Dissolved Organic Matter Composition Research Articles

Removing DOM from chloride modified hydrochar could improve Cu2+ adsorption capacity from aqueous solution

The behavior and composition of hydrochar-based dissolved organic matter (DOM) would affect the efficiency of copper (Cu) removal from wastewater through adsorption. In this study, the reed was hydrolyzed in the presence of feedwater with and without ZnCl2, FeCl3, and SnCl4 to produce pristine hydrochars (PHCs), which were named H2O-HC, ZnCl2-HC, FeCl3-HC, and SnCl4-HC. After removal of DOM, washed hydrochars (WHCs) were obtained, labelled as W–H2O-HC, W-ZnCl2-HC, W-FeCl3-HC, and W-SnCl4-HC. The release dynamics of DOM from PHCs were analyzed, and the adsorption behaviors of Cu2+ on both PHCs and WHCs were investigated. The results showed that chloride-modifications were beneficial for the porosity, specific surface area (SSA), and functional groups of WHCs. Meanwhile, the quantity of hydrochar-based DOM was significantly affected by chloride-modifications. In particular, the relative contents of Ar–P and Fa-L in the DOM released from hydrochars varied with time and modification. Furthermore, the Qe of Cu2+ adsorption on WHCs followed the order of W-SnCl4-HC > W-FeCl3-HC > W-ZnCl2-HC > W–H2O-HC at 15 °C. Compared to PHCs, the adsorption capacity of Cu2+ on WHCs was improved by 7.15–119.77% at the temperature of 35 °C. Simultaneously, the adsorption capacity of Cu2+ in WHCs showed a significant correlation with the SSA via physical adsorption (P < 0.05). Moreover, XPS analysis revealed that Cu2+ adsorption also occurred via complexation and chelation through newly formed Cu–O group between W-SnCl4-HC and Cu2+. Notably, the increase of Cu2+ adsorption in WHCs was significantly correlated with the release of Fa-L and Ar–P from PHCs (P < 0.05). This study found that the content and composition of hydrochar-based DOM could be a major driving factor for Cu2+ adsorption.

Effects of dissolved organic matter removal and molecular transformation in different water treatment processes on formation of disinfection byproducts

Alterations in molecular composition of dissolved organic matter (DOM) during water treatments can influence the composition and toxicity of disinfection by-products (DBPs) in subsequent chlorination disinfection process. In this study, the impacts of DOM composition after various water treatment techniques (coagulation, adsorption, nanofiltration, biological aerated filter (BAF), and their integrated processes) on the generation mechanisms of DBPs were comprehensively explored by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in combination with GC–MS and LC-MS analysis. The results indicated that coagulation preferentially removed unsaturated (low H/C) and oxidized (high O/C) compounds, whereas adsorption was prone to remove the reduced (low O/C) component that was more reactive with chlorine, leading to lower yields (μg DBP/mg DOC) of trihalomethanes (THMs) and haloacetic acids (HAAs) during subsequent chlorination. The coagulation-adsorption technique exhibited a relatively high removal of both known and unknown DBPs, demonstrating that coagulation and adsorption were complementary for DOM removal at the molecular level. Nanofiltration selectively removed molecules with relatively high O/C, however, those with very low O/C that were more reactive with chlorine could pass through the nanofiltration membrane, resulting in the highest yields of THMs and HAAs. Although BAF was inefficient in removing DBPs precursors, it could convert molecules with low degree of oxidation and unsaturation into highly oxidized and unsaturated ones, thereby significantly enhancing the removal of DBPs precursors in the subsequent coagulation-adsorption process. These findings are instrumental in developing and selecting more effective techniques to minimize the formation of DBPs in water treatment.

