Lake Chemodiversity Driven by Natural and Anthropogenic Factors

Molecular characterization and environmental response of dissolved organic matter in reserve quiescent groundwater wells of the North China plain: Insights from spectroscopy and mass spectrometry

Dissolved organic matter (DOM) plays a critical role in aquatic ecosystems. However, the characteristics of DOM in groundwater source wells and interactions with environmental factors remain poorly understood. This study investigated the spectral properties, molecular composition, and environmental drivers across vertical groundwater gradients in Shijiazhuang using spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS), multivariate statistics and molecular network analysis. Three components were identified: two humic-like substances (C1, C3) and one protein-like component (C2) Humic-like substances exhibited significant vertical stratification, with bottom groundwater DOM showing higher humification and autochthonous characteristics. Multivariate statistical analysis indicated that NO3--N and dissolved oxygen (DO) were keystone factors influencing the vertical differences of DOM. Surface-layer DOM was driven by dissolved total phosphorus (DTP), pH, DO and NO3--N, while the bottom layer was jointly regulated by pH, total phosphorus (TP), total nitrogen (TN) and NO3--N. DOM components correlated significantly with fluorescence index (FI), humification index (HIX), chemical oxygen demand (CODMn) and dissolved total nitrogen (DTN). FT-ICRMS analysis revealed that DOM molecular composition was dominated by CHO (38.71 %-52.07 %) and CHON (22.30 %-34.44 %) compounds, with lignin-like (LIG) (60.91 %-80.56 %) serving as the core molecular formulae. Redundancy analysis (RDA) identified that TN, DO, and NH4+-N were key drivers regulating the DOM molecules distribution. Furthermore, molecular network analysis demonstrated that LIG molecular formulae played a crucial role in the network, significantly enhancing the chemical stability of the DOM molecular network. These findings elucidate DOM dynamics in groundwater systems at a molecular scale, providing critical insights for resource protection and risk management.

Decomposing the molecular complexity and transformation of dissolved organic matter for innovative anaerobic bioprocessing

The sustainable transformation and management of dissolved organic matter (DOM) are crucial for advancing organic waste treatment towards resource-oriented processes. However, the intricate molecular complexity of DOM poses significant challenges, impeding a comprehensive understanding of the underlying biochemical processes. Here, we focus on the chemical "dark matter" mining using ultra-high resolution mass spectrometry technologies to elucidate the molecular diversity and transformation in anaerobic bioprocessing of food waste. We developed an analytical framework that reveals the persistence of DOM in the final effluent is mainly determined by its molecular properties, such as carbon chain length, aromaticity, unsaturation, and redox states. Our in-depth characterization and quantitative analysis of key biochemical reactions unveils the evolution of DOM composition, providing valuable insights into the targeted conversion of persistent molecules toward full utilization. Additionally, we establish a correlation between the redox state and energy density of a broad range of DOM molecules, enabling us to comprehend and evaluate their biodegradability. These insights enhance the mechanistic understanding of DOM transformation, guiding the rational design and regulation of sustainable organic waste treatment strategies.

Chemodiversity and molecular traits of dissolved organic matter driven by cascade reservoirs regulations in the upper Yangtze River

Dissolved organic matter (DOM) plays a fundamental role in biogeochemical cycles within riverine ecosystems. However, the construction and impoundment of dams disrupt the natural biophysical gradients in rivers, potentially leading to alterations and turnover of DOM compositions. This research investigated the composition and chemodiversity of DOM in the cascade reservoirs along the upper Yangtze River. Our findings reveal that DOM in the cascade reservoirs are predominantly composed of lignin-like, lipid-like, and protein-like compounds. The DOM chemodiversity exhibited a significant decrease during the flood season, but recovered during the dry season. Additionally, a general decline in DOM chemodiversity was observed along the longitudinal gradient in the cascade reservoirs. The molecular traits of DOM results indicated high bioavailability and lability of DOM molecules during the flood season. A random forest analysis identified physicochemical indicators as the most important factor influencing DOM chemodiversity. During the flood season, the selected variables were mostly strongly correlated with DOM molecular traits and categories. Specifically, Qin and Qout showed significant negative correlations with easily degradable compounds such as carbohydrates and lipids, as well as with molecular traits. Conversely, during the dry season, few physicochemical indicators and hydrological parameters show significant correlations with proteins and aromatic compounds. Structural equation model further demonstrated that hydrological parameters exert a strong positive influence on DOM molecular categories. This study provides a foundational framework for further evaluation cascade damming effects on DOM biogeochemical processes.

