Dissolved Organic Matter Pool Research Articles

Molecular insights into the impacts of acid mine drainage on dissolved organic matter dynamics in pit lakes

Pit lakes are artificial hydrological features created by mining operations that typically suffer from acid mine drainage (AMD), which not only endangers water quality but also exacerbates carbon loss. However, the impacts of AMD on the fate and role of dissolved organic matter (DOM) in pit lakes remain unclear. This study employed negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) combined with biogeochemical analysis to examine DOM molecular variations and environmental controls across the AMD-induced acidic and metalliferous gradients in five pit lakes. The results demonstrated distinct DOM pools in pit lakes characterized by the prevalence of smaller aliphatic compounds compared to other waterbodies. AMD-induced geochemical gradients promoted DOM heterogeneity among pit lakes, with acidic pit lakes containing more lipid-like compounds. Acidity and metals enhanced DOM photodegradation, reducing the content, chemo-diversity and aromaticity. Organic sulfur was detected in high abundance, potentially from sulfate photo-esterification and mineral flotation agent. Furthermore, microbial involvements in carbon cycling were revealed by DOM-microbe correlation network, but microbial contributions to the DOM pools decreased under acidic and metal stresses. These findings highlight abnormal carbon dynamics caused by AMD pollution and integrate DOM fate into pit lake biogeochemistry, thereby contributing to management and remediation.

Seasonal variations of dissolved organic matter chemistry in a semi-enclosed and eutrophic coastal bay in southeastern China: Implications for carbon cycling

As the widely distributed transition zones connecting the terrestrial and marine ecosystems, coastal bays have long water retention times to reconstruct the DOM pool affecting global carbon cycling. However, we still have a limited understanding of mechanisms shaping the carbon cycling of coastal bays at regional scales. To address this issue, we combined stable carbon isotopes, ultraviolet–visible absorption, fluorescence spectroscopy, and ultrahigh-resolution mass spectrometry to investigate the spatiotemporal chemistry of dissolved organic matter (DOM) in Xiangshan Bay, a semi-enclosed, eutrophic bay in southeastern China. The DOM in Xiangshan Bay was characterized by a higher degree of humification and aromaticity during winter, whereas the inverse signature during summer. Results showed that the significant seasonal variation of DOM chemistry was mainly modulated by intense primary productivity stimulated by nutrients during summer, and other factors including sediment pore water DOM release and photochemical reactions. Additionally, based on the microbial carbon pump (MCP) hypothesis, marine microorganisms may synthesize biologically recalcitrant DOM with millennia turnover time. The efficient MCP associated with reduced nutrients probably acted as a critical factor in Xiangshan Bay, leading to the formation of additional refractory molecular formulae (carboxyl-rich alicyclic molecules) and a more recalcitrant signature of DOM during winter. Incorporated with seven other coastal bays across China, it is revealed that anthropogenic inputs substantially contributed to Chinese coastal bays. Further studies need to better constrain the biogeochemical implications of these anthropogenic inputs. More importantly, decreasing the anthropogenic input of nutrients to coastal bays may enhance their functions in long-term carbon sequestration.

A Tropical Cocktail of Organic Matter Sources: Variability in Supraglacial and Glacier Outflow Dissolved Organic Matter Composition and Age Across the Ecuadorian Andes

AbstractThe biogeochemistry of rapidly retreating Andean glaciers is poorly understood, and Ecuadorian glacier dissolved organic matter (DOM) composition is unknown. This study examined molecular composition and carbon isotopes of DOM from supraglacial and outflow streams (n = 5 and 14, respectively) across five ice capped volcanoes in Ecuador. Compositional metrics were paired with streamwater isotope analyses (δ18O) to assess if outflow DOM composition was associated with regional precipitation gradients and thus an atmospheric origin of glacier DOM. Ecuadorian glacier outflows exported ancient, biolabile dissolved organic carbon (DOC), and DOM contained a high relative abundance (RA) of aliphatic and peptide‐like compounds (≥27%RA). Outflows were consistently more depleted in Δ14C‐DOC (i.e., older) compared to supraglacial streams (mean −195.2 and −61.3‰ respectively), perhaps due to integration of spatially heterogenous and variably aged DOM pools across the supraglacial environment, or incorporation of aged subglacial OM as runoff was routed to the outflow. Across Ecuador, Δ14C‐DOC enrichment was associated with decreased aromaticity of DOM, due to increased contributions of organic matter (OM) from microbial processes or atmospheric deposition of recently fixed and subsequently degraded OM (e.g., biomass burning byproducts). There was a regional gradient between glacier outflow DOM composition and streamwater δ18O, suggesting covariation between regional precipitation gradients and the DOM exported from glacier outflows. Ultimately, this highlights that atmospheric deposition may exert a control on glacier outflow DOM composition, suggesting regional air circulation patterns and precipitation sources in part determine the origins and quality of OM exported from glacier environments.

