Processing Of Dissolved Organic Matter Research Articles

Linking Dissolved Organic Matter Composition to Landscape Properties in Wetlands Across the United States of America

AbstractWetlands are integral to the global carbon cycle, serving as both a source and a sink for organic carbon. Their potential for carbon storage will likely change in the coming decades in response to higher temperatures and variable precipitation patterns. We characterized the dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition from 12 different wetland sites across the USA spanning gradients in climate, landcover, sampling depth, and hydroperiod for comparison to DOM in other inland waters. Using absorption spectroscopy, parallel factor analysis modeling, and ultra‐high resolution mass spectroscopy, we identified differences in DOM sourcing and processing by geographic site. Wetland DOM composition was driven primarily by differences in landcover where forested sites contained greater aromatic and oxygenated DOM content compared to grassland/herbaceous sites which were more aliphatic and enriched in N and S molecular formulae. Furthermore, surface and porewater DOM was also influenced by properties such as soil type, organic matter content, and precipitation. Surface water DOM was relatively enriched in oxygenated higher molecular weight formulae representing HUPHigh O/C compounds than porewaters, whose DOM composition suggests abiotic sulfurization from dissolved inorganic sulfide. Finally, we identified a group of persistent molecular formulae (3,489) present across all sites and sampling depths (i.e., the signature of wetland DOM) that are likely important for riverine‐to‐coastal DOM transport. As anthropogenic disturbances continue to impact temperate wetlands, this study highlights drivers of DOM composition fundamental for understanding how wetland organic carbon will change, and thus its role in biogeochemical cycling.

Dearomatization drives complexity generation in freshwater organic matter

Dissolved organic matter (DOM) is one of the most complex, dynamic and abundant sources of organic carbon, but its chemical reactivity remains uncertain1–3. Greater insights into DOM structural features could facilitate understanding its synthesis, turnover and processing in the global carbon cycle4,5. Here we use complementary multiplicity-edited 13C nuclear magnetic resonance (NMR) spectra to quantify key substructures assembling the carbon skeletons of DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We find that one type of reaction mechanism, oxidative dearomatization (ODA), widely used in organic synthetic chemistry to create natural product scaffolds6–10, is probably a key driver for generating structural diversity during processing of DOM that are rich in suitable polyphenolic precursor molecules. Our data suggest a high abundance of tetrahedral quaternary carbons bound to one oxygen and three carbon atoms (OCqC3 units). These units are rare in common biomolecules but could be readily produced by ODA of lignin-derived and tannin-derived polyphenols. Tautomerization of (poly)phenols by ODA creates non-planar cyclohexadienones, which are subject to immediate and parallel cycloadditions. This combination leads to a proliferation of structural diversity of DOM compounds from early stages of DOM processing, with an increase in oxygenated aliphatic structures. Overall, we propose that ODA is a key reaction mechanism for complexity acceleration in the processing of DOM molecules, creation of new oxygenated aliphatic molecules and that it could be prevalent in nature.

Seven genome sequences of bacterial, environmental isolates from Pony Lake, Antarctica.

Dissolved organic matter (DOM) in Antarctic inland waters is unique in that its precursor molecules are microbially derived and lack the chemical signature of higher plants. Here, we report the genomic sequences of seven environmental, bacterial isolates from Pony Lake, Antarctica, to explore the genetic potential linked to DOM processing.

Utilization and transformation of Chrysotila dentata-derived dissolved organic matter by phycosphere bacteria Marinobacter hydrocarbonoclasticus and Bacillus firmus.