Spatio-temporal dynamics variation of dissolved organic matter and water quality parameters in Giheung Reservoir: A vertical perspective

Water reservoirs in densely populated areas play a crucial role in water supply, irrigation, and flood regulation. This study investigated the spatiotemporal dynamics of dissolved organic matter (DOM) and its interactions with water quality parameters in Giheung Reservoir, South Korea. Monthly water samples were collected at three depths, measuring fifteen physicochemical properties alongside DOM characterization using Excitation-Emission Matrix Parallel Factor Analysis (EEM-PARAFAC), identifying four DOM components: C1 (microbial protein-like), C2 (humic-like), C3 (terrestrial humic-like), and C4 (tyrosine/tryptophan-like). Seasonal stratification influenced DOM distribution, with humic-like DOM (C2, C3) accumulating in deeper layers during summer, while protein-like DOM (C1, C4) dominated surface waters due to enhanced biological productivity, and mixing periods homogenizing DOM across depths. The dimictic thermal regime, with moderate summer stratification and full mixing in spring and autumn, governed these patterns. 2D correlation spectroscopy (2D-COS) identified sequential DOM variation across layers, with humic-like DOM increasing at depth during stratification. Fluorescence indices-Fluorescence Index (FI), Humification Index (HIX), and Biological Index (BIX)-were integrated into structural equation modeling (SEM), showing that temperature and dissolved oxygen strongly drive protein-like DOM, while nitrogen and phosphorus compounds shape DOM reactivity. These findings enhance understanding of DOM biogeochemical processes and provide valuable insights for reservoir management to maintain water quality.

Unravelling the complex influence of dissolved organic matter on microbial diversity in a salinized lake

Ecosystems in cold and arid regions, such as Dai Lake - a typical inland, salinized lake in the semi-arid region of northern China - face severe environmental challenges, including salinization and biodiversity loss. This study investigates the chemical composition of dissolved organic matter (DOM) and the structure of microbial communities in lake water and sediments, offering novel insights into the ecosystem's dynamics. In winter, DOM in the lake water is primarily derived from decaying plant and animal matter, while sediment DOM is predominantly associated with microbial activity. Statistical analyses revealed significant correlations between DOM components and key microbial species, including Firmicutes, Actinobacteria, α-Proteobacteria, and Acidobacteria (p < 0.05), highlighting the critical role of microbial communities in DOM transformation. Partial least squares structural equation modeling (PLS-SEM), a latent variable approach integrating multiple regression analysis, demonstrates that DOM composition, along with environmental factors such as salinity, temperature, and nutrient levels, significantly influences microbial diversity and community dynamics in the lake. These findings highlight the importance of seasonally adjusting management strategies - such as controlling nutrient inputs, monitoring water quality, and managing sediments - to protect key microbial communities and sustain ecosystem health.

Keystone taxa drive the synchronous production of methane and refractory dissolved organic matter in inland waters

The production of both methane (CH4) and refractory dissolved organic matter (RDOM) depends on microbial consortia in inland waters, and it is unclear yet the link of these two processes and the underlying microbial regulation mechanisms. Therefore, a large-scale survey was conducted in China's inland waters, with the measurement of CH4 concentrations, DOM chemical composition, microbial community composition, and relative environmental parameters mainly by chromatographic, optical, mass spectrometric, and high-throughput sequencing analyses, to clarify the abovementioned questions. Here, we found a synchronous production of CH4 and RDOM linked by microbial consortia in inland waters. The increasing microbial cooperation driven by the keystone taxa (mainly Fluviicola and Polynucleobacter) could promote the transformation of labile DOM into RDOM and meanwhile benefit methanogenic microbial communities to produce CH4. As such, CH4 and RDOM showed consistent spatial differences, which were mainly influenced by total nitrogen and dissolved oxygen concentrations. This finding deepened the understanding of microbial-driven carbon transformation and will help to more accurately evaluate the carbon source-sink relationship in inland waters.