Molecular and optical signatures of photochemical transformation of dissolved organic matter: Nonnegligible role of suspended particulate matter in urban river

Natural dissolved organic matter (DOM) is one of the Earth's dynamic carbon pools and a key intermediate in the global carbon cycle. Photochemical processes potentially affect DOM composition and activity in surface water. Suspended particulate matter (SPM) is the integral component of slow-moving rivers, and holds the potential for photochemical reactivity. To further investigate the influence of SPM on DOM photochemical transformation, this study conducted experiments comparing samples with and without SPM irradiated under simulated sunlight. Surface water samples from slow-moving urban rivers were collected. DOM optical characteristics and molecular features obtained by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were investigated. Photolabile DOM was enriched in unsaturated and highly aromatic terrestrial substances. Photoproduced DOM had low aromaticity and was dominated by saturated aliphatics, protein-like substances, and carbohydrates. Study results indicated that the presence of SPM had a nonnegligible impact on the molecular traits of DOM, such as composition, molecular diversity, photolability, and bioavailability during photochemical reactions. In the environment affected by SPM, molecules containing heteroatoms exhibit higher photosensitivity. SPM promotes the photochemical transformation of a wider range of chemical types of photolabile DOM, particularly nitrogen-containing compounds. This study provides an essential insight into the more precise simulation of photochemical reactions of DOM influenced by SPM occurring in natural rivers, contributing to our understanding of the global carbon cycle from new theoretical perspectives.

Assessing spatiotemporal variability in the concentration and composition of dissolved organic matter and its impact on iron solubility in tropical freshwater systems through a machine learning approach

Dissolved organic matter (DOM) plays important roles not only in maintaining the productivity and functioning of aquatic ecosystems but also in the global carbon cycle, although the sources and biogeochemical functions of terrestrially derived DOM have not been fully elucidated, particularly in the tropics and subtropics. This study aimed to evaluate the factors influencing spatiotemporal variability in (i) the concentration and composition of DOM, including dissolved organic carbon (DOC), ultraviolet absorption coefficient at 254-nm wavelength (a254), and components identified by fluorescence excitation–emission matrix coupled with parallel factor analysis (EEM-PARAFAC), and (ii) the concentration of dissolved iron (DFe) across freshwater systems (rivers, forested streams, and dam reservoirs) on a tropical island (Ishigaki Island, Japan) based on the results of water quality monitoring at 2-month intervals over a 2-year period. Random forests (RF) machine learning algorithm was employed, with the catchment characteristics (land use, soil type) and water temperature as the predictor variables for DOM and the composition of DOM (EEM-PARAFAC components) and hydrochemistry (water temperature, pH, and concentrations of divalent cations) as the predictor variables for DFe. The RF models for DOC, a254, and three humic-like components exhibited excellent predictive performance, indicating that these DOM properties are not only seasonally variable but also strongly influenced by the compositions of land uses and soil types in the upstream watershed. Poorly drained riparian lowland soil (Gleyic Fluvisols) was identified as the most important catchment parameter that positively influences these DOM variables. The RF model also explained a large portion of the variation in DFe, while terrestrial humic-like components were the most important parameters, emphasizing their significance as organic ligands for iron. These results improve our understanding of the impacts of terrestrial DOM and iron loadings on tropical and subtropical coastal ecosystems as well as on regional and global carbon budgets.

Release of glucose from dissolved and mineral‐bound organic matter by enzymatic hydrolysis

AbstractSorption of dissolved organic matter (DOM) by poorly crystalline minerals during their formation may protect large amounts of carbon in soils from mineralization. We investigated the bioavailability of carbohydrates in DOM and after co‐precipitation with short‐range ordered aluminosilicates. Carbohydrates originated from soil solutions collected in situ at two depths of a Dystric Cambisol, and from litter extracts. Quantification of substrate‐specific degradability was achieved by the addition of β‐glucosidase at an optimal concentration and subsequent determination of glucose release. Depending on DOM composition, 0.6–41.4 mg g−1 C−1 of glucose was enzymatically released from dissolved carbohydrates. Co‐precipitated carbohydrates were partially accessible, resulting in a glucose release of 0.7–5.2 mg g−1 C−1. Restricted enzymatic depolymerization due to co‐precipitation may contribute to accumulation of easily degradable substrates in soils.

Hydrophilic Species Are the Most Biodegradable Components of Freshwater Dissolved Organic Matter.