The dissolved organic matter (DOM) released from the cocoolithophores (Chrysotila dentata) was studied in laboratory experiments after co-culturing C. dentata with bacteria. Marinobacter hydrocarbonoclasticus (CA6)-γ-Proteobacteria and Bacillus firmus (CF2) were used to investigate the utilization and processing of the DOM derived from C. dentata, utilizing fluorescence excitation-emission matrix (EEM) combined with parallel factor analysis (EEM-PARAFAC), while measuring algal abundance and photosynthetic parameters. The experimental groups consisted of axenic C. dentata groups, filter cultured with bacteria (CA6 or CF2) groups, C. dentata co-cultured with bacteria (CA6 or CF2) groups and axenic bacteria (CA6 or CF2) groups. We then evaluated the processing of DOM by determining four fluorescence indices. The number of C. dentata cells and the photosynthetic capacity of microalgae were enhanced by CA6 and CF2. The main known fluorophores, including humic-like components and protein-like components, were present in all sample. The protein-like component of algal-bacterial co-cultures was effectively utilized by CA6 and CF2. The humic-like components increased at the end of the culture time for all cultures. Meanwhile, the average fluorescence intensity of protein-like in CA6 co-culture with algae was lower than that in CF2 co-culture with algae over time. On the other hand, the average fluorescence intensity of humic-like in CA6 was higher than CF2. However, the total change in fluorescence in humic-like and protein-like of axenic CF2 cultures was lower than that of CA6. Hence, the ability of CA6 to transform microalgal-derived DOM was superior to that of CF2, and CF2's ability to consume bacterial-derived DOM was superior to that of CA6.

Disentangling Biological Transformations and Photodegradation Processes from Marine Dissolved Organic Matter Composition in the Global Ocean.

Dissolved organic matter (DOM) holds the largest amount of organic carbon in the ocean, with most of it residing in the deep for millennia. Specific mechanisms and environmental conditions responsible for its longevity are still unknown. Microbial transformations and photochemical degradation of DOM in the surface layers are two processes that shape its molecular composition. We used molecular data (via Fourier transform ion cyclotron resonance mass spectrometry) from two laboratory experiments that focused on (1) microbial processing of fresh DOM and (2) photodegradation of deep-sea DOM to derive independent process-related molecular indices for biological formation and transformation (Ibio) and photodegradation (Iphoto). Both indices were applied to a global ocean data set of DOM composition. The distributions of Iphoto and Ibio were consistent with increased photodegradation and biological reworking of DOM in sunlit surface waters, and traces of these surface processes were evident at depth. Increased Ibio values in the deep Southern Ocean and South Atlantic implied export of microbially reworked DOM. Photodegraded DOM (increased Iphoto) in the deep subtropical gyres of Atlantic and Pacific oceans suggested advective transport in warm-core eddies. The simultaneous application of Iphoto and Ibio disentangled and assessed two processes that left unique molecular signatures in the global ocean.

Life after a fiery death: Fire and plant biomass loading affect dissolved organic matter in experimental ponds.

Drier and hotter conditions linked with anthropogenic climate change can increase wildfire frequency and severity, influencing terrestrial and aquatic carbon cycles at broad spatial and temporal scales. The impacts of wildfire are complex and dependent on several factors that may increase terrestrial deposition and the influx of dissolved organic matter (DOM) from plants into nearby aquatic systems, resulting in the darkening of water color. We tested the effects of plant biomass quantity and its interaction with fire (burned vs. unburned plant biomass) on dissolved organic carbon (DOC) concentration and degradation (biological vs. photochemical) and DOM composition in 400 L freshwater ponds using a gradient experimental design. DOC concentration increased nonlinearly with plant biomass loading in both treatments, with overall higher concentrations (>56 mg/L) in the unburned treatment shortly after plant addition. We also observed nonlinear trends in fluorescence and UV-visibleabsorbance spectroscopic indices as a function of fire treatment and plant biomass, such as greater humification and specific UV absorbance at 254 nm (a proxy for aromatic DOM) over time. DOM humification occurred gradually over time with less humification in the burned treatment compared to the unburned treatment. Both burned and unburned biomass released noncolored, low molecular weight carbon compounds that were rapidly consumed by microbes. DOC decomposition exhibited a unimodal relationship with plant biomass, with microbes contributing more to DOC loss than photodegradation at intermediate biomass levels (100-300 g). Our findings demonstrate that the quantity of plant biomass leads to nonlinear responses in the dynamics and composition of DOM in experimental ponds that are altered by fire, indicating how disturbances interactively affect DOM processing and its role in aquatic environments.

