Dissolved Organic Matter Pool Research Articles

High-resolution mass spectrometric characterization of dissolved organic matter from warm and cold periods in the NEEM ice core

Dissolved organic matter (DOM) is an important component of ice cores but is currently poorly characterized. DOM from one Holocene sample (HS, aged at 1600–4500 B.P.) and one Last Glacial Maximum sample (LS, aged at 21000–25000 B.P.) from the North Greenland Eemian Ice Drilling (NEEM) ice core were analyzed by ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). CHO compounds contributed 50% of the compounds identified in negative-ionization mode in these two samples, with significant contributions from organic N, S, and P compounds, likely suggesting that marine DOM was an important source in these samples. Overall, the chemical compositions are similar between these two samples, suggesting their consistent DOM sources. However, subtle differences in the DOM between these two samples are apparent and could indicate differences in source strength or chemistry occurring through both pre- and post-depositional processes. For example, higher relative amounts of condensed carbon compounds in the HS DOM (5%), compared to the LS DOM (2%), suggest potentially important contributions from terrestrial sources. Greater incorporation of P in the observed DOM in the LS DOM (22%), compared to the HS DOM (13%), indicate more active microbiological processes that likely contribute to phosphorus incorporation into the DOM pool. Although these two samples present only a preliminary analysis of DOM in glacial/interglacial periods, the data indicate a need to expand the analysis into a broader range of ice-core samples, geographical locations, and glacial/interglacial periods.

Understanding Dissolved Organic Matter Reactivity and Composition in Lakes and Streams Using Proton-Transfer-Reaction Mass Spectrometry (PTR-MS)

Here, we present a novel approach for investigating dissolved organic matter (DOM) composition using thermal desorption proton-transfer-reaction mass spectrometry (PTR-MS), a technique that provides insight into the molecular composition of DOM < m/z 500 (termed “PTR-DOC”). The applicability of PTR-MS for understanding the relationship between DOM composition and reactivity has yet to be explored. We present results from a synoptic sampling campaign of streams and lakes in a Swedish forest catchment where we measured DOM composition using PTR-MS and traditional optical methods and conducted DOM biodegradability assays. PTR-DOC comprised ≤12% of the total DOC pool. We found significant relationships between PTR-DOC and DOM degradability; reduced chemodiversity and low concentrations of PTR-DOC were both associated with the total DOM pool being more susceptible to microbial degradation. Furthermore, molecular differences were apparent among headwater lakes, headwater streams, and lakes further down the catc...

Eutrophication Leads to Accumulation of Recalcitrant Autochthonous Organic Matter in Coastal Environment

AbstractAnthropogenic nutrient enrichment is changing the structure and the function of coastal ecosystems. These coastal zones are transitions between freshwater and marine systems where multiple biogeochemical processes remove, produce, and transform organic matter. The extent to which the coastal zone is merely a conduit for terrestrial (allochthonous) organic matter versus a distinct source of autochthonous organic matter fueled by eutrophication is unclear. To address this issue, we characterized the freshwater and marine dissolved organic matter (DOM) pools in a eutrophic estuary with a long water residence time (Roskilde Fjord, Denmark) over an annual cycle. We combined elemental, optical (absorbance and fluorescence), and isotopic analyses to obtain insight about the bulk properties of the DOM pool during this period. We also used sediment traps to analyze the changes related to the exchange of organic matter between the particulate organic matter and DOM fractions. The results showed that labile autochthonous DOM from in situ primary production was rapidly transformed to more recalcitrant DOM that accumulated in the estuary despite continuous exchange with the open sea. Also, parts of the particulate organic matter pool were degraded rapidly (within 24 hr) and transformed into the DOM pool. Accumulated DOM was characterized by relatively low molecular size and stable carbon isotopic value and by high protein‐like fluorescence. These results indicate that autotrophic material can be a major source of specific recalcitrant DOM in eutrophic coastal waters, contributing significantly to the flux of organic carbon to the ocean.