Influence of spectral and molecular composition of dissolved organic matter on labile Cd mobility in riparian soils in the Three Gorges Reservoir, China

The periodic anti-seasonal inundation of the Three Gorges Reservoir (TGR) leads to changes in the molecular composition of dissolved organic matter (DOM) in riparian soils, further impacting the geochemical processes and ecological risk of heavy metals. However, the intrinsic driving mechanisms of DOM influencing the cadmium (Cd), a major pollutant in riparian soils in TGR, at the molecular level remain unclear. In this study, the DOM molecular composition, labile Cd in riparian soils and the key driving mechanism before and after flooding were explored using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), the diffusive gradients in thin films (DGT) and partial least squares path modeling (PLS-PM). A spectral analysis revealed that after flooding, the relative abundance of terrestrial humic-like substances decreased whereas that of microbial humic-like substances increased. Furthermore, FT-ICR MS analysis revealed that the relative abundance of lignin, the main molecular components of DOM in riparian soils, increased after flooding. The linkage of DOM with the concentration and kinetic processes of labile Cd indicated that the higher aromaticity and unsaturation, larger molecular weight, and higher humification level of DOM promoted the mobility of labile Cd from the soil solid phase to the liquid phase. In particular, our findings indicated that at the molecular level, the most significant factor influencing the mobility of labile Cd was lignin, which was primarily governed by the complexation of lignin with labile Cd. The complexation mechanism between lignin and labile Cd resulted in increased ecotoxicological risk of labile Cd after flooding, while the overall ecotoxicological risk was low in riparian soils in TGR. This study provides better insight into the geochemical cycling and fate of toxic elements in reservoir ecosystems under the change of hydrological regime.

Organic matter in geothermal springs and its association with the microbial community

Organic matter (OM) plays an important role in the biogeochemical cycles of carbon, nitrogen, and other elements, shaping the structure of the microbiome and vice versa. However, the molecular composition of OM and its impact on the microbial community in terrestrial geothermal environments remain unclear. In this study, we characterized the OM in water and sediment from a typical geothermal field using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. By combining high-throughput amplicon sequencing and multivariate analyses, we deciphered the association between OM components and microbial community. A surprisingly high chemodiversity of OM was observed in the waters (11,088 compounds) and sediments (7772 compounds) in geothermal springs. Sulfur-containing organic compounds, a characteristic molecular signature of geothermal springs, accounted for 21 % ± 5 % in waters and 33 % ± 4 % in sediments. Multivariate analyses revealed that both labile and recalcitrant fractions of OM (e.g., carbohydrates intensity and tannins chemodiversity) influenced the structure and function of the microbial community. Co-occurrence networks showed that Proteobacteria and Crenarchaeota accounted for most of the connections with OM in waters (33 % and 15 %, respectively) and sediments (15 % and 12 %, respectively), highlighting their key roles in carbon cycling. This study expands our understanding of the molecular compositions of OM in geothermal springs and highlights its potentially important role in global climate change through microbial carbon cycling.

A comprehensive conceptual framework for signaling in-lake CO2 through dissolved organic matter

Organic carbon (C) and CO2 pools are closely interactive in aquatic environments. While there are strong indications linking freshwater CO2 to dissolved organic matter (DOM), the specific mechanisms underlying their common pathways remain unclear. Here, we present an extensive investigation from 20 subtropical lakes in China, establishing a comprehensive conceptual framework for identifying CO2 drivers and retrieving CO2 magnitude through co-trajectories of DOM evolution. Based on this framework, we show that lake CO2 during wet period is constrained by a combination of biogeochemical processes, while photo-mineralization of activated aromatic compounds fuels CO2 during dry period. We clearly determine that biological degradation of DOM governs temporal variations in CO2 rather than terrestrial C inputs within the subtropical lakes. Specifically, our results identify a shared route for the uptake of atmospheric polycyclic aromatic compounds and CO2 by lakes. Using machine learning, in-lake CO2 levels are well modelled through DOM signaling regardless of varying CO2 mechanisms. This study unravels the mechanistic underpinnings of causal links between lake CO2 and DOM, with important implications for understanding obscure aquatic CO2 drivers amidst the ongoing impacts of global climate change.