Dissolved organic matter and nutrient processing in organic-rich subterranean estuaries: Implications for future land use and climate scenarios

The subterranean estuary (STE) is a recognized reactive biogeochemical zone at the groundwater–seawater interface in coastal regions. However, few studies have explored dissolved organic matter (DOM) and nutrient processes in buried organic-rich STEs (with up to 20 % of organic matter content), despite their potential contributions to coastal water hypoxia and greenhouse gas emissions. This study conducted controlled laboratory experiments to investigate the physicochemical controls on DOM and nutrient processing in organic-rich STEs, considering variations in groundwater nitrate (NO3–) concentrations, salt concentration, seawater ions, and incubation times. Results showed elevated levels of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in pore water, primarily from particulate organic matter (POM) degradation that originated from decayed vascular plants. POM-to-DOM degradations further resulted in the accumulation of ammonium (NH4+) and highly unsaturated compounds in all treatments. The addition of high-ionic-strength treatments (e.g., seawater and sodium chloride [NaCl] solution) reduced DOC solubility and selectively removed dissolved aromatic compounds in porewater. The cation-rich seawater shifted the porewater NH4+ generation pathway from predominantly biological (re)mineralization in freshwater to abiotic (desorption via ion exchange) pathways in the saline water setup. Orthophosphate (PO43−) dynamics were likely to be influenced by the iron-rich sediment surface in both freshwater and saline water scenarios. Our experiments further indicated that the long-term incubation enriched DOM concentration with a high degradation state. This work demonstrated that organic-rich STEs are significant sources of less labile DOM and NH4+ to coastal waters. In the context of global climate change, our results illustrate the sensitivity of DOM and nutrient biogeochemical cycling in STEs to salinization and altered residence time, consequently affecting the quantity and quality of DOM and nutrients transported to coastal waters.

Co-occurrence of arsenic and iodine in the middle-deep groundwater of the Datong Basin: From the perspective of optical properties and isotopic characteristics

Redox processes can induce arsenic (As) and iodine (I) transformation and thus change As and I co-occurrence, yet there is no evidence that Fe–C–S coupled redox processes have such an impact on the co-occurrence of As and I. To fill this gap, middle-deep groundwater from the Datong Basin were samples for the purpose of exploring how dissolved organic matter (DOM) reactivity affects As and I enrichment and how iron reduction and sulfate reduction processes influence As and I co-occurrence. We identified three DOM components: reduced and oxidized quinone compounds (C1 and C3) and a labile DOM from terrestrial inputs (C2). Two pathways of DOM processing take place in the aquifer, including the degradation of labile DOM to HCO3− and the transformation of oxidized quinone compounds to reduced quinone compounds. Electrons transfer drives the reduction of the terminal electron acceptors. The supply of electrons promotes the reduction of iron and sulfate by microbes, enhancing As and I co-enrichment in groundwater. Thus, the reduction processes of iron and sulfate triggered by the dual roles of DOM affect dissolved As and I co-enrichment. As and I biogeochemical cycling interacts with C, Fe, and S cycling. These results provide isotopic and fluorescence evidence that explains the co-occurrence of arsenic and iodine in middle-deep aquifers.

Dissolved organic matter in peat and marl marshes varies with nutrient enrichment and restored hydrology

Dissolved organic matter (DOM) drives biogeochemical processes in aquatic ecosystems. Yet, how hydrologic restoration in nutrient‐enriched ecosystems changes DOM and the consequences of those changes for the carbon cycle remain unclear. To predict the consequences of hydrologic restoration on carbon cycling in restored wetlands, we need to understand how local environmental factors influence production, processing, and transport of DOM. We collected surface water samples along transects in restored peat (organic‐rich, macrophyte‐dominated) and marl (carbonate, periphyton‐dominated) freshwater marshes in the Everglades (Florida, U.S.A.) that varied in environmental factors (water depth, phosphorus [P] concentrations [water, macrophytes, periphyton, and soil], and primary producer biomass) to understand drivers of dissolved organic carbon (DOC) concentrations and DOM composition. Higher water depths led to a “greening” of DOM, due to increasing algal contributions, with decreasing concentrations of DOC in peat wetlands, and a “browning” of DOM, due to increasing humic contributions, with increasing DOC concentrations in marl wetlands. Soil total P was positively correlated with DOC concentrations and microbial contributions to DOM in peat wetlands, and periphyton total P was positively correlated with algal contributions to DOM in marl wetlands. Despite large variations in both vegetation biomass and periphyton biovolume across transects and sites, neither were predictors of DOC concentrations or DOM composition. Hydrologic restoration differentially alters DOM in peat and marl marshes and interacts with nutrient enrichment to shift proportions of green and brown contributions to surface water chemistry, which has the potential to modify wetland food webs, as well as the processing of carbon by micro‐organisms.