Solid phase extraction of ocean dissolved organic matter with PPL cartridges: efficiency and selectivity

Our current knowledge of the chemical composition of ocean dissolved organic matter (DOM) is limited, mainly because of its extreme molecular diversity, low concentration of individual compounds and the elevated ionic strength of ocean waters. As a result, many analytical methods require a previous extraction step. The efficiency and selectivity of the extraction method defines the representativeness of the extracted DOM fraction. Nowadays, the most widespread procedure for concentrating DOM is solid phase extraction (SPE) using styrene divinyl benzene polymer cartridges (PPL). Here, we investigate the effect of SPE-PPL on DOM elemental and optical properties to assess the efficiency and selectivity of this extraction method on water samples from the main intermediate and deep water masses of Arctic, Mediterranean and Antarctic origin present in the Cape Vert Frontal Zone (CVFZ, NW Africa). Furthermore, North and South Atlantic Central waters converge in this area and coastal DOM is injected by the giant upwelling filament of Cape Blanc. On one side, the colored fraction of DOM (CDOM) presented extraction efficiencies comparable to that of the bulk dissolved organic carbon (DOC), but decreased significantly with increasing wavelength, suggesting an affinity of PPL cartridges for low molecular weight organic compounds. While the protein-like fluorescent fraction of DOM (FDOM) was also extracted with the same efficiency than DOC, the extraction efficiency of the humic-like fraction was comparatively much higher. On the other side, dissolved organic nitrogen (DON) extraction efficiencies were about half that of DOC. These contrasting extraction efficiencies of the different DOM pools indicated that the extracts were enriched in N-poor, low molecular weight and recalcitrant DOM, therefore showing less variability than the corresponding bulk DOM. Furthermore, DOC, DON, CDOM and FDOM extracted were not homogeneous through the water column but displayed certain significant differences among water masses in both efficiency and selectivity.

Characterization of a Newly Available Coastal Marine Dissolved Organic Matter Reference Material (TRM-0522).

Recent methodological advances have greatly increased our ability to characterize aquatic dissolved organic matter (DOM) using high-resolution instrumentation, including nuclear magnetic resonance (NMR) and mass spectrometry (HRMS). Reliable DOM reference materials are required for further method development and data set alignment but do not currently exist for the marine environment. This presents a major limitation for marine biogeochemistry and related fields, including natural product discovery. To fill this resource gap, we have prepared a coastal marine DOM reference material (TRM-0522) from 45 m deep seawater obtained ∼1 km offshore of Sweden's west coast. Over 3000 molecular formulas were assigned by direct infusion HRMS, confirming sample diversity, and the distribution of formulas in van Krevelen space was typical for a marine sample, with the majority of formulas in the region H/C 1-1.5 and O/C 0.3-0.7. The extracted DOM pool was more nitrogen (N)- and sulfur (S)-rich than a typical terrestrial reference material (SRFA). MZmine3 processing of ultrahigh-performance liquid chromatography (UPLC)-HRMS/MS data revealed 494 resolvable features (233 in negative mode; 261 in positive mode) over a wide range of retention times and masses. NMR data indicated low contributions from aromatic protons and, generally speaking, low lignin, humic, and fulvic substances associated with terrestrial samples. Instead, carboxylic-rich aliphatic molecules were the most abundant components, followed by carbohydrates and aliphatic functionalities. This is consistent with a very low specific UV absorbance SUVA254 value of 1.52 L mg C-1 m-1. When combined with comparisons with existing terrestrial reference materials (Suwannee River fulvic acid and Pony Lake fulvic acid), these results suggest that TRM-0522 is a useful and otherwise unavailable reference material for use in marine DOM biogeochemistry.