Natural levels and photo-production rates of hydrogen peroxide (H2O2) in Andean Patagonian aquatic systems: Influence of the dissolved organic matter pool

In aquatic environments the reactive oxygen species hydrogen peroxide (H2O2) is produced through photochemical reactions involving chromophoric dissolved organic matter (CDOM). Andean Patagonian freshwaters experience challenging underwater UV levels, which promote high levels of photochemical weathering. In this investigation, we study natural H2O2 levels and experimentally address the photochemical formation of H2O2 in stream and lake water with a range of dissolved organic matter (DOM) concentrations and quality. The screening of different pristine aquatic systems of Patagonia revealed that H2O2 concentration fluctuates between 8 and 60 nM. Laboratory incubation of different water types in PAR + UV showed photo-production of H2O2. The H2O2 formation rate increased linearly with dissolved organic carbon (DOC) in streams (13.5–20.5 nM h-1) and shallow lakes (25.7–37.8 nM h-1). In contrast, the H2O2 formation rate in deep lakes was much lower (2.1–7.1 nM h-1), and decreased with DOC. The natural potential for H2O2 formation was related to the concentration and quality of the DOM pool. At higher DOC levels, such as those present in shallow lakes, H2O2 production was directly related to DOC, whereas at low DOC levels in deep lakes and streams, two patterns were distinguished in relation to their DOM pool quality. Stream DOM, composed of high molecular weight/size humic compounds, proved to be a reactive substrate, as reflected by their high H2O2 formation rates. On the other hand, deep lake DOM, with its higher relative contribution of small and more processed compounds, was found to be a less reactive substrate, affording lower H2O2 formation rates.

Photo-biochemical transformation of dissolved organic matter on the surface of the coastal East Antarctic ice sheet

Recent studies have highlighted the composition and complexity of dissolved organic matter (DOM) in glacial environments. Climate-induced changes to glacier runoff are projected to be an important source of DOM to coastal ecosystems. Photochemical and microbial (termed photo-biochemical) degradation of DOM would determine its fate on the glacier surface and in recipient coastal ecosystems. In order to understand the molecular imprints of photo-biochemical alteration of DOM, in situ field experiments were conducted over a period of 35 days in a coastal Antarctic site and DOM molecularly characterised using ultrahigh-resolution mass spectrometry. We show that the biogeochemistry of DOM is highly complex and intimately connected with microbial and photochemical processes operating individually or in combination. Photo-biochemical processes resulted in shifts in the nitrogen, sulfur, and phosphorous content of the DOM. These processes are also an important mechanism for transforming refractory DOM, like dissolved black carbon and carboxylic rich alicyclic molecules from the snow surface. This study is unique, as it provides new molecular-level information on compounds that comprise the photo- and bio-labile, photo- and bio-refractory, as well as photo- and bio-produced fractions of the supraglacial DOM pool. These insights into the interactions between microbes, light, and specific components of the DOM pool highlight the need for studies focused on the biogeochemistry of supraglacial carbon and its response to a changing climate.

Origin and Dynamics of Dissolved Organic Matter in a Mariculture Area Suffering From Summertime Hypoxia and Acidification

Based on six cruises from March to September in 2016, we investigated monthly distributions of dissolved organic matter (DOM) and ancillary water chemistry parameters in a mariculture area in the Northern Yellow Sea, where summertime hypoxia and seawater acidification were observed. The most severe oxygen depletion (hypoxia covered approximately one third of the aquaculture area) and the largest pH decrease (8.07 ± 0.05 in surface layer vs. 7.66 ± 0.07 in bottom layer) were revealed in August. Concentration of dissolved organic carbon (DOC) and the absorption properties of chromophoric DOM (CDOM) were used to characterize DOM. Results showed that DOM was mainly originated from marine in situ processes. In March, a DOM pool with the lowest DOC concentration of 2.53 ± 0.28 mgC L-1 and nearly uniform optical characteristics were presented in the well-mixed water column. In August, however, DOC increased to 4.33 ± 0.35 mgC L-1 in the surface layer and 4.10 ± 0.30 mgC L-1 in the bottom layer. Two nonlinear relationships between the absorption coefficient at 355 nm (aCDOM(355)) and the absorption spectral slope over 275–295nm (S275-295) were revealed. According to modelling results, the nonlinear relationships were mostly caused by the conservative mixing of a refractory CDOM pool with a freshly-produced CDOM pool. Apparent oxygen utilization in August was positively related to DOC but not to aCDOM(355) and S275-295, presumably due to multiple sources of CDOM in bottom waters. The absorption properties of CDOM respond largely to varied pH, however, they would be less affected by ocean acidification since the latter process leads to a limited pH decline.

