moisture, catchment slope, and catchment stocks of OC or BC, respectively) but varied divergently across gradients of temperature and soil properties. DBC concentrations also varied strongly with aerosol BC deposition rate, indicating further potential for deviation of DBC fluxes from those of DOC due to secondary inputs of DBC from this unmatched source. Overall, this study identifies the specific drivers of BC dynamics in river catchments and fundamentally enhances our understanding of refractory DOC export to the global oceans. Key Points: Common hydrological factors explain variability in riverine dissolved organic carbon and dissolved black carbon concentrations, however;Variation in soil properties, temperature, antecedent rainfall, and aerosol deposition may drive divergence in their relative abundanceAt an unprecedented geographic scale, we find that aerosol BC contributes significantly to riverine fluxes of DBC [ABSTRACT FROM AUTHOR]

Hydrology, especially extreme hydrological events, has been recognized as an important driver of the land‐to‐ocean export of terrigenous dissolved organic matter (tDOM). Nevertheless, how various types of tDOM that differ in source and reactivity respond to changes in hydrology is not known. Seasonal and event exports of dissolved organic carbon (DOC), dissolved black carbon (DBC), and dissolved lignin were studied in a small subtropical river. We found that seasonal variations in DBC concentration were significantly related to hydrology, while DOC and dissolved lignin were not. In contrast, DOC, DBC, and dissolved lignin changed similarly during an extreme rain event. The variation magnitudes of DOC, DBC, and dissolved lignin concentrations were in the lower end compared to other rivers, which may be related to the limited coverage of wetlands and riparian vegetation and poor development of organic‐rich soil. Dilution effects were observed when the runoff exceeded 0.4 mm/hr, and the fluxes of both DBC and dissolved lignin decreased during the runoff peak, which was caused by surface flow and potentially by removal processes during peak discharge. Our results suggest that the influence of hydrology varies with tDOM source and reactivity and that high enough runoff (e.g., 0.7 mm/hr in the Jiulong River) may not enhance the export rate of tDOM. However, our study was carried out in a small watershed with limited wetlands and riparian vegetation, and more studies are needed to verify whether this trend is consistent among global rivers. Key Points: Seasonal variations in DBC and dissolved lignin were controlled by different factorsDBC and dissolved lignin showed similar variation trends during a rain eventStrong dilutions of DBC and dissolved lignin were observed during an extreme rain event [ABSTRACT FROM AUTHOR]

Marine dissolved organic matter (DOM) contains one of the largest exchangeable organic carbon pools on Earth. Riverine input represents an important source of DOM to the oceans, yet much remains to be learned about the fate of the DOM linking terrestrial to oceanic carbon cycles through rivers at the global scale. Here we use ultrahigh-resolution mass spectrometry to identify 184 molecular formulae that are indicators of riverine inputs (referred to as t-Peaks) and to track their distribution in the deep North Atlantic and North Pacific Oceans. The t-Peaks were found to be enriched in the Amazon River, to be highly correlated with known tracers of terrigenous input, and to be observed in all samples from four different rivers characterized by vastly different landscapes and vegetation coverage spanning equatorial (Amazon and Congo), subtropical (Altamaha), and Arctic (Kolyma) regions. Their distribution reveals that terrigenous organic matter is injected into the deep ocean by the global meridional overturning circulation, indicating that a fraction of the terrigenous DOM introduced by rivers contributes to the DOM pool observed in the deep ocean and to the storage of terrigenous organic carbon. This novel molecular approach can be used to further constrain the transfer of DOM from land to sea, especially considering that Fourier transform ion cyclotron resonance mass spectrometer analysis is becoming increasingly frequent in studies characterizing the molecular composition of DOM in lakes, rivers, and the ocean. [ABSTRACT FROM AUTHOR]

Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air-sea CO2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River-ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50-76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large-scale circulation and potentially contribute to the long-term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean. [ABSTRACT FROM AUTHOR]

Bacterial biomarkers (D-amino acids and muramic acid) were investigated in surface sediments (0-1 cm) at 22 stations in the Peru margin. Concentrations were used to quantitatively estimate the relative importance of peptidoglycan in the preservation of prokaryotic remains during diagenesis. The bacterial imprint in organic matter was also evident from low molar ratios (average of 1.6) of glucosamine and galactosamine. Estimates of the fraction of biomarkers associated with intact peptidoglycan showed that peptidoglycan became a progressively less important component of prokaryotic remains with ongoing diagenesis, whereas other prokaryotic biomolecules increased in importance. The exact nature of these biomolecules remains unknown, but constant ratios between individual D-amino acids and amino sugars observed in this study provide novel information on their molecular composition. Examination of total hydrolyzable amino acids revealed systematic compositional changes with increasing water depth and age of the sediment, reflecting increased diagenetic alteration of the organic matter. The time scale integrated in the upper 1 cm of the sediment was ca. 2 yr at the shallow sites and up to 20 yr at the deeper stations. The alteration stages of organic matter ranged from coastal and ocean margin sediments at shallow water depth to organic matter as degraded as hemipelagic surface sediments at greater water depths. [ABSTRACT FROM AUTHOR]