Calcium regulates the interactions between dissolved organic matter and planktonic bacteria in Erhai Lake, Yunnan Province, China

In recent years, the global carbon cycle has garnered significant research attention. However, details of the intricate relationship between planktonic bacteria, hydrochemistry, and dissolved organic matter (DOM) in inland waters remain unclear, especially their effects on lake carbon sequestration. In this study, we analyzed 16S rRNA, chromophoric dissolved organic matter (CDOM), and inorganic nutrients in Erhai Lake, Yunnan Province, China. The results revealed that allochthonous DOM (C3) significantly regulated the microbial community, and that autochthonous DOM, generated via microbial mineralization (C2), was not preferred as a food source by lake bacteria, and neither was allochthonous DOM after microbial mineralization (C4). Specifically, the correlation between the fluorescence index and functional genes (FAPRPTAX) showed that the degree of utilization of DOM was a critical factor in regulating planktonic bacteria associated with the carbon cycle. Further examination of the correlation between environmental factors and planktonic bacteria revealed that Ca2+ had a regulatory influence on the community structure of planktonic bacteria, particularly those linked to the carbon cycle. Consequently, the utilization strategy of DOM by planktonic bacteria was also determined by elevated Ca2+ levels. This in turn influenced the development of specific recalcitrant autochthonous DOM within the high Ca2+ environment of Erhai Lake. These findings are significant for the exploration of the stability of DOM within karst aquatic ecosystems, offering a new perspective for the investigation of terrestrial carbon sinks.

Polystyrene microplastics facilitate formation of refractory dissolved organic matter and reduce CO2 emissions

Microplastics, as a type of anthropogenic pollution in aquatic ecosystems, affect the carbon cycle of organic matter. Although some studies have investigated the effects of microplastics on dissolved organic matter (DOM), the impact of alterations in the chemical properties of microplastics on refractory DOM and carbon release remains unclear. Here, we observed that microplastic treatments (e.g., polystyrene, PS) altered the composition and function of microbial community, notably increasing the abundance of microbial families involved in consuming easily degradable organic matter. During the process in which microbial community decomposed organic matter into DOM, PS underwent surface oxidation. The oxidized PS aggregated with DOM and microorganisms through electrostatic interactions and chemical bonds. Moreover, these interactions between oxidized PS and microbial community affect the utilization of organic matter, resulting in a significant decrease in CO2 emissions. Specifically, total CO2 emissions decreased by approximately 23.76 % with 0.1 mg/L PS treatment and by 44.97 % with 10 mg/L PS treatment compared to those in PS-free treatments over the entire reaction. These findings underscored the significance of the chemical properties of PS in the interactions among DOM and microorganisms, emphasizing the potential impact of PS microplastics on the carbon cycle in ecosystems.

Using the Potential Transformation of Dissolved Organic Matter to Understand Carbon Emissions from Inland Rivers

Understanding the transformation of river dissolved organic matter (DOM) is important for assessing the emissions of greenhouse gases (GHGs) in inland waters. However, the relationships between the variations in DOM components and GHGs remain largely unknown. Here, parallel factor analysis (PARAFAC) was applied to investigate the DOM components in 46 inland rivers in China. We found that the GHG emissions in peri-urban rivers were 1.10-2.15 times greater than those in urban rivers. Microbial and environmental factors (e.g., living cell numbers, microbial activity and pH) explained more than 70% of the total variance in GHG emissions in rivers. DOM variations relationships between different components ware revealed based on compositional data principal component analysis (CoDA-PCA). Microbial-mediated DOM production and degradation were quantified, and the degradation levels in peri-urban rivers were 11.8-25.2% greater than those in urban rivers. Differences in carbon emission potential between urban and peri-urban rivers were related to DOM variances and transformations and were affected by water chemistry (e.g., NH4-N and As). This study clarifies the regulatory effects of DOM composition variations and transformations on GHG emissions, and enhances the understanding of the DOM biogeochemical cycle.

Spatial patterns and environmental functions of dissolved organic matter in grassland soils of China

Soil dissolved organic matter (DOM) is crucial to atmospheric, terrestrial and aquatic environments as well as human life. Here, by characterizing DOM from 89 grassland soils throughout China, we reveal the spatial association between DOM geochemistry in the dry season vs annual ecosystem exchange and cancer cases. The humic-like and high molecular weight (3.4-25 kDa) fractions with lower biodegradability, decline from the northern to the southern regions of China, and are correlated with lower soil respiration and net ecosystem productivity at the continental scale. The <1.2 kDa and proteinaceous fractions could serve as a geographical indicator of nasopharyngeal cancer incidence and mortality, while the 3.4-25 kDa and humified fractions are potentially associated with pancreatic cancer cases (P < 0.05). Our findings highlight that exploiting the environmental functions of soil DOM and mitigating the negative impacts are necessary, and require actions tailored to local soil DOM conditions.