Dissolved organic matter defines microbial communities during initial soil formation after deglaciation

Ecosystem succession and pedogenesis reshuffle the composition and turnover of dissolved organic matter (DOM) and its interactions with soil microbiome. The changes of these connections are especially intensive during initial pedogenesis, e.g. in young post-glacial areas. The temporal succession and vertical development of DOM effects on microbial community structure remains elusive. Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), high-throughput sequencing, and molecular ecological networks, we characterized the molecular diversity of water-extractable DOM and identified its links to microbial communities in soil profiles along deglaciation chronosequence (12, 30, 40, 52, 80, and 120 years) in the southeastern Tibetan Plateau. Low-molecular-weight compound content decreased, whereas the mid- and high-molecular-weight compounds increased with succession age and soil depth. This was confirmed by the increase in double bond equivalents and averaged oxygen-to‑carbon ratios (O/C), and decrease in hydrogen-to‑carbon ratios (H/C), which reflect DOM accumulation and stabilization. Microbial community succession shifted towards the dominance of oligotrophic Acidobacteria and saprophytic Mortierellomycota, reflecting the increase of stable DOM components (H/C < 1.5 and wider O/C). Less DOM-bacterial positive networks during the succession reduced specialization of labile DOM production (such as lipid- and protein-like compounds), whereas more DOM-fungal negative networks increased specialization of stable DOM decomposition (such as tannin- and condensed aromatic-like compounds). Consequently, DOM stability is not intrinsic during initial pedogenesis: stable DOM compounds remain after fast bacterial utilization of labile DOM compounds, whereas fungi decompose slowly the remaining DOM pools.

Multiple spectral comparison of dissolved organic matter in the drainage basin of a reservoir in Northeast China: Implication for the interaction among organic matter, iron, and phosphorus

Dissolved organic matter (DOM) plays a major role in ecological systems, affecting the fate and transportation of iron (Fe) and phosphorus (P). To better understand the geochemical cycling of these components, soil and sediment samples were collected around a reservoir downstream of a typical temperate forest in Northeast China. The DOM fractions from these soils, river, and reservoir sediments were extracted and then characterized by spectroscopic techniques. Comparative characterization data showed that the DOM pool of the Xishan Reservoir was partly autochthonous and derived from runoff and deposition of material in terrestrial ecosystems upstream. The upper reaches of the reservoir had significantly lower total Fe (TFe) content in the DOM extracts than those found in the reservoir (p < 0.05). Within the DOM, TFe was correlated with the amino acid tryptophan (p < 0.01). There was also a strong positive correlation between total P (TP) concentrations in DOM and tyrosine (p < 0.01). Organic P (Po) comprised most of the DOM TP, and was related to dissolved organic carbon (DOC) content and the amino acid tyrosine (p < 0.01). The interaction among DOM, Fe, and P appears to be due to complexation with tryptophan (Fe) and tyrosine (P). This suggests that the formation of Fe-DOM-P would be produced more readily than DOM-Fe-P complexes under optimal conditions. The interaction among DOM, Fe, and P can promote the coordinated migration, transformation, and ultimate fate of components that are complex with DOM from riverine and reservoir ecosystems, ultimately leading to accumulation within a reservoir and transport to downstream regions when reservoir dams are released. Reservoir dams can effectively intercept DOM and minerals prevent its flow downstream; however, it is important to understand the co-cycling of DOM, Fe and P within reservoirs, downstream rivers, and ultimately oceans. The involvement of amino acid components of DOM, tyrosine and tryptophan, in DOM complexation is an issue that requires further study.