Aquatic dissolved organic matter (DOM) is a crucial component of the global carbon cycle, and the extent to which DOM escapes mineralization is important for the transport of organic carbon from the continents to the ocean. DOM persistence strongly depends on its molecular properties, but little is known about which specific properties cause the continuum in reactivity among different dissolved molecules. We investigated how DOM fractions, separated according to their hydrophobicity, differ in biodegradability across three different inland water systems. We found a strong negative relationship between hydrophobicity and biodegradability, consistent for the three systems. The most hydrophilic fraction was poorly recovered by solid-phase extraction (SPE) (3-28% DOC recovery) and was thus selectively missed by mass spectrometry analysis during SPE. The change in DOM composition after incubation was very low according to SPE-ESI (electrospray ionization)-mass spectrometry (14% change, while replicates had 11% change), revealing that this method is sub-optimal to assess DOM biodegradability, regardless of fraction hydrophobicity. Our results demonstrate that SPE-ESI mass spectrometry does not detect the most hydrophilic and most biodegradable species. Hence, they question our current understanding of the relationships between DOM biodegradability and its molecular composition, which is built on the use of this method.

Impacts of autochthonous dissolved organic matter on the accumulation of methylmercury by phytoplankton and zooplankton in a eutrophic coastal ecosystem

The bioaccumulation of methylmercury (MeHg) within the pelagic food webs is a crucial determinant of the MeHg concentration in the organisms at higher trophic levels. Dissolved organic matter (DOM) is recognized for its influence on mercury (Hg) cycling in the aquatic environment because of its role in providing metabolic substrate for heterotrophic organism and serving as a strong ligand for MeHg. However, the impact of DOM on MeHg bioaccumulation in pelagic food chains remain controversial. Here, we explored MeHg bioaccumulation within a pelagic food web in China, in the eutrophic Bohai Sea and adjacent seas, covering a range of DOM concentrations and compositions. Our findings show that elevated concentrations of dissolved organic carbon (DOC) and phytoplankton biomass may contribute to a reduction in MeHg uptake by phytoplankton. Moreover, we observe that a higher level of autochthonous DOM in the water may result in more significant MeHg biomagnification in zooplankton. This can be explained by alterations in the structure of pelagic food webs and/or an increase in the direct consumption of DOM and particulate organic matter (POM) containing MeHg. Our study offers direct field monitoring evidence of dual roles played by DOM in regulating MeHg transfers from water to phytoplankton and zooplankton in coastal pelagic food webs. A thorough understanding of the intricate interactions is essential for a more comprehensive evaluation of ecological risks associated with MeHg exposure in coastal ecosystems.

Tracing the pathways by which flood duration impacts soil bacteria through soil properties and water-extractable dissolved organic matter: A soil column experiment

Soil microbial communities control biogeochemical processes, nutrient cycling, and organic carbon storage and release in wetlands, which are influenced by flooding. To predict soil nutrient function in wetland ecosystems, understanding the effect of flooding on soil biogeochemical cycling and energy flux, including soil properties, dissolved organic matter (DOM), and microbial communities is essential. This study investigated how different flood durations (1, 3, 8, 16, and 30 d) affect the interactions between physicochemical properties and bacterial communities in a river wetland. The DOM composition was measured using ultraviolet/visible spectrophotometry coupled with fluorescence spectroscopy, and the bacterial communities were identified using 16S rRNA sequencing. Simpson's diversity index varied from 0.92 to 0.94, indicating high bacterial diversity throughout the treatments; the highest and lowest bacterial diversities were found at 1 and 8 flooding days, respectively. The abundance of Desulturomonadales, Clostridiales, Bacteroidales, and Gaiellales was positively correlated with pH, electrical conductivity, water-extractable dissolved organic carbon (WEOC), and water-extractable total dissolved nitrogen (TDN) but negatively correlated with dissolved oxygen (DO) and soil organic matter (SOM), suggesting complex interactions among these factors in response to flooding. Structural equation model revealed that flooding directly increased TDN but indirectly increased WEOC through increasing soil pH; and directly decreased DO and SOM, leading to decreases in total protein-like fraction. Three significant pathways were identified, showing the impacts of flooding on bacterial diversity: (1) flood duration decreased DO, resulting in decreased bacterial diversity; (2) flood duration decreased SOM, leading to increased bacterial diversity; and (3) flood duration decreased DO and SOM, leading to increased bacterial diversity via decreased total protein-like fraction. This study indicated that prolonged flooding has both positive and negative impacts on bacterial diversity, depending on environmental factors. It highlights the importance of flooding in shaping soil bacterial communities, with implications for nutrient cycling and carbon storage in wetlands.