Unveiling Microbial Nitrogen Metabolism in Rivers using a Machine Learning Approach

Microbial nitrogen metabolism is a complicated and key process in mediating environmental pollution and greenhouse gas emissions in rivers. However, the interactive drivers of microbial nitrogen metabolism in rivers have not been identified. Here, we analyze the microbial nitrogen metabolism patterns in 105 rivers in China driven by 26 environmental and socioeconomic factors using an interpretable causal machine learning (ICML) framework. ICML better recognizes the complex relationships between factors and microbial nitrogen metabolism than traditional linear regression models. Furthermore, tipping points and concentration windows were proposed to precisely regulate microbial nitrogen metabolism. For example, concentrations of dissolved organic carbon (DOC) below tipping points of 6.2 and 4.2 mg/L easily reduce bacterial denitrification and nitrification, respectively. The concentration windows for NO3--N (15.9-18.0 mg/L) and DOC (9.1-10.8 mg/L) enabled the highest abundance of denitrifying bacteria on a national scale. The integration of ICML models and field data clarifies the important drivers of microbial nitrogen metabolism, supporting the precise regulation of nitrogen pollution and river ecological management.

Source apportionment based on EEM-PARAFAC combined with microbial tracing model and its implication in complex pollution area, Wujin District, China

Further improving the quality of surface water is becoming more difficult after the control of main point-sources, especially in the complex pollution area with mixed industrial and agricultural productions, whereas the pollution source apportionment might be the key to quantify different pollution sources and developing some effective measures. In this study, a technical framework for source apportionment based on three-dimensional fluorescence and microbial traceability model is developed. Based on screening of the main environmental factors and their spatiotemporal characteristics, potential pollution sources have been tentatively identified. Then, the pollution sources are further tested based on the analysis of fluorescence excitation-emission matrix (EEM) and the similarity of fluorescence components in surface water and potential pollution sources. At the same time, the correlation between microbial species and pollution sources is constructed by analyzing the spatiotemporal characteristics of microbial composition and the response of main species to environmental factors. Therefore, pollution source apportionment is quantified using PCA-APCS-MLR, Fast Expectation-maximization for Microbial Source Tracking (FEAST), and Bayesian community-wide culture-independent microbial source tracking (SourceTracker). PCA-APCS-MLR could not effectively distinguish the contributions of different industrial sources in the complex environment of this study, and the contribution of unknown sources was high (average 39.60%). In contrast, the microbial traceability model can accurately identify the contribution of 7 pollution sources and natural sources, effectively reduce the proportion of unknown sources (average of FEAST is 19.81%, SourceTracker is 16.72%), and show better pollution identification and distribution capabilities. FEAST exhibits a more sensitive potential in source apportionment and shorter calculation time than SourceTracker, thus might be used to guide the precise regional pollution control, especially in the complex pollution environments.

Unique dissolved organic matter molecules and microbial communities in rhizosphere of three typical crop soils and their significant associations based on FT-ICR-MS and high-throughput sequencing analysis

Using cucumber, maize, and ryegrass as model plants, the diversity and uniqueness of the molecular compositions of dissolved organic matter (DOM) and the structures of microbial communities in typical crop rhizosphere soils, as well as their associations, were investigated based on high-resolution mass spectrometry combined with high-throughput sequencing. The results showed that the rhizosphere contained 2200 organic molecules that were not identified in the non-rhizosphere soils, as characterized by FT-ICR-MS. The rhizosphere DOM molecules generally contained more N, S, and P heteroatoms, stronger hydrophilicity, and more refractory organic matter, representing high and complex chemical diversity characteristics. 16SrRNA sequencing results demonstrated that Proteobacteria, Actinomycetes and Firmicutes were the dominant flora in the soils. Plant species could significantly change the composition and relative abundance of rhizosphere microbial populations. The microbial community structures of rhizosphere and non-rhizosphere soils showed significant differences at both the phylum and class levels. Multiple interactions between the microorganisms and DOM compositions formed a complex network of relationships. There were strong and remarkable positive or negative couplings between different sizes and categories of DOM molecules and the specific microbial groups (P < 0.05, |R| ≥ 0.9) in the rhizosphere soils as shown by network profiles. The correlations between DOM molecules and microbial groups in rhizosphere soils had plant species specificity. The results above emphasized the relationship between the heterogeneity of DOM and the diversity of microbial communities, and explored the molecular mechanisms of the biochemical associations in typical plant rhizosphere soils, providing a foundation for in-depth understanding of plant-soil-microbe interactions.