Dissolved organic matter transformations in a freshwater rivermouth

River-to-lake transitional areas are biogeochemically active ecosystems that can alter the amount and composition of dissolved organic matter (DOM) as it moves through the aquatic continuum. However, few studies have directly measured carbon processing and assessed the carbon budget of freshwater rivermouths. We compiled measurements of dissolved organic carbon (DOC) and DOM in several water column (light and dark) and sediment incubation experiments conducted in the mouth of the Fox river (Fox rivermouth) upstream from Green Bay, Lake Michigan. Despite variation in the direction of DOC fluxes from sediments, we found that the Fox rivermouth was a net sink of DOC where water column DOC mineralization outweighed the release of DOC from sediments at the rivermouth scale. Although we found DOM composition also changed during our experiments, alterations in DOM optical properties were largely independent of the direction of sediment DOC fluxes. We found a consistent decrease in humic-like and fulvic-like terrestrial DOM and a consistent increase in the overall microbial composition of rivermouth DOM during our incubations. Moreover, greater ambient total dissolved phosphorus concentrations were positively associated with the consumption of terrestrial humic-like, microbial protein-like, and more recently derived DOM but had no effect on bulk DOC in the water column. Unexplained variation indicates that other environmental controls and water column processes affect the processing of DOM in this rivermouth. Nonetheless, the Fox rivermouth appears capable of substantial DOM transformation with implications for the composition of DOM entering Lake Michigan.

Seasonal Trends of DOM Character in Soils and Stream Change With Snowmelt Timing

AbstractIn many watersheds, runoff occurring with snowmelt brings the largest pulse of dissolved organic matter (DOM) from soils to streams. Yet, exactly how soil and stream DOM fractions are altered as snowmelt timing changes is not well understood. We studied the optical character of DOM as it moved through a first‐order northern hardwood watershed through the snowmelt period. While stream dissolved organic carbon (DOC; mean = 4.6 mg C L−1) concentrations were not correlated with snowmelt flushing, stream DOM appeared humic‐like throughout the study period (humification index, HIX; mean = 10.9). Within the stream, we measured seasonal increases in an inverse index of DOM molecular weight and processing (spectral slope, S275–295; range = 0.0102–0.0214) and in an index of recently derived DOM (Biological Index, BIX; range = 0.2–1.1). We measured a decrease in an inverse index of DOM oxidation (ratio of Peak C to Peak A, or C/A; range = 0.59–2.01) through the snowmelt period. Stream BIX and C/A tracked those measured in the north aspect soils throughout the study. C/A was unique as it decreased from source to stream, likely reflecting the diagenesis of DOM through soils. These data suggest that changes in the timing of snowmelt and hydrologic connectedness within a watershed affect seasonal changes in stream DOM character. We suggest that watersheds with deep glacial till parent materials have stream connectivity limited by subsurface flow paths, with stream DOM character being determined largely by slow melt and a longer period of infiltration.

Dissolved Organic Matter Sources in High Arsenic Groundwater From a Sand‐Gravel Confined Aquifer