Advancing chemical lability assessments of organic matter using a synthesis of FT-ICR MS data across diverse environments and experiments

Patterns in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) data provide perspective on how organic matter (OM) character is shaped by natural processes. Here, we reevaluate and update the molecular lability boundary (MLB) approach to assess OM lability using a synthesis of FT-ICR MS data. Now included are MLBL (biolability) indices (i.e., the OM proportion of hydrogen-to-carbon ratios ≥ 1.5) of freshwaters, plant litter and biochar leachates, soils, and oils across the US, trends in OM and dissolved OM (DOM) lability from laboratory experiments examining isolated microbial, photochemical, and thermal degradation, and biogeochemical interpretations of biolability patterns across ecosystems influenced by human activity (e.g., fertilizer addition and agricultural management). Photo-processing increased the hydrogen saturation of OM, describing products of higher biolability (>MLBL values). DOM MLBL values decreased during microbial and thermal degradation processes. Laboratory experiments with isolated treatments showed larger changes in MLBL than exhibited by field samples from systems managed by humans. We interpret changes in N- and/or S-containing DOM above the MLB as the production or use of heterogeneously biolabile material. We interpret heterogeneously labile DOM composition to indicate autochthonous production, biomass growth, nutrient accumulation, or biological degradation in stream and soil samples. The most recent data in our synthesis suggest that the MLB approach should be reframed to provide lability indices for biotic, thermal, and photochemical processes. A broader perspective on lability provides a useful tool to decompose large, complex, and process-driven DOM data into more simple and informative indicators of trends in ecosystem form and function.

Molecular characterization of DOM: A full-scale case study of an advanced DWTP from Jintan, China

Since the variation of dissolved organic matter (DOM) is complicated in the aquatic environment and water treatment processes, adverse effects on human and water quality may be triggered. Nowadays, more drinking water treatment plants (DWTPs) begin using pretreatment and advanced treatment processes for removing refractory DOM to pursue high-quality drinking water, including preozonation, and combined ozonation and biological activated carbon (BAC) treatment. The combination of Fourier transform ion cyclotron resonance mass spectrometer and fluorescence spectroscopy could comprehensively unveil the DOM composition and characteristics in water treatments. This study aims to provide information on the molecular and spectroscopic characteristics of DOM in a DWTP with pretreatments and advanced treatment processes and summarize the removal efficiency of different treatment processes on DOM. Results showed that the removal rate of low-molecular-weight acids (LMW-acids) by BAC was as high as 98 %, reflecting the synergistic effect of ozonation/BAC unit. Pre-ozonation enhanced coagulation process had a good removal effect on CHONS. The diversity of DOM increased and the relative abundance of DOM intensity was reduced in preozonation/coagulation/sedimentation and O3/BAC units. The preozonation-coagulation/sedimentation unit reduced the aromaticity or unsaturation of DOM while the advanced treatment process had a higher removal rate of fluorescent components.

The migration and transformation processes of dissolved organic matter in rainwater- drip water- phreatic water of a typical karst spring catchment, in South China