Transformations of dissolved organic matter in the Marmara Sea traced by its optical signature

Abstract The Marmara Sea is a semi-enclosed basin connecting the Black and the Mediterranean Seas through the Bosporus and Dardanelles Straits (Turkish Straits System- TSS). It is characterized by a permanent two layered counter-flow caused by the exchange of water masses between the Black and Mediterranean Seas. In this study we focus on the role of the Marmara Sea in altering the optical characteristics of dissolved organic matter (DOM) during its transport through the TSS. We use detailed absorbance and fluorescence measurements followed by parallel factor analysis (PARAFAC). PARAFAC identified four components: a primary terrestrial humic C1 (ex/em 255/448 nm), a marine humic C2 (ex/em 300/402 nm), a secondary terrestrial humic C3 (ex/em 280, 370/480 nm) and a protein like component C4 (ex/em 275/336 nm). The above halocline waters (≤25 m) of Black Sea origin are characterized by high DOC (114–181 μmol L−1); high CDOM absorption (aCDOM(300): 1.97–2.92 m−1), low absorption spectral slope (S275–295: 0.0280–0.0307 nm−1) and high humic content. In this water layer, interrelationships with salinity, DOC and optical parameters revealed that although bulk DOC is diluted towards the west, transformations of the DOM pool are related to several processes. In the sub-halocline waters (>25–1250 m) of Mediterranean origin, all measured parameters are significantly lower (DOC: 55–65 μmo L−1; aCDOM(300): 0.46–0.80 m−1; S275–295: 0.0241–0.0376 nm−1). No relationship of DOC with apparent oxygen utilization (AOU) could be established, likely due to the proliferation of particles. Nevertheless, qualitative changes of DOM could be traced by the negative relationship of S275–295 with AOU and the two fold increase of the intensities of the humic-like components towards the eastern part of the basin. This suggests that the optical signature of the Mediterranean DOM is altered in the deep Marmara Sea despite its constant DOC content.

Identifying labile DOM components in a coastal ocean through depleted bacterial transcripts and chemical signals.

Understanding which compounds comprising the complex and dynamic marine dissolved organic matter (DOM) pool are important in supporting heterotrophic bacterial production remains a major challenge. We eliminated sources of labile phytoplankton products, advected terrestrial material and photodegradation products to coastal microbial communities by enclosing water samples in situ for 24 h in the dark. Bacterial genes for which expression decreased between the beginning and end of the incubation and chemical formulae that were depleted over this same time frame were used as indicators of bioavailable compounds, an approach that avoids augmenting or modifying the natural DOM pool. Transport- and metabolism-related genes whose relative expression decreased implicated osmolytes, carboxylic acids, fatty acids, sugars and organic sulfur compounds as candidate bioreactive molecules. FT-ICR MS analysis of depleted molecular formulae implicated functional groups ~ 30-40 Da in size cleaved from semi-polar components of DOM as bioreactive components. Both gene expression and FT-ICR MS analyses indicated higher lability of compounds with sulfur and nitrogen heteroatoms. Untargeted methodologies able to integrate biological and chemical perspectives can be effective strategies for characterizing the labile microbial metabolites participating in carbon flux.

Dissolved organic matter binding with Pb(II) as characterized by differential spectra and 2D UV–FTIR heterospectral correlation analysis