AbstractSources of dissolved organic matter (DOM) triggering arsenic enrichment in deep confined groundwater is a highly debated issue. Along these lines, the spectroscopic and molecular characteristics of DOM were monitored in both shallow unconfined and deep confined groundwater from the Songnen Basin, China. Results demonstrated that deep groundwater with higher dissolved arsenic concentrations was characterized by more abundant labile organic compounds than shallow groundwater. The three‐end‐member mixing model estimated that shallow groundwater had more fraction of surface water, while deep groundwater was primarily recharged by lateral groundwater flow. Groundwater DOM sources were estimated from the fluorescence index (FI) values, which suggests that shallow groundwater DOM was primarily sourced from surface water with stronger terrestrial signatures. However, deep groundwater DOM was mainly sourced from in situ sedimentary organic matter release or by‐products from microbial activity with stronger microbial signatures. Higher values of degradation index (I_deg) indicate that deep groundwater DOM had undergone greater degradation extents than the shallow one. The microbial DOM degradation in anoxic environments mainly occurred via a pathway where more saturated and low‐molecular‐mass compounds were preferentially degraded to produce more mid‐molecular‐mass compounds with unsaturated, aromatic, and microbial signatures. This explained the significant positive correlations between groundwater arsenic concentrations and FI and I_deg values. A stronger DOM degradation state in deep groundwater was conducive to more arsenic enrichment than that in the shallow one. This work successfully identified distinct sources of shallow and deep groundwater DOM, and proposed a paradigm of DOM processing pathways during arsenic enrichment under anoxic environments.

Data evaluation strategy for identification of key molecular formulas in dissolved organic matter as proxies for biogeochemical reactivity based on abundance differences from ultrahigh resolution mass spectrometry

The molecular composition of dissolved organic matter (DOM) is of relevance for global carbon cycling and important for drinking water processing also. The detection of variation of DOM composition as function of time and space from a methodological viewpoint is essential to observe DOM processing and was addressed so far. High resolution concerning DOM quality was achieved with Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS).However almost none of the existing FTICR-MS data sets were evaluated addressing the fate of single mass features / molecular formulas (MFs) abundance during experiments. In contrast to former studies we analyze the function of MF abundance of time and space for such MFs which are present in all samples and which were formerly claimed as recalcitrant in not all but a great number of studies. For the first time the reactivity of MFs was directly compared by their abundance differences using a simple equation, the relative intensity difference (δRI). Search strategies to find out the maximum δRI values are introduced. The corresponding MFs will be regarded as key MFs (KEY-MFs). In order to test this new approach data from a recent photo degradation experiment were combined with monitoring surveys conducted in two drinking water reservoirs. The δRI values varied over one order of magnitude (more than five-fold). MFs like C9H12O6 and C10H14O6 revealed high biogeochemical reactivity as photo products. Some of the KEY-MFs were identical with MFs identified as disinfection byproducts precursors in recent studies. Other KEY-MFs were oxygen-rich and relatively unsaturated (poly-phenol-like) and hence relevant to flocculation procedures.

Organic matter cycling in a model restored wetland receiving complex effluent

Wetlands have been used to treat anthropogenic effluents for decades due to their intense biogeochemical processes that transform and uptake nutrients, organic matter, and toxins. Despite these known functions, we lack generalizable knowledge of effluent-derived dissolved organic matter (DOM) cycling in wetlands. Here, we quantify the cycling of DOM in one of Canada’s more economically important wetland complexes (Frank Lake, Alberta), restored to hydrologic permanence in the 1980s using urban and agro-industrial effluents. Optical analyses and PARAFAC (parallel factor analysis) modelling showed a clear compositional change from more bioavailable and protein-like DOM at effluent input sites to more aromatic and humic-like at the wetland outflow, likely due to DOM processing and inputs from marsh plants and wetland soils. Microbial incubations showed that effluent DOM was rapidly consumed, with the half-life of DOM increasing from as low as 35 days for effluent, to 462 days at the outflow, as a function of compositional shifts toward aromatic, humic-like material. Long-term averaged dissolved organic carbon (DOC) export was low compared to many wetlands (10.3 ± 2.0 g C m−2 yr−1). Consistent with predictions based on water residence time, our mass balance showed Frank Lake was a net source of DOM across all measured years, but shifted from a source to sink among wet and drought years that respectively shortened or lengthened the water residence and DOM processing times. Overall, Frank Lake processes and transforms effluent DOM, despite being a longer-term net source of DOM to downstream environments.