The dissolved organic matter (DOM) migration and transformation in karst aquifer are of great significance for geochemical processes but remain poorly understood. In this study, we selected the Guilin Yaji Karst Experimental Site as a natural laboratory and collected rainwater (RW), drip water (DW), and phreatic water (PW) from September 2019 to September 2021 to study the DOM migration and transformation in a typical bare karst spring catchment. Hydrochemical and stable hydrogen and oxygen isotopes were applied to study the water cycle of the aquifer. Absorbance and fluorescence spectroscopy were used to characterize the DOM quality. Three-dimensional fluorescence combined with fluorescence regional integration was used for identifying the DOM compositions. The RW DOM had a weak humification degree and strong terrestrial characteristics. It was affected by the monsoon and showed higher humification in the dry season than in the wet season. The higher rainfall intensity had a dilution effect on the DOM fluorescence components levels when the rain intensity was>10 mm/day (not the threshold value). Continuous rain events led to the continuous dilution effect on RW DOM in the dry season, but such a phenomenon was not obvious in the wet season. The DW DOM inherited weak humification characteristic that was close to RW DOM but was altered by the hydrogeology conditions and plant root and microbial activities intensities of the upper vadose zone. The higher humification degree of PW DOM may be related to the higher degree of microbial processing caused by the longer groundwater residence time. The humic and hydrophobic degree of DOM for fissure PW was weaker than that for conduit PW because fissure media provides a superior condition for DOM to adsorb to the mineral surface. In the wet season, when the preferential channel of the aquifer was activated, the PW DOM showed similar characteristics to RW DOM, indicating a significant hydrologic effect on the PW DOM. Our results provide a valuable reference to advance understanding of DOM behavior in karst aquifer and highlight the importance of hydrogeologic conditions and hydrologic processes on DOM migration and transformation.

Photochemical transformation of terrestrial dissolved organic matter derived from multiple sources in tropical plantations

Photochemical transformation is a critical geochemical fate of terrestrial dissolved organic matter (DOM) that drains from forests to downstream water bodies. However, the photo-degradability and photo-transformation of terrestrial DOM from various sources are yet to be explored. Here, optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry were used to analyze photochemical change in spectroscopic and molecular-level signatures of DOM derived from throughfall, stemflow, runoff, and soil pore water at depths of 20 and 40 cm in tropical typical Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) plantations. The dissolved organic carbon concentrations were generally higher in the EU plantation than in the AA plantation and higher in stemflow than in other sample types. The greatest DOM degradation occurred in soil pore water and the least degradation occurred in stemflow. Similarly, the common photo-transformation of DOM, including decreased aromaticity and unsaturation degrees, was greatest for soil pore water and the least for stemflow DOM. Most of the photochemical products were high H/C and O/C molecules for throughfall, stemflow, and pore water, but high H/C and low O/C molecules for runoff because of strong decarboxylation. These findings reveal the highly varied molecular composition and photo-degradability of DOM from different terrestrial sources and highlight that stemflow is an important contributor to photo-resistant DOM among terrestrial DOM sources.

Soil microbial community, dissolved organic matter and nutrient cycling interactions change along an elevation gradient in subtropical China

To identify possible dominating processes involved in soil microbial community assembly, dissolved organic matter (DOM) and multi-nutrient cycling (MNC) interactions and contribute to understanding of climate change effects on these important cycles, we investigated the interaction of soil chemistry, DOM components and microbial communities in five vegetation zones - ranging from evergreen broad-leaved forest to alpine meadow - along an elevation gradient of 290–1960 m in the Wuyi Mountains, Fujian Province, China. Soil DOM composition and microbial community assembly were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and Illumina MiSeq high-throughput sequencing, respectively. Sloan's neutral model and the modified stochasticity ratio were used to infer community assembly processes. Key microbial drivers of the soil MNC index were identified from partial least squares path models. Our results showed that soil DOM composition is closely related to the vegetation types along an elevation gradient, the structure and composition of the microbial community, and soil nutrient status. Overall, values of the double bond equivalent (DBE), modified aromaticity index (AImod) increased, and H/C ratio and molecular lability boundary (MLBL) percentage decreased with elevation. Lignins/CRAM-like structures compounds dominated soil DOM in each vegetation type and its relative abundance decreased with elevation. Aliphatic/protein and lipids components also decreased, but the relative abundance of aromatic structures and tannin increased with elevation. The alpha diversity index of soil bacteria gradually decreased with elevation, with deterministic processes dominating the microbial community assembly in the highest elevation zone. Bacterial communities were conducive to the decomposition of labile degradable DOM compounds (H/C ≥ 1.5) at low elevation. In the cooler and wetter conditions at higher-elevation sites the relative abundance of potentially resistant soil DOM components (H/C < 1.5) gradually increased. Microbial community diversity and composition were important predictors of potential soil nutrient cycling. Although higher elevation sites have higher nutrient cycling potential, soil DOM was assessed to be a more stable carbon store, with apparent lower lability and bioavailability than at lower elevation sites. Overall, this study increases understanding of the potential linkage between soil microbial community, multiple nutrient cycling and DOM fate in subtropical mountain ecosystems that can help predict the effect of climate change on soil carbon sequestration and thus inform ecosystem management.