Fluorescent properties of marine phytoplankton exudates and lability to marine heterotrophic prokaryotes degradation

AbstractExudates by the diatom Phaeodactylum tricornutum were incubated with a natural community of marine heterotrophic prokaryotes for 24 d in order to investigate the link between the biological lability and the molecular weight, fluorescence, and polarity of phytoplankton dissolved organic matter (DOM). Dissolved organic carbon (DOC) removal, changes in fluorescence and in the heterotrophic prokaryote abundance were followed over time both in the total exudates and in the low‐ and high‐molecular‐weight fractions. To detect changes in the polarity of proteins, reverse‐phase high‐performance liquid chromatography (HPLC) was applied to the high‐molecular‐weight fraction. Our results indicate that freshly produced phytoplankton DOM exhibits a dynamic pattern of degradation that is accompanied by large changes in the growth efficiency of the bacterial community that are likely related to changes in DOM quality. Approximately 20% of high‐molecular‐weight DOM and 40% of fluorescence attributed to protein‐like DOM were degraded over the first days of the incubation indicating that protein‐like DOM is likely a labile component of phytoplankton exudates. In contrast, fluorescence measurements suggest that humic‐like substances are resistant to bacterial degradation over the 24 d of the experiment. Despite fluctuations in the short‐term rates of high‐molecular‐weight and low‐molecular‐weight DOM removal, the relative contributions of these fractions to DOM pool were similar in the fresh exudates and at the end of our incubation experiments.

The priming effects of plant leachates on dissolved organic matter degradation in water depend on leachate type and water stability

The modification of dissolved organic matter (DOM) degradation by plant carbon inputs represents a critical biogeochemical process that controls carbon dynamics. However, the priming effects (PEs) different plant tissues induce on the degradation of DOM pools with different stabilities remain unknown. In this study, PEs, induced by different tissue leachates of Phragmites australis, were evaluated via changes in DOM components and properties of both fresh and tidal water (with different stabilities). The results showed that DOM derived from different plant tissue leachates differed in composition and bioavailability. Inputs of tissue leachates induced PEs with different intensities and directions (negative or positive) on DOM degradation of fresh and tidal water. In fresh water, the PEs of leaf and root leachates were significantly higher than those of stem and rhizome leachates. The PE direction changed for DOM degradation between fresh and tidal water. The addition of leaf and root leachates tended to induce positive PEs on DOM degradation of fresh water, while resulting in negative PEs on DOM degradation of tidal water. Negative PEs for tidal water DOM may be due to preferential utilization of microbes, high salinity, and/or the promotion of exogenous DOM production from plant tissues. The results indicate that intensity and direction of PEs induced by plant leachates depend on both leachate type and water stability. The findings highlight the necessity to examine the nature of exogenous and native DOM when interpreting the interactive processes that regulate DOM degradation.

The removal of dissolved organic matter by marine sponges is a function of its composition and concentration: An in situ seasonal study of four Mediterranean species

Sponges are unique among metazoans in their ability to use dissolved organic matter (DOM), the largest pool of organic matter in the ocean, as a major food source. The effect of variations in DOM abundance and composition on its uptake by sponges has rarely been studied. We examined, in situ, the seasonal uptake of DOM by four sponges [2 species with high microbial abundance (HMA) and 2 with low microbial abundance (LMA)] in the northwestern Mediterranean. Dissolved organic carbon (DOC) showed a strong seasonality with 3-fold higher concentrations in fall-winter (256 ± 16 μmol L−1, mean ± SE) than in spring-summer (88 ± 3 μmol L−1). Dissolved organic nitrogen (DON) showed the opposite trend, with higher summer concentrations (8.9 ± 0.4 μmol L−1) and mean concentrations of 2.5–3.4 μmol L−1 in the other seasons. DOC removal by all sponge species increased linearly with its ambient concentration, but only above a DOC removal threshold that was threefold higher in fall-winter (198 μmol L−1) than in spring-summer (74 μmol L−1). All species showed a concentration-dependent DON removal, but LMA sponges removed more DON than HMA sponges. The DOC removal rate (normalized to sponge volume) was 2–3 times higher in fall-winter, when ambient DOC levels were high, than in spring-summer. Sponges efficiently removed clusters of the fluorescent DOM (FDOM) associated with protein-rich DOM, but not those associated with humic material. The clear threshold for DOC removal and the protein-like FDOM uptake pattern suggest that the quality and quantity of DOM control its removal and transformation by marine sponges. Our results indicate that marine sponges transform the composition of the coastal DOM pool, thereby affecting its fate. It is postulated that the DOM excreted by the sponges is more recalcitrant; consequently, sponge activity enhances carbon sequestration in benthic habitats in a similar fashion to that of the oceanic ‘microbial pump’.