Comprehensive assessment of dissolved organic matter processing in the Amazon River and its major tributaries revealed by positive and negative electrospray mass spectrometry and NMR spectroscopy

Rivers are natural biogeochemical systems shaping the fates of dissolved organic matter (DOM) from leaving soils to reaching the oceans. This study focuses on Amazon basin DOM processing employing negative and positive electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI[±] FT-ICR MS) and nuclear magnetic resonance spectroscopy (NMR) to reveal effects of major processes on the compositional space and structural characteristics of black, white and clear water systems. These include non-conservative mixing at the confluences of (1) Solimões and the Negro River, (2) the Amazon River and the Madeira River, and (3) in-stream processing of Amazon River DOM between the Madeira River and the Tapajós River. The Negro River (black water) supplies more highly oxygenated and high molecular weight compounds, whereas the Solimões and Madeira Rivers (white water) contribute more CHNO and CHOS molecules to the Amazon River main stem. Aliphatic CHO and abundant CHNO compounds prevail in Tapajos River DOM (clear water), likely originating from primary production. Sorption onto particles and heterotrophic microbial degradation are probably the principal mechanisms for the observed changes in DOM composition in the Amazon River and its tributaries.

Microbial Interactions with Dissolved Organic Matter Are Central to Coral Reef Ecosystem Function and Resilience.

To thrive in nutrient-poor waters, coral reefs must retain and recycle materials efficiently. This review centers microbial processes in facilitating the persistence and stability of coral reefs, specifically the role of these processes in transforming and recycling the dissolved organic matter (DOM) that acts as an invisible currency in reef production, nutrient exchange, and organismal interactions. The defining characteristics of coral reefs, including high productivity, balanced metabolism, high biodiversity, nutrient retention, and structural complexity, are inextricably linked to microbial processing of DOM. The composition of microbes and DOM in reefs is summarized, and the spatial and temporal dynamics of biogeochemical processes carried out by microorganisms in diverse reef habitats are explored in a variety of key reef processes, including decomposition, accretion, trophictransfer, and macronutrient recycling. Finally, we examine how widespread habitat degradation of reefs is altering these important microbe-DOM interactions, creating feedbacks that reduce reef resilience to global change.

The glacial–terrestrial–fluvial pathway: A multiparametrical analysis of spatiotemporal dissolved organic matter variation in three catchments of Lake Nam Co, Tibetan Plateau

The Tibetan Plateau (TP) is a sensitive alpine environment of global importance, being Asia's water tower, featuring vast ice masses and comprising the world's largest alpine grasslands. Intensified land-use and pronounced global climate change have put pressure on the environment of the TP. We studied the tempo-spatial variability of dissolved organic matter (DOM) to better understand the fluxes of nutrients and energy from terrestrial to aquatic ecosystems in the TP. We used a multiparametrical approach, based on inorganic water chemistry, dissolved organic carbon (DOC) concentration, dissolved organic matter (DOM) characteristics (chromophoric DOM, fluorescence DOM and δ13C of DOM) in stream samples of three catchments of the Nam Co watershed and the lake itself. Satellite based plant cover estimates were used to link biogeochemical data to the structure and degradation of vegetation zones in the catchments. Catchment streams showed site-specific DOM signatures inherited from glaciers, wetlands, groundwater, and Kobresia pygmaea pastures. By comparing stream and lake samples, we found DOM processing and unification by loss of chromophoric DOM signatures and a change towards an autochthonous source of lake DOM. DOM diversity was largest in the headwaters of the catchments and heavily modified in terminal aquatic systems. Seasonality was characterized by a minor influence of freshet and by a very strong impact of the Indian summer monsoon on DOM composition, with more microbial DOM sources. The DOM of Lake Nam Co differed chemically from stream water samples, indicating the lake to be a quasi-marine environment in regards to the degree of chemical modification and sources of DOM. DOM proved to be a powerful marker to elucidate consequences of land use and climatic change on biogeochemical processes in High Asian alpine ecosystems.