Linking the source, molecular composition, and reactivity of dissolved organic matter in major rivers across the pearl river delta

Dissolved organic matter (DOM) constitutes a substantial portion of the global carbon cycle and serves as the largest reservoir of reactive carbon on Earth. Studying DOM's sources, molecular compositions, and reactivity is central to understanding the fate and environmental effects of organic carbon, yet, links between DOM composition and reactivity remain to be established. In this study, stable carbon isotopic composition and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) were combined with degradation experiments to comprehensively study DOM's properties in the major rivers across the Pearl River Delta (PRD) in South China. The increasing δ13C-DOC along the Pearl River at the PRD suggests that terrestrial DOM from upstream was gradually diluted by local processes. Although affected by different interaction patterns of multiple sources, a major component of DOM, enriched in CHO and lignins, is molecularly indistinguishable in different rivers (molecular Bray-Curtis dissimilarity of 15%–31%). The photo-degradation removed 34% (PDOC%) of initial DOM within 14 days, twice that by biodegradation (BDOC%). In contrast to earlier research, the distinctive integration of molecular characterization with DOM degradation kinetics indicates that both regional (common shared molecules) and local (partial shared and unique molecules) derived DOM play a collective role in governing the bio-degradation kinetics of DOM, while the photo-degradation kinetics of DOM are barely correlated to molecular composition. The thorough investigation of DOM molecular composition and its correlation with bulk DOM properties in the PRD region, though requiring further investigation into its mechanisms, offers valuable insights into the role of riverine DOM in coastal hypoxia and the global carbon cycle.

Molecular characterization of dissolved organic matter in urban stormwater pond and municipal wastewater discharges transformed by the Florida red tide dinoflagellate Karenia brevis

Karenia brevis blooms occur almost annually in southwest Florida, imposing significant ecological and human health impacts. Currently, 13 nutrient sources have been identified supporting blooms, including nearshore anthropogenic inputs such as stormwater and wastewater outflows. A 21-day bioassay was performed, where K. brevis cultures were inoculated with water sourced from three stormwater ponds along an age gradient (14, 18, and 34 yrs.) and one municipal wastewater effluent sample, with the aim of identifying biomolecular classes and transformations of dissolved organic matter (DOM) compounds used by K. brevis. All sample types supported K. brevis growth and showed compositional changes in their respective DOM pools. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) catalogued the molecular composition of DOM and identified specific compound classes that were biodegraded. Results showed that K. brevis utilized species across a wide range of compositions that correspond to amino sugars, humic, and lignin-like biomolecular classes. The municipal wastewater and the youngest stormwater pond (SWP 14) effluent contained the largest pools of labile DOM compounds which were bioavailable to K. brevis, which indicates younger stormwater pond effluents may be as ecologically important as wastewater effluents to blooms. Conversely, generation of DOM compounds of greater complexity and a wide range of aromaticity was observed with the older (SWP 18 and SWP 34) stormwater pond treatments. These data confirm the potential for stormwater ponds and/or wastewater to contribute nutrients which can potentially support K. brevis blooms, revealing the need for improved nutrient retention strategies to protect coastal waters from the potential ill effects of urban effluent.

Plant invasion strengthens the linkages between dissolved organic matter composition and the microbial community in coastal wetland soils