Dissolved organic matter (DOM) in aquatic environment significantly influences the behavior and fate of heavy metals via binding, complexation and thus changes the metal speciation; however detailed interfacial processes and mechanisms are still unclear. Here, differential absorbance and fluorescence spectra and two dimensional UV–FTIR heterospectral correlation analysis were applied to probe into the Pb(II)–DOM interaction at a wide range of pH and ionic strength (IS). The absorbance of DOM molecules under all conditions increased with metal addition, while the different extents of absorbance variations along the wavelength range in the differential zero-order and log-transformed absorbance spectra indicated the site heterogeneity within the DOM pool for metal binding. Spectral parameters, namely differential fluorescent components 1 and 2 (DFC1 and DFC2) and differential slopes of log-transformed absorbance in the range of wavelength 350–400 nm (DSlope350–400) were found to be highly correlated with the total amounts of DOM-bound Pb(II) predicted by the NICA-Donnan model, while the differential absorbance spectra at 235 nm (DA235) was related to the extent of Pb(II) bound by carboxylic groups. Thus, these parameters are an indicator or proxy for the in situ Pb(II)–DOM interaction extent. Aryl C–H gave the fastest response to Pb(II) binding at lower pH and IS (e.g., pH 4.7 and IS = 0.01 M), followed by carboxyl C=O and polysaccharide C–OH and then chromophoric groups at 265 nm (CDOM265). However, the CDOM265 bound to Pb(II) prior to aryl C–H and polysaccharide C–OH groups at higher pH and IS (6.0 and 0.1 M, respectively), showing that the binding sequences were highly dependent on solution chemistry. Differential spectra combined with two dimensional UV–FTIR heterospectral correlation analysis can be used as a promising approach to elucidate metal–DOM interaction processes, including site heterogeneity, binding sensitivity and sequence at the functional group level.

Dissolved organic matter in coastal rainwater: Concentration, bioavailability and depositional flux to seawater in southeastern China

Forty-seven precipitation samples were collected between March 2011 and May 2012 in Xiamen, southeastern China to examine the concentration, bioavailability and depositional flux of dissolved organic matter (DOM) in rainwater. Fluorescence excitation-emission (EEM) spectra of DOM were also determined to trace compositional changes during biodegradation experiments to assess which components of the DOM pool are biolabile. Rainwater dissolved organic carbon (DOC) concentrations ranged from 8.5 to 932 μM, with a volume-weighted average concentration of 128 μM. The fraction of biolabile DOC (BDOC%) ranged from 16 to 91% with an average of 46 ± 17%. The biodegradation rate of rainwater DOC followed a first-order exponential curve. During biodegradation experiments, protein-like fluorescence decreased and humic-like fluorescence remained constant or increased. Both DOC and BDOC concentration showed a dilution effect in relation to rainfall amount. Rainwater with continental trajectories typically had higher DOC concentrations than from marine trajectories, while, in contrast, the BDOC% didn't show significant differences among trajectories. Winter rainwater DOC concentration was significantly higher, while BDOC% was lower than other seasons, which may be related to greater relative fossil fuel inputs, particularly from coal burning in northern China. The depositional flux of rainwater DOC into Xiamen Bay was estimated to be 2.1 Gg C yr−1, and the rainwater BDOC flux to Xiamen Bay was approximately 35% of that discharged from the Jiulong River. Our results highlight that precipitation inputs play a significant role in supplying BDOC to the ocean with ramifications for marine food webs.

Bacterioplankton drawdown of coral mass-spawned organic matter

Coral reef ecosystems are highly sensitive to microbial activities that result from dissolved organic matter (DOM) enrichment of their surrounding seawater. However, the response to particulate organic matter (POM) enrichment is less studied. In a microcosm experiment, we tested the response of bacterioplankton to a pulse of POM from the mass-spawning of Orbicella franksi coral off the Caribbean coast of Panama. Particulate organic carbon (POC), a proxy measurement for POM, increased by 40-fold in seawater samples collected during spawning; 68% degraded within 66 h. The elevation of multiple hydrolases presumably solubilized the spawn-derived POM into DOM. A carbon budget constructed for the 275 µM of degraded POC showed negligible change to the concentration of dissolved organic carbon (DOC), indicating that the DOM was readily utilized. Fourier transform ion cyclotron resonance mass spectrometry shows that the DOM pool became enriched with heteroatom-containing molecules, a trend that suggests microbial alteration of organic matter. Our sensitivity analysis demonstrates that bacterial carbon demand could have accounted for a large proportion of the POC degradation. Further, using bromodeoxyuridine immunocapture in combination with 454 pyrosequencing of the 16S ribosomal RNA gene, we surmise that actively growing bacterial groups were the primary degraders. We conclude that coral gametes are highly labile to bacteria and that such large capacity for bacterial degradation and alteration of organic matter has implications for coral reef health and coastal marine biogeochemistry.

Microbial Interactions With Dissolved Organic Matter Drive Carbon Dynamics and Community Succession.