Chemodiversity of riverine dissolved organic matter: Effects of local environments and watershed characteristics

Riverine dissolved organic matter (DOM) is crucial to global carbon cycling and aquatic ecosystems. However, the geographical patterns and environmental drivers of DOM chemodiversity remain elusive especially in the waters and sediments of continental rivers. Here, we systematically analyzed DOM molecular diversity and composition in surface waters and sediments across 97 broadly distributed rivers using data from the Worldwide Hydrobiogeochemistry Observation Network for Dynamic River Systems (WHONDRS) consortium. We further examined the associations of molecular richness and composition with geographical, climatic, physicochemical variables, as well as the watershed characteristics. We found that molecular richness significantly decreased toward higher latitudes, but only in sediments (r = -0.24, p < 0.001). The environmental variables like precipitation and non-purgeable organic carbon showed strong associations with DOM molecular richness and composition. Interestingly, we identified that less-documented factors like watershed characteristics were also related to DOM molecular richness and composition. For instance, DOM molecular richness was positively correlated with the soil sand fraction for waters, while with the percentage of forest for sediments. Importantly, the effects of watershed characteristics on DOM molecular richness and composition were generally stronger in waters than sediments. This phenomenon was further supported by the fact that 11 out of 13 watershed characteristics (e.g., the percentages of impervious area and cropland) showed more positive than negative correlations with molecular abundance especially in waters. As the percentage of forest increased, there was a continuous accumulation of the compounds with higher molecular weight, aromaticity, and degree of unsaturation. In contrast, human activities accumulated the compounds with lower molecular weight and oxygenation, and higher bioavailability. Our findings imply that it may be possible to use a small set of broadly available data types to predict DOM molecular richness and composition across diverse river systems. Elucidation of mechanisms underlying these relationships will provide further enhancements to such predictions, especially when extrapolating to unsampled systems.

The ratio but not individual of fragile to refractory DOM affects greenhouse gases release in different trophic level lakes

Inland shallow lakes are recognized as an important source of greenhouse gases (GHGs), and their contribution is expected to increase due to global eutrophication. The generation and release of GHGs involved multiple variables, leading to many uncertain potential factors. This study examined the emission characteristics of GHGs at the water-air interface in 12 shallow lakes categorized into four eutrophic levels in the Yangtze River basin. The average emission rates of CH4, CO2 and N2O were 1.55, 3.43, 18.13 and 30.47 mg m-2 h-1, 4.12, 14.64, 25.11 and 69.84 mg m-2 h-1, and 0.2, 0.25, 0.43 and 0.79 mg m-2 day-1 in the oligotrophic, mesotrophic, eutrophic and hypereutrophic lakes, respectively. There were significant correlations between eutrophic levels and the emission rates of CH4 and CO2 (p < 0.05). Redundancy analysis and Mantel test were conducted to further examine the key factors influencing carbon emissions from eutrophic water. It was found that the presence of algae and nutrients in the overlying water played a crucial role in the release of GHGs, indicating the importance of ecosystem productivity in the carbon budget of the lake. In order to assess the bioavailability of organic matter, a new indicator called R(P/H) was proposed. This indicator represents the ratio of protein and humus-like components, which were obtained through EEMs-PARAFAC modeling. The relationship between R(P/H) and CH4 was found to be exponential (R2 = 0.90). Additionally, R(P/H) showed a linear relationship with CO2 and N2O (R2 = 0.68, R2 = 0.75). Therefore, it is crucial to consider R(P/H) as an important factor in accurately estimating global GHG emission fluxes in the future, especially with advancements in the database.