Exotic plant invasion dramatically alters soil carbon transformation and shapes the structure and diversity of the microbial community in coastal wetlands. However, it remains unclear how the dissolved organic matter (DOM) optical components and chemical composition and their linkages with soil microorganisms change with increasing time since invasion. Here, we explored the impacts of Spartina alterniflora (S. alterniflora) invasion on DOM compositions and relationships with microbial communities along a short-term chronosequence (i.e., 3-, 5-,10-, and 15-year) in the Yellow River Delta. The results showed an increase in the soil water content, soil organic carbon, dissolved organic carbon, and C/N ratio along the invasion chronosequence, whereas bulk density showed the opposite trend. Moreover, invaded wetland soils had a higher abundance of total phospholipid fatty acids, gram-positive bacteria (G + ), gram-negative bacteria (G-), and fungi than tidal flat soils. The DOM in invaded wetland soils was mainly composed of endogenous protein-like components, and the proportion of autochthonous DOM increased over invasion time. The DOM was characterized by low molecular weight, low aromatic content, and low hydrophobic compounds in S. alterniflora wetland soils, whereas in tidal flat soils, the humification degree and DOM molecular weight were higher. The soil microbial communities exhibited stronger linkages with various environmental variables and DOM fluorescence components in invaded wetlands based on redundancy analysis (RDA) and correlation analysis using Mantel tests. Structural Equation Model (SEM) revealed that the soil microbial abundance and community structure induced by increasing S. alterniflora invasion time were the most important drivers of DOM content and chemical composition. The findings of this work provide a better understanding of labile carbon pool turnover in coastal wetlands invaded by S. alterniflora.

Spatial and seasonal controls on dissolved organic matter composition in shallow aquifers under the rapidly developing city of Patna, India

The distribution and composition of dissolved organic matter (DOM) affects numerous (bio)geochemical processes in environmental matrices including groundwater. This study reports the spatial and seasonal controls on the distribution of groundwater DOM under the rapidly developing city of Patna, Bihar (India). Major DOM constituents were determined from river and groundwater samples taken in both pre- and post-monsoon seasons in 2019, using excitation-emission matrix (EEM) fluorescence spectroscopy. We compared aqueous fluorescent DOM (fDOM) composition to satellite-derived land use data across the field area, testing the hypothesis that the composition of groundwater DOM, and particularly the components associated with surface-derived ingress, may be controlled, in part, by land use. In the pre-monsoon season, the prominence of tryptophan-like components likely generated from recent biological activity overwhelmed the humic-like and tyrosine-like fluorescence signals. Evidence from fluorescence data suggest groundwater in the post-monsoon season is composed of predominantly i) plant-derived matter and ii) anthropogenically influenced DOM (e.g. tryptophan-like components). Organic tracers, as well as Eh and Cl−, suggest monsoonal events mobilise surface-derived material from the unsaturated zone, causing dissolved organic carbon (DOC) of more microbial nature to infiltrate to >100 m depth. A correlation between higher protein:humic-like fluorescence and lower vegetation index (NDVI), determined from satellite-based land use data, in the post-monsoon season, indicates the ingression of wastewater-derived OM in groundwater under the urban area. Attenuated protein:humic-like fluorescence in groundwater close to the river points towards the mixing of groundwater and river water. This ingress of surface-derived OM is plausibly exacerbated by intensive groundwater pumping under these areas. Our approach to link the composition of aqueous organics with land use could easily be adapted for similar hydrogeochemical settings to determine the factors controlling groundwater DOM composition in various contexts.

Analyzing the Pollution Sources and Mechanisms of Urban Rivers Based on Identifying the Molecular Signature of Dissolved Organic Matter

Dissolved organic matter (DOM) is an essential component of river pollutants. Under the new situation of black water treatment in urban areas of China, in view of the widespread problem of unclear sources of multiple pollutants, further analysis of DOM components in urban rivers from the molecular level is a key link to deeply explore the sources, causes, and mechanism of river pollution so as to achieve efficient management. In this study, the urban rivers in the central city were selected as the research object, and a total of five rivers were selected that were seriously affected by the discharge sewage of four combined and separated sewer systems, respectively. Using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), this study identified the molecular formulae and analyzed the elemental composition and compound groups of DOM in water and sediment samples at each site in dry and wet weather. The results showed that:①although CHO molecules and lignins were the main compounds in the urban river DOM, the high proportion of lipids, proteins, and heteroatomic compounds (especially CHOS molecules) revealed the anthropogenic pollution in rivers, which also led to the increase in DOC, TN, and NH+4-N. ②Surfactants such as C17H28O3S and C18H30O3S were ubiquitous in all urban rivers, which could be used as markers of domestic wastewater pollution. ③In wet weather, the rainfall inputs, storm runoffs, and hydraulic disturbance jointly led to the increase in the proportion of CHO molecules and lignin compounds; the decrease in proteins and lipids; the rise of DOC, TN, and NH+4-N concentrations in river water; and the decrease in DOC, TN, and NH+4-N concentrations and proteins and lipids in river sediments. ④The abundance of multi-heteroatomic compounds and condensed aromatics in the combined sewer system was higher than that in the separated sewer system, which may have been more severely polluted by domestic wastewater and storm runoff, especially kitchen wastewater. This study provides new insight for clarifying the critical causes of pollution in the new stage and provides an essential basis for further precision prevention and control of water pollution.