Biological and photochemical reactivity of dissolved organic matter in a large temperate river

AbstractLarge rivers are critical conduits from continents to oceans as they receive, produce and process huge amounts of dissolved organic matter (DOM). Yet, the relative influence of intrinsic DOM properties and extrinsic environmental properties on these processes at the ecosystem‐level is rarely studied. We assessed DOM optical properties as well as bioreactivity and photoreactivity at 40 sites along a >200 km transect of the freshwater portion of the St. Lawrence River through a series of standardized microbial incubations and exposure to simulated sunlight, and then estimated in situ areal rates of processing. We found that biological and photochemical processes preferentially targeted contrasting pools of DOM, but that DOM composition had an undiscernible effect on in situ degradation rates compared to other environmental factors. Total daily processing across the whole water column ranged from 36.7 to 892.1 mg C m−2. In situ photochemical degradation was largely driven by intrinsic DOM photoreactivity rather than environmental drivers in the water. In contrast, we found a relatively constant baseline pool of biolabile DOM that appeared to be independent from changes in concentration and environmental conditions. In situ DOM processing was mostly driven by biological degradation (on average 95%), and disproportionately high biodegradation rates (2.5–4x the average) were found in a few shallower sites near effluents or islands, potentially driven by local increases in nutrient concentration and in the proportion of protein‐like DOM. These results illustrate how DOM composition and degradability interact with ambient environmental and morphological properties to dictate an ecosystem‐level reactivity of DOM.

Trapped Under Ice: Spatial and Seasonal Dynamics of Dissolved Organic Matter Composition in Tundra Lakes

AbstractArctic lakes store, modify, and transport large quantities of carbon from terrestrial environments to the atmosphere; however, the spatial and temporal relationships between quantity and composition of dissolved organic matter (DOM) have not been well characterized across broad arctic regions. Moreover, most arctic lake DOM compositions have been examined during the ice‐free summer, whereas DOM cycling between the ice‐covered winter months and summer have not been addressed. To resolve these spatial and seasonal uncertainties in DOM cycling, we sampled a series of arctic lakes from the North Slope of Alaska across a latitudinal gradient in the winter and summer over 3 years. Samples were analyzed for dissolved organic carbon concentration and DOM composition was characterized using optical and fluorescence properties combined with molecular‐level analysis using Fourier transform‐ion cyclotron resonance mass spectrometry. Tundra lake DOM properties including aromaticity and molecular stoichiometries were similar to other northern high‐latitude lakes, but optical parameters related to aromaticity and molecular weight were greater in major arctic rivers and in coastal lakes in the North Slope region. DOM composition was highly seasonal, with ice exclusion concentrating microbially processed DOM in the winter water columns, potentially influencing DOM cycling the following summer. However, the greatest variations in DOM composition were related to lake depth and likely other physical features including morphology and bathymetry. As the Arctic warms, we expect changes in hydrology and ice cover to enhance under‐ice microbial DOM processing, early summer photodegradation, and ultimately carbon fluxes to the atmosphere after ice‐out.

Characterization of dissolved organic matter processing between surface sediment porewater and overlying bottom water in the Yangtze River Estuary

Dissolved organic matter (DOM) exchange in the sediment-water interface of estuaries is essential for the global elemental cycle. To clarify the interface DOM processing, this study applies optical techniques and ultrahigh-resolution mass spectrometry to assess DOM composition of surface sediment porewater and bottom (overlying) water across the Yangtze River Estuary (YRE). Results suggested that DOM exchange in the sediment-water interface mainly followed from sediment porewater to bottom water driven by a significant dissolved organic carbon concentration gradient and hydrodynamic force. We also characterized two porewater DOM sources, including microbial production and byproducts of processed sediments. High microbial activities resulted in the enrichment of protein-like fluorescent components and N-bearing compounds in porewater, potentially decreasing the oxygen concentration of bottom water due to the high lability. And the deamination of N-bearing compounds in the sediment-water interface could likely serve as a N-bearing nutrient source to bottom water. Moreover, due to sediment-specific features in different areas driven by hydrologic sorting and local phytoplankton supply, porewater DOM of muddy areas accumulated more aromatic substances from the degradation of terrestrial organic matter. The release and oxic transformation of oxygen-deficient aromatic compounds could contribute to the refractory carbon pool of estuarine water (carboxyl-rich alicyclic molecules, CRAM), modulating the quality of organic carbon mobilized from the land to the coastal ocean. Considering strong hydrodynamic force in numerous estuaries worldwide, DOM exchange and processing at the sediment-water interface has a meaningful influence on the biogeochemistry of estuarine water columns, which warrants further studies.