Instability in a carbon pool driven by multiple dissolved organic matter sources in a eutrophic lake basin: Potential factors for increased greenhouse gas emissions

Lakes serve as vital reservoirs of dissolved organic matter (DOM) and play pivotal roles in biogeochemical carbon cycles. However, the sources and compositions of DOM in freshwater lakes and their potential effects on lake sediment carbon pools remain unclear. In this study, seven inflowing rivers in the Lake Taihu basin were selected to explore the potential effects of multi-source DOM inputs on the stability of the lake sediment carbon pool. The results showed the high concentrations of dissolved organic carbon in the Lake Taihu basin, accompanied by a high complexity level. Lignins constituted the majority of DOM compounds, surpassing 40% of the total, while the organic carbon content was predominantly composed of humic acids (1.02-3.01 g kg-1). The high amounts of lignin oxidative cleavage led to CHO being the main molecular structure in the DOM of the seven rivers. The carbon constituents within the sediment carbon reservoir exhibited a positive correlation with dissolved CH4 and CO2, with a notable emphasis on humic acid and dissolved CH4 (R2 = 0.86). The elevated concentration of DOM, coupled with its intricate composition, contributed to the increases in dissolved greenhouse gases (GHGs). Experiments showed that the mixing of multi-source DOM can accelerate the organic carbon mineralization processes. The unit carbon emission efficiency was highest in the mixed group, reaching reached 160.9 μmol∙Cg-1, which also exhibited a significantly different carbon pool. The mixed decomposition of DOM from different sources influenced the roles of the lake carbon pool as source and sink, indicating that the multi-source DOM of this lake basin was a potential driving factor for increased carbon emissions. These findings have improved our understanding of the sources and compositions of DOM in lake basins and revealed their impacts on carbon emissions, thereby providing a theoretical basis for improving assessments of lake carbon emissions.

Mitigation of soil N2O emissions by decomposed straw based on changes in dissolved organic matter and denitrifying bacteria

The return of decomposed straw represents a less explored potential option for reducing N2O emissions. However, the mechanisms underlying the effects of decomposed straw return on soil N2O mitigation are still not fully clear. Therefore, we used a helium atmosphere robotized continuous flow incubation system to compare the soil N2O and N2 emissions from four treatments: CK (control: no straw), WS (wheat straw), IWS (wheat straw decomposed with Irpex lacteus), and PWS (wheat straw decomposed with Phanerochaete chrysosporium). All the treatments have been fertilized with the same amount of KNO3. Furthermore, we also analyzed i) the chemodiversity of soil dissolved organic matter (DOM), ii) the nirS, nirK, and nosZ gene copies and relative abundances of denitrifying bacterial communities (DBCs), and iii) the specific linkages between N2O emissions and DOM and DBC. The results showed that the WS, IWS and PWS treatments increased N2O emissions compared to the CK treatment. However, applying decomposed straw to soil, especially straw treated with P. chrysosporium, effectively decreased the soil N2O and increased N2 emissions compared to WS and IWS. Moreover, the IWS and PWS treatments increased the CHO composition, but they decreased the CHON and CHOS compositions of heteroatomic compounds of DOM compared with the WS and CK treatments. Furthermore, the WS, IWS and PWS treatments all significantly increased the nirS and nosZ gene copies compared with the CK treatment. Additionally, compared with the other treatments, the PWS treatment significantly shaped the DBC and led to a higher relative abundance of Pseudomonas with nirS and nosZ genes. Meanwhile, Network analysis showed that the mitigation of N2O was closely related to particular DOM molecules, and specific DBC taxa. These results highlight the potential for decomposed straw amendments to mitigate of soil N2O emissions not only by changing soil DOM but also mediating the soil DBC.

Unraveling heterogeneity of dissolved organic matter in highly connected natural water bodies at molecular level