Accumulation of dissolved organic matter in the transition from fresh to aged seasonal snow in an industrial city in NE China.

Dissolved organic matter (DOM) plays a significant role in the reduction of snow albedo and the acceleration of snowmelt, but its accumulation in snow remains poorly understood. This study investigated the accumulation of DOM in seasonal snow including its accumulation rate, molecular characteristics, and biological and chemical processing. Sixteen snow samples of both fresh and aged snow were collected at one-day interval in Changchun, a typical industrial city in NE China. The snow DOM contents increased linearly with accumulation time at a rate of 30.3 μg L-1 d-1. The optical properties, including fluorescence intensity and optical absorption coefficient, of snowmelt increased exponentially with time owing to the rapid accumulation of terrestrial humic-like fluorophores through snow-soil exchange and deposition of soil-derived substances. Fourier transform-ion cyclotron resonance-mass spectrometry highlighted the properties of DOM at a molecular level, indicating that compounds derived from underlying soil and vascular plants make the largest contribution to DOM. Microbe-derived compounds contribute 35.5 % to the DOM pool. Degrees of saturation and oxidation increase slightly after accumulation, with the impacts of photo- and bio-chemistry on DOM molecules being non-negligible. This study provides a new perspective concerning the accumulation and fate of organic contaminants in snow ecosystems.

Variation in riverine dissolved organic matter (DOM) optical quality during snowmelt- and rainfall-driven events in a forested wetland watershed

The spring snowmelt period is an important time for organic matter mobilization and export in peatland and wetland dominated systems. The hydrologic flowpaths that transport melting snow and soil water to surface waters can vary over relatively short time periods (several days to weeks) depending on the timing and rate of melt and the occurrence of freeze–thaw cycles and rain-on-snow events. Dynamic flowpaths can alter dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) optical properties in surface waters due to differential melt and mobilization of snow, ice, and soil waters. In 2018 and 2020, we measured the DOC concentration and DOM optical properties in a woody wetland-dominated watershed in northern Minnesota during spring snowmelt (2018 & 2020) and early growing season (2018 only) events. We examined patterns in riverine DOC, DOM, and hydrochemical tracers across event hydrographs to infer changes in hydrologic connectivity across this seasonal transition. Measurements of riverine DOM optical properties, calcium concentration, and water isotopic composition indicated that DOM shifted from a primarily groundwater source and more fresh character during the dormant season to a more shallow wetland soil source and recalcitrant character during the early growing season. Seasonal variations in DOM optical properties and highly complex hysteresis patterns of DOM indices (e.g., Fluorescence Index, Humification Index, β/α, E250:E365, and the fluorescence peak ratio T:C) during individual events may reflect spatial heterogeneity in soil thawing throughout the watershed and temporal variations in water table elevation, affecting flowpath depths and connectivity to DOM pools within the soil profile. This study provides an important seasonal and event-scale baseline against which to evaluate potential changes in riverine DOM quantity and quality associated with predicted changes in climate and snow cover in organic matter-rich ecosystems.

Origins and transformations of terrigenous dissolved organic matter in a transgressive coastal system