Knowledge of dynamic interactions between natural organic matter (NOM) and microbial communities is critical not only to delineate the routes of NOM degradation/transformation and carbon (C) fluxes, but also to understand microbial community evolution and succession in ecosystems. Yet, these processes in subsurface environments are usually studied independently, and a comprehensive view has been elusive thus far. In this study, we fed sediment-derived dissolved organic matter (DOM) to groundwater microbes and continually analyzed microbial transformation of DOM over a 50-day incubation. To document fine-scale changes in DOM chemistry, we applied high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and soft X-ray absorption spectroscopy (sXAS). We also monitored the trajectory of microbial biomass, community structure and activity over this time period. Together, these analyses provided an unprecedented comprehensive view of interactions between sediment-derived DOM and indigenous subsurface groundwater microbes. Microbial decomposition of labile C in DOM was immediately evident from biomass increase and total organic carbon (TOC) decrease. The change of microbial composition was closely related to DOM turnover: microbial community in early stages of incubation was influenced by relatively labile tannin- and protein-like compounds; while in later stages the community composition evolved to be most correlated with less labile lipid- and lignin-like compounds. These changes in microbial community structure and function, coupled with the contribution of microbial products to DOM pool affected the further transformation of DOM, culminating in stark changes to DOM composition over time. Our study demonstrates a distinct response of microbial communities to biotransformation of DOM, which improves our understanding of coupled interactions between sediment-derived DOM, microbial processes, and community structure in subsurface groundwater.

Relationship between dissolved organic matter quality and microbial community composition across polar glacial environments.

Vast expanses of Earth's surface are covered by ice, with microorganisms in these systems affecting local and global biogeochemical cycles. We examined microbial assemblages from habitats fed by glacial meltwater within the McMurdo Dry Valleys, Antarctica and on the west Greenland Ice Sheet (GrIS), evaluating potential physicochemical factors explaining trends in community structure. Microbial assemblages present in the different Antarctic dry valley habitats were dominated by Sphingobacteria andFlavobacteria, while Gammaproteobacteria and Sphingobacteria prevailed in west GrIS supraglacial environments. Microbial assemblages clustered by location (Canada Glacier, Cotton Glacier and west GrIS) and were separated by habitat type (i.e. ice, cryoconite holes, supraglacial lakes, sediment and stream water). Community dissimilarities were strongly correlated with dissolved organic matter (DOM) quality. Microbial meltwater assemblages were most closely associated with different protein-like components of the DOM pool. Microbes in environments with mineral particles (i.e. stream sediments and cryoconite holes) were linked to DOM containing more humic-like fluorescence. Our results demonstrate the establishment of distinct microbial communities within ephemeral glacial meltwater habitats, with DOM-microbe interactions playing an integral role in shaping communities on local and polar spatial scales.

Photoreactivities of two distinct dissolved organic matter pools in groundwater of a subarctic island

Abstract Groundwater is a potentially significant source of dissolved organic matter (DOM) to coastal oceans where it is subject to photochemical transformation and thus possibly influences major marine biogeochemical processes. Furthermore, groundwater DOM usually receives little prior light exposure, making it suitable for probing the photoreactivity of source organic materials. In this study we collected two DOM pools in beach and inland groundwater of the Iles-de-la-Madeleine in the Gulf of St. Lawrence, characterized them with absorbance and fluorescence spectroscopy , and compared their photoreactivities. Beach groundwater (BGW) primarily comprised old, highly colored terrestrial DOM having high molecular weight (MW), strong humification , and low protein contents, whereas inland groundwater (IGW) largely contained fresh, less colored microbial-derived DOM with low molecular weight (MW), weaker humification, and higher protein contents. For both BGW and IGW, exposure to solar-simulated radiation led to increases in the E 2/E3 quotient, biological index (BIX), and ammonium (NH4+ ) and decreases in absorbance, specific absorption coefficient at 254 nm (SUVα 254 ), fluorescence index (FI), humification index (HIX), MW, and dissolved organic carbon (DOC). The irradiation also reduced the intensities of humic-like fluorescence peaks and PARAFAC-modeled humic components. On a fractional-absorption-loss basis, the photochemically induced fractional changes in BGW were comparable to those in IGB for SUVα 254 and E2/E3, considerably larger for MW, BIX, and DOC, and substantially smaller for FI and HIX. On an absorbed-photon basis, the efficiencies of absorbance photobleaching and DOC photomineralization for DOM in BGW were 8.3 times and 2.0 times those of the respective photoprocesses for DOM in IGW, while the efficiency of photoammonification for DOM in BGW was 41% of that for DOM in IGW. Results from this study, in combination with those reported previously for surface waters, lead to a tentative paradigm: terrigenous DOM is more prone to absorbance photobleaching but less prone to photoammonification than microbial-derived DOM while the two DOM pools are comparably reactive with respect to DOC photomineralization.