Seasonal changes of chemodiversity along with microbial succession in a municipal wastewater treatment plant

The relationship between chemodiversity and microbial succession in wastewater treatment plants (WWTPs) is highly intricate and bidirectional. The specific contribution of the microbial community to changes in the composition of dissolved organic matter (DOM) within different biological treatment units remains unclear, as does the reciprocal influence of DOM composition on microbial succession. In this study, spectroscopy ((Excitation-emission matrix) EEM-PARAFAC, Ultraviolet (UV)-spectrum, Fourier transform infrared spectrometer (FT-IR)), Liquid chromatograph mass spectrometer (LC‒MS) and Fourier transform ion cyclotron resonance (FT-ICR) MS along with high-throughput sequencing technology were used to explore the relationship between chemodiversity and microbial succession in WWTPs concerning seasonal changes. The results showed that WWTPs with anaerobic/anoxic/oxic (A2O) processes can metabolize and transform most of the wastewater DOM, and the anaerobic unit has the highest removal rate for fluorescence DOM (FDOM, 14.07%-64.43%); the anaerobic unit increased aliphatic/proteins and lignin-like molecules but decreased relative intensity, while the anoxic unit removed unsaturated hydrocarbons, aromatic structures, and lignin-like substances. The impact of seasonal changes on the composition and removal of FDOM and DOM in wastewater treatment is significant, and the variations that occur during different seasons affect microbial activity, as well as the production, degradation, and transformation of organic compounds throughout the wastewater treatment process. Network analysis shows that Parcubacteria_genera_incertae_sedis plays a crucial role in DOM chemodiversity, highlighting the crucial contribution of microbial communities to both the structure and operation of the entire DOM network. The results in this study could provide some theoretical and practical basis for guiding the process optimization of WWTPs.

Distinct variations in fluorescent DOM components along a trophic gradient in the lower Fox River-Green Bay as characterized using one-sample PARAFAC approach

Variations in molecular weight distributions of dissolved organic matter (DOM) and PARAFAC-derived fluorescent components were investigated along a transect in the seasonally hypereutrophic lower Fox River-Green Bay using the one-sample PARAFAC approach coupling flow field-flow fractionation for size-separation with fluorescence excitation-emission matrix (EEM) and PARAFAC analysis. Concentrations of dissolved organic carbon and nitrogen, chromophoric-DOM, specific UV absorbance at 254 nm, and humification index all decreased monotonically from river to open bay, showing a strong river-dominated DOM source and a dynamic change in DOM quality along the river-lake transect. The relative abundance of colloidal DOM (>1 kDa) derived from ultrafiltration exhibited minimal variation, averaging 71 ± 4 % of the bulk DOM, across the entire estuarine transect although the colloidal concentration decreased in general. Using the one-sample EEM-PARAFAC approach, the identified major fluorescent components were distinct between stations along the river-estuary-open bay continuum, with four components in river/upper-estuary but three components in open bay waters. Among the four common fluorescent components (C475, C410, C320 and C290), the most abundant and refractory humic-like component, C475, behaved conservatively and its relative abundance (%ΣFmax) remained fairly constant (50 ± 4 %) along the transect, while the semi-labile humic-like component, C410, consistently decreased from river to estuary and eventually vanished in open Green Bay. In contrast, the two autochthonous protein-like components (C320 and C290) increased from river to open bay along the trophic gradient. The new results presented here provide an improved understanding of the diverse and fluctuating characteristics in DOM composition, lability, and estuarine mixing behavior across the river-lake interface and demonstrate the efficacy of the one-sample PARAFAC approach.