Hydrogeological, geochemical, and optical approaches were combined to explore how a transgressive coastal system, where 2-kyr-old organic matter is preserved, contributes to dissolved organic matter (DOM) and dissolved organic carbon (DOC) flux to coastal waters. The sand spit system, located on the St. Lawrence shore (Chaleur Bay, Québec, Canada), was mainly composed of saline groundwater from seawater recirculation, and we estimated submarine groundwater discharge (SGD) velocities fluctuating from 0 during the rising tide to 106 cm d−1 during the ebb tide. The radon-222 (222Rn) activities revealed the presence of two distinct water masses in the spit: a surface recirculated saline groundwater cell with a short residence time and low or no 222Rn activity, and a deeper recirculated saline groundwater with a longer residence time, from a few hours to a few days, and thus exhibited higher 222Rn activities. At the falling tide, the upward flow of this 222Rn-rich groundwater contributes to volumetric discharges ranging from ∼6 to ∼17 m3 d−1, corresponding between 10 and 30% of the total SGD. Both DOC and DOM were produced in the subsurface. However, despite the concentrations being high in the discharge zone, the DOC export remained weak, with mean fluxes of 60.7 (±15.5) and 55.8 (±36.5) mol C yr− 1 in 2018 and 2019, respectively. While surface recirculated saline groundwater dominates the SGD, absorbance and fluorescent indices indicated a strong terrestrial character in the DOM pool likely originated from material sources with a high degree of humification. An EEM-PARAFAC model reveals the dominance of humic-like components of high molecular weights and the occurrence of degraded material, whatever the tidal regime. When the residence time of the groundwater was longer, with higher radon activities, protein-like components with lower molecular weights were microbially produced, probably enhancing the bioavailability of the DOM exported to surrounding seawater. Our results suggest that transgressive coastal systems can be a hot spot for terrigenous DOM transformation which could affect the optical and chemical properties of coastal waters.

A study of peatland-derived dissolved organic matter from headstream to sea using multiple analytical tools.

The River Thurso, North Scotland, receives substantial terrestrial deliveries of dissolved organic matter (DOM) leached from Europe's most extensive blanket bogs. The relatively short distance between peatlands and coastal ocean offers potential for research to investigate source-to-sea processing of terrigenous dissolved organic carbon (DOC). Here, we determined DOC concentrations in the bulk (< 0.4μm), truly dissolved (< 5kDa), and colloidal fraction (5kDa - 0.4μm) as well as DOM absorbance and fluorescence spectra during two river catchment surveys and two corresponding coastal plume surveys, in early spring (1st sampling period) and late spring (2nd sampling period). DOC concentrations ranged from 79 to 3799μM in early spring and from 115 to 5126μM in late spring. DOM exhibited conservative mixing across the plume in both surveys, but the plume extended further offshore in the second survey due to a pulse of freshwater caused by recent rainfall. Fluorescence excitation-emission matrices (EEMs) and fluorescence indices revealed that the flushed DOM was humic-like, recently synthesized DOM. Coupled with C/N ratio analyses and molecular weight fractionation, the fluorescence indices also provided evidence for the gradual altering of DOM characteristics along the bog - headstream - loch - river continuum. The same analytical tools revealed that seasonal variations occurred within the DOM pool of marine origin, i.e., greater abundance of low-molecular weight bacterial or algal DOM in the late spring survey. The time scale of such variations relative to the flushing time of water through the aquatic continuum should be taken into account when interpreting the DOM property-salinity distributions of major river plumes.

Seasonally variable interactions between dissolved organic matter and mineral particles in an agricultural river

Streams and rivers form an important link in the global carbon cycle by transporting and transforming large amounts of carbon imported from terrestrial ecosystems to the oceans. Since streams in agricultural areas often experience increased concentrations of suspended mineral particles from soil erosion, they are important sites where dissolved organic carbon (DOC) may be adsorbed to particles and retained in the sediment. As the extent of adsorption varies with the molecular composition of dissolved organic matter (DOM), which is seasonally variable, we expect also the fraction of organic material that adsorbs to mineral particles to fluctuate over time. We sampled the agriculturally impacted River Fyrisån (Sweden) monthly during 1 year, and measured DOC concentration and DOM composition based on several optical properties. At each sampling occasion, we estimated the potential for adsorption by exposing the samples to a reference clay. The potential for adsorption was greatest when riverine DOM had the most terrestrial character, as this fraction of the DOM pool was selectively adsorbed to clay surfaces. The extent of adsorption was negatively related to the concentration of base cations, most notably calcium. We suggest that the observed relationships between the potential for adsorption, DOM composition and base cations are linked by discharge. A bioavailability test at one sampling occasion suggested that DOM remaining after exposure to clay particles was more biodegradable. This implies that adsorption may alter the degradation potential of DOM remaining in solution, which could have far reaching effects on the fate of organic carbon.