Regional and intra‐annual stability of dissolved organic matter composition and biolability in high‐latitude Alaskan rivers

AbstractNutrient availability and molecular composition of dissolved organic matter (DOM) determine whether the carbon (C) associated with DOM is respired during decomposition or exported from stream networks. High‐latitude ecosystems are changing rapidly due to permafrost thaw, shifts in vegetation, and increasing runoff, which alter sources of DOM and nutrients to rivers. To address how changes in the composition of riverine DOM and nutrients may influence C cycling, we quantified biolability of DOM across a latitudinal gradient in Alaska and assessed temporal patterns associated with intra‐annual variation in hydrology. We measured biolability under ambient and fertilized conditions and characterized optical and chemical properties of DOM to determine the relative influence of nutrients and molecular composition of DOM on its fate in rivers. Biolability was low, comprising 4% of the DOM pool on average and 0–42% across all sites and seasons. Optical indicators of the molecular composition of the DOM pool were more constant in space and time than bulk concentrations of dissolved C and nutrients, and described a DOM pool dominated by humic‐like functional groups. Phosphorus limited biolability at freshet, but biolability was not linked to DOM composition. Strong hydrologic connection of high‐latitude rivers to a large pool of soil organic matter likely results in export of recalcitrant DOM to receiving rivers and coasts in this region, contributing to stability of the high‐latitude C cycle. However, increased inputs of phosphorus or DOM of greater biolability will likely cause increased respiration of riverine DOM and flux of CO2 to the atmosphere.

Increasing dominance of terrigenous organic matter in circumpolar freshwaters due to permafrost thaw

AbstractClimate change and permafrost thaw are unlocking the vast storage of organic carbon held in northern frozen soils. Here, we evaluated the effects of thawing ice‐rich permafrost on dissolved organic matter (DOM) in freshwaters by optical analysis of 253 ponds across the circumpolar North. For a subset of waters in subarctic Quebec, we also quantified the contribution of terrestrial sources to the DOM pool by stable isotopes. The optical measurements showed a higher proportion of terrestrial carbon and a lower algal contribution to DOM in waters affected by thawing permafrost. DOM composition was largely dominated (mean of 93%) by terrestrial substances at sites influenced by thawing permafrost, while the terrestrial influence was much less in waterbodies located on bedrock (36%) or with tundra soils unaffected by thermokarst processes (42%) in the catchment. Our results demonstrate a strong terrestrial imprint on freshwater ecosystems in degrading ice‐rich permafrost catchments, and the likely shift toward increasing dominance of land‐derived organic carbon in waters with ongoing permafrost thaw.

Influence of labile dissolved organic matter on nitrate reduction in a seepage face.

Seepage faces, the outer rim of subterranean estuaries, are an important reaction node for SGD-borne nitrate (NO3-) on a global scale. Labile dissolved organic matter (DOM) has been suggested to be a key factor constraining the NO3- removal rate in aquifer systems. To determine whether and to what extent the availability of labile DOM affects benthic NO3- reduction in seepage faces, a series of flow-through reactor (FTR) experiments with sandy sediment collected from a seepage face was conducted under oxic conditions. Experimental results revealed that the addition of labile DOM (glucose) to porewater did not trigger a significant enhancement in NO3- reduction rate. In contrast, the aerobic respiration was boosted from ca. 50 to 90μmoldm-3 sediment h-1 by glucose amendments, accounting for approximately 70% consumption of the labile DOM pool. This rapid consumption may increase the NO3- reducing capability within the sediment, but only indirectly. Together with fluorescent DOM (FDOM) analyses, it can be inferred that NO3- reducers tend to choose sediment organic matter the prime electron donor under the experimental conditions. As a result, enrichment of DOM in seepage faces, depending on composition, might only stimulate aerobic respiration and nitrification, thus promoting the increase of ensuing NO3- fluxes to adjacent coastal waters.