Phytoplankton assemblage shifts with dissolved organic matter properties: A functional perspective for sentinel lakes

Abstract Climate change is altering both dissolved organic matter (DOM) properties and phytoplankton dynamics in lakes. High‐altitude lakes are good sentinels of global change. However, their value as sentinels depends on how well we understand their functioning. This study analysed physical, chemical and biological data during three pre‐defined periods running from complete ice‐cover right to the end of the ice‐free season, in a high‐altitude oligotrophic lake. A functional approach was used to assess how relationships between DOM and the planktonic community vary over time. Phytoplankton functional change was found to occur with shifts in DOM. During the ice‐influenced period, from February to the end of the thaw, the phytoplankton community was dominated by small autotrophs and mixotrophic flagellates and DOM from sediment and terrestrial origin dominated the DOM pool of the lake. Phytoplankton diversity and richness increased during the post‐snowmelt overturn period, when terrestrial DOM dominated the DOM pool. Finally, large siliceous autotrophs, competitive under low nitrogen concentrations and high temperature, dominated almost exclusively during the late summer period (LSP). Increased phytoplankton biomass meant that phytoplankton‐derived DOM was dominant during the LSP. These phenological changes in the phytoplankton community resulted in functional shifts at the base of the food web. Based on the relationships between the variables in the study, it can be deduced that the nature of the relationship between phytoplankton and bacteria progressively shifted from strong top‐down control exerted by phytoplankton over bacteria towards predominantly bottom‐up control at the end of the ice‐free season. Synthesis. Using a field survey starting from complete ice‐cover and lasting right to the end of the ice‐free season in a sentinel lake, we show seasonal shifts in the link between DOM properties and plankton community traits. Differences in environmental conditions and DOM origin explained variation in phytoplankton community structure and function, pointing to seasonal shifts in microbial food web interactions. Our research suggests that, under climate change, the balance between mixotrophy and autotrophy and the control exerted by phytoplankton over bacterioplankton will change in oligotrophic high‐altitude lakes with large, vegetated catchments.

Characterization of dissolved black carbon and its binding behaviors to ceftazidime and diclofenac pharmaceuticals: Employing the molecular weight fractionation

As the ubiquitous component of the aquatic environment, dissolved organic matter (DOM) readily bind with residual pharmaceutical contaminants (PCs) and influence their environmental behaviors. However, the binding mechanisms between dissolved black carbon (DBC), a vital part of the natural DOM pool, and PCs were poorly researched. In this study, the bulk DBC was divided into four fractions in molecular weight (MW) via an ultrafiltration system, and the properties of DBC and their binding interaction with two kinds of typical PCs (ceftazidime (CAZ) and diclofenac (DCF)) were explored concretely. The results showed that low MW component was the main contributor to bulk DBC, and the aromaticity increased with the increase of MW. The categories of chemical structures and fluorescent substances in different MW DBC were similar. Multispectral techniques showed that the oxygen-enriched compounds in DBC had the higher affinity to CAZ/DCF. The –NH–, –COOH, –NH2 groups in CAZ molecules appeared to form the hydrogen bond with DBC. Fluorescence quenching experiments were analyzed, and the binding mechanisms were specifically expounded from the thermodynamic perspective. The fluorophore of fulvic acid-like compounds (FA) were quenched by both static and dynamic quenching mechanisms, while only static quenching occurred for humic acid-like compounds (HA). For bulk DBC, the hydrogen bond and van der Waals force were the major forces in the HA-CAZ system, while the hydrophobic force made the primary contribution to the HA-DCF system, which might be ascribed to the higher hydrophobic nature of DCF. Notably, with the increase of HA MW, the main binding mode of HA-CAZ/DCF changed from hydrophobic force to hydrogen bond and van der Waals force gradually, which also directly proved that various noncovalent interactions co-driven the binding processes. Our findings are beneficial to better assess the fate of DBC and PCs and the corresponding complexes in the aquatic environment.

Contrasting seasons and land uses alter riverine dissolved organic matter composition

Different sources and fates control riverine dissolved organic matter (DOM) composition in catchments of contrasting land use and climate. However, assessing the changes in DOM composition together with nutrient forms along rivers exposed to these gradients remains rare. Here we quantified the spatial and temporal patterns in DOM components and nutrient forms along the mainstem of a 5th order river through sequential forested, urban, and agricultural reaches during low flow moments in summer and winter, and two contrasting springs, one with a historically rare flooding event. There were widespread abrupt shifts in the composition of DOM in low flow seasons that coincided with changes in land use whose sources could be inferred by endmember samples and nutrient changes. DOM pools considered bio- (microbial-like) and photo-labile (Peak C), along with reactive nutrient forms (ammonium, dissolved phosphorus) tended to accumulate during the winter compared to summer. This implied higher processing during summer as microbial-humic-like DOM and nitrate dominated. DOM composition remained relatively stable under typical spring high flow conditions with reduced retention time and processing, but major shifts were observed during an extreme flood year, pointing to unusual loadings of highly labile DOM. Overall, we found that despite relatively small changes in the quantity of DOM along the axis of flow in this north temperate river, there were major spatial and temporal shifts in its composition, associated with different nutrient forms. These changes inferred contrasting loading and processing potential depending on land use and seasonal patterns in temperature and hydrology.

Viruses direct carbon cycling in lake sediments under global change

Global change is altering the vast amount of carbon cycled by microbes between land and freshwater, but how viruses mediate this process is poorly understood. Here, we show that viruses direct carbon cycling in lake sediments, and these impacts intensify with future changes in water clarity and terrestrial organic matter (tOM) inputs. Using experimental tOM gradients within sediments of a clear and a dark boreal lake, we identified 156 viral operational taxonomic units (vOTUs), of which 21% strongly increased with abundances of key bacteria and archaea, identified via metagenome-assembled genomes (MAGs). MAGs included the most abundant prokaryotes, which were themselves associated with dissolved organic matter (DOM) composition and greenhouse gas (GHG) concentrations. Increased abundances of virus-like particles were separately associated with reduced bacterial metabolism and with shifts in DOM toward amino sugars, likely released by cell lysis rather than higher molecular mass compounds accumulating from reduced tOM degradation. An additional 9.6% of vOTUs harbored auxiliary metabolic genes associated with DOM and GHGs. Taken together, these different effects on host dynamics and metabolism can explain why abundances of vOTUs rather than MAGs were better overall predictors of carbon cycling. Future increases in tOM quantity, but not quality, will change viral composition and function with consequences for DOM pools. Given their importance, viruses must now be explicitly considered in efforts to understand and predict the freshwater carbon cycle and its future under global environmental change.

Changes in dissolved organic pool and regulation of associated nutrients during green tides: A case study of Ulva prolifera bloom in the southern Yellow Sea.

To explore the spatial and temporal distributions of dissolved organic matter (DOM) affected by Ulva prolifera bloom and the regulation of DOM released by the addition of nitrate (NO3-N) nutrients, the variation in DOM was analyzed by three cruises in the coastal waters of the southern Yellow Sea (SYS) off Qingdao during a 2018 U. prolifera bloom. A mesocosm experiment was conducted during the decay of U. prolifera. The U. prolifera bloom resulted in higher concentrations of the bulk DOM except for total dissolved amino acids (TDAA), compared with levels recorded during non-bloom periods. Three different fluorescent substances of chromophoric dissolved organic matter (CDOM) were identified, which were also affected by the bloom. Concentrations of total dissolved carbohydrates (TCHO), monosaccharides (MCHO), and polysaccharides (PCHO) peaked during the late bloom, with average levels of 49.78 ± 9.73, 14.23 ± 3.32, and 35.55 ± 9.19 μmol C L-1, respectively, accompanied by an increase of DOC, indicating release of DOM with U. prolifera decay. In contrast, the concentrations of CDOM and TDAA declined during the late bloom, with averages of 0.10 ± 0.10 m-1 and 1.16 ± 0.82 μmol L-1, suggesting that the yields of CDOM and TDAA were regulated by the U. prolifera bloom and other mechanism. These findings were confirmed by the mesocosm experiment. DOM production was obviously improved by NO3-N addition except for TDAA and CDOM. The release of protein-like CDOM was primarily regulated by NO3-N nutrients, however, it was not obvious with humic-like CDOM. Our findings provide evidence supporting the role of U. prolifera blooms in the coastal DOM pool and its significant effect on the marine carbon cycle.