Organic matter processing by microbial communities throughout the Atlantic water column as revealed by metaproteomics

The phylogenetic composition of the heterotrophic microbial community is depth stratified in the oceanic water column down to abyssopelagic layers. In the layers below the euphotic zone, it has been suggested that heterotrophic microbes rely largely on solubilized particulate organic matter as a carbon and energy source rather than on dissolved organic matter. To decipher whether changes in the phylogenetic composition with depth are reflected in changes in the bacterial and archaeal transporter proteins, we generated an extensive metaproteomic and metagenomic dataset of microbial communities collected from 100- to 5,000-m depth in the Atlantic Ocean. By identifying which compounds of the organic matter pool are absorbed, transported, and incorporated into microbial cells, intriguing insights into organic matter transformation in the deep ocean emerged. On average, solute transporters accounted for 23% of identified protein sequences in the lower euphotic and ∼39% in the bathypelagic layer, indicating the central role of heterotrophy in the dark ocean. In the bathypelagic layer, substrate affinities of expressed transporters suggest that, in addition to amino acids, peptides and carbohydrates, carboxylic acids and compatible solutes may be essential substrates for the microbial community. Key players with highest expression of solute transporters were Alphaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria, accounting for 40%, 11%, and 10%, respectively, of relative protein abundances. The in situ expression of solute transporters indicates that the heterotrophic prokaryotic community is geared toward the utilization of similar organic compounds throughout the water column, with yet higher abundances of transporters targeting aromatic compounds in the bathypelagic realm.

Predicting apparent singlet oxygen quantum yields of dissolved black carbon and humic substances using spectroscopic indices

Dissolved black carbon (DBC) is ubiquitous in aquatic systems, being an important subgroup of the dissolved organic matter (DOM) pool. Nevertheless, its aquatic photoactivity remains largely unknown. In this study, a range of spectroscopic indices of DBC and humic substance (HS) samples were determined using UV–Vis spectroscopy, fluorescence spectroscopy, and proton nuclear magnetic resonance. DBC can be readily differentiated from HS using spectroscopic indices. It has lower average molecular weight, but higher aromaticity and lignin content. The apparent singlet oxygen quantum yield (Φsinglet oxygen) of DBC under simulated sunlight varies from 3.46% to 6.13%, significantly higher than HS, 1.26%–3.57%, suggesting that DBC is the more photoactive component in the DOM pool. Despite drastically different formation processes and structural properties, the Φsinglet oxygen of DBC and HS can be well predicted by the same simple linear regression models using optical indices including spectral slope coefficient (S275-295) and absorbance ratio (E2/E3) which are proxies for the abundance of singlet oxygen sensitizers and for the significance of intramolecular charge transfer interactions. The regression models can be potentially used to assess the photoactivity of DOM at large scales with in situ water spectrophotometry or satellite remote sensing.

Nutrient availability as major driver of phytoplankton-derived dissolved organic matter transformation in coastal environment

Incubation experiments were performed to examine the processing of fresh autochthonous dissolved organic matter (DOM) produced by coastal plankton communities in spring and autumn. The major driver of observed DOM dynamics was the seasonally variable inorganic nutrient status and characteristics of the initial bulk DOM, whereas the characteristics of the phytoplankton community seemed to have a minor role. Net accumulation of dissolved organic carbon (DOC) during the 18-days experiments was 3.4 and 9.2 µmol l−1 d−1 in P-limited spring and N-limited autumn, respectively. Bacterial bioassays revealed that the phytoplankton-derived DOC had surprisingly low proportions of biologically labile DOC, 12.6% (spring) and 17.5% (autumn). The optical characteristics of the DOM changed throughout the experiments, demonstrating continuous heterotrophic processing of the DOM pool. However, these temporal changes in optical characteristics of the DOM pool were not the same between seasons, indicating seasonally variable environmental drivers. Nitrogen and phosphorus availability is likely the main driver of these seasonal differences, affecting both phytoplankton extracellular release of DOM and its heterotrophic degradation by bacteria. These findings underline the complexity of the DOM production and consumption by the natural planktonic community, and show the importance of the prevailing environmental conditions regulating the DOM pathways.