The exploring of molecular-level heterogeneity of dissolved organic matter (DOM) in highly connected water bodies is of great importance for pollution tracing and lake management, and provides new perspectives on the transformations and fate of DOM in aquatic systems. However, the inherent homogeneity of DOM in connected water bodies poses challenges for its heterogeneity analysis. In this work, an innovative method combining fluorescence spectroscopy, high-resolution mass spectrometry (HRMS), and cluster analysis was developed to reveal the heterogeneity of DOM in highly connected water bodies at the molecular level. We detected 4538 molecules across 36 sampling sites in Chaohu Lake using HRMS. Cluster analysis based on excitation-emission matrix (EEM) data effectively divided the sampling sites into four clusters, representing the water bodies from West Chaohu Lake, East Chaohu Lake, agricultural land, and urban areas. Analysis of DOM in the western and eastern parts of the lake revealed that aerobic degradation led to a decrease in CHOS and aliphatic compounds, alongside an increase in CHO and highly unsaturated and phenolic compounds. Furthermore, we unveiled the characteristics and sources of heterogeneity in DOM from agricultural land and urban areas. Our method accurately captured the heterogeneous distribution of DOM in the lake and revealed the heterogeneous composition of DOM at molecular level. This work underscores the importance of integrating complementary spectroscopic analyses with HRMS in DOM research with similar compositions.

Aqueous-Phase Reactions of Anthropogenic Emissions Lead to the High Chemodiversity of Atmospheric Nitrogen-Containing Compounds during the Haze Event

Nitrogen-containing organic compounds (NOCs), a type of important reactive-nitrogen species, are abundant in organic aerosols in haze events observed in Northern China. However, due to the complex nature of NOCs, the sources, formation, and influencing factors are still ambiguous. Here, the molecular composition of organic matters (OMs) in hourly PM2.5 samples collected during a haze event in Northern China was characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We found that CHON compounds (formulas containing C, H, O, and N atoms) dominated the OM fractions during the haze and showed high chemodiversity and transformability. Relying on the newly developed revised-workflow and oxidation-hydrolyzation knowledge for CHON compounds, 64% of the major aromatic CHON compounds (>80%) could be derived from the oxidization or hydrolyzation processes. Results from FT-ICR MS data analysis further showed that the aerosol liquid water (ALW)-involved aqueous-phase reactions are important for the molecular distribution of aromatic-CHON compounds besides the coal combustion, and the ALW-involved aromatic-CHON compound formation during daytime and nighttime was different. Our results improve the understanding of molecular composition, sources, and potential formation of CHON compounds, which can help to advance the understanding for the formation, evolution, and control of haze.

Difference Analysis of the Composition of Iron (Hydr)Oxides and Dissolved Organic Matter in Pit Mud of Different Pit Ages in Luzhou Laojiao and Its Implications for the Ripening Process of Pit Mud

Long-term production practice proves that good liquor comes out of the old cellar, and the aged pit mud is very important to the quality of Luzhou-flavor liquor. X-ray diffraction, Fourier transform ion cyclotron resonance mass spectrometry, and infrared spectroscopy were used to investigate the composition characteristics of iron-bearing minerals and dissolved organic matter (DOM) in 2-year, 40-year, and 100-year pit mud and yellow soil (raw materials for making pit mud) of Luzhou Laojiao distillery. The results showed that the contents of total iron and crystalline iron minerals decreased significantly, while the ratio of Fe(II)/Fe(III) and the content of amorphous iron (hydr)oxides increased significantly with increasing cellar age. DOM richness, unsaturation, and aromaticity, as well as lignin/phenolics, polyphenols, and polycyclic aromatics ratios, were enhanced in pit mud. The results of the principal component analysis indicate that changes in the morphology and content of iron-bearing minerals in pit mud were significantly correlated with the changes in DOM molecular components, which is mainly attributed to the different affinities of amorphous iron (hydr)oxides and crystalline iron minerals for the DOM components. The study is important for understanding the evolution pattern of iron-bearing minerals and DOM and their interactions during the aging of pit mud and provides a new way to further understand the influence of aged pit mud on Luzhou-flavor liquor production.

Mechanisms shaping dissolved organic matter and microbial community in lake ecosystems

Lakes are active components of the global carbon cycle and host a range of processes that degrade and modify dissolved organic matter (DOM). Through the degradation of DOM molecules and the synthesis of new compounds, microbes in aquatic environments strongly and continuously influence chemodiversity, which can feedback to influence microbial diversity. Developing a better understanding of the biodiversity patterns that emerge along spatial and environmental gradients is one of the key objectives of community ecology. A changing climate may affect ecological feedback, including those that affect microbial communities. To maintain the function of a lake ecosystem and predict carbon cycling in the environment, it is increasingly important to understand the coupling between microbial and DOM diversity. To unravel the biotic and abiotic mechanisms that control the structure and patterns of DOM and microbial communities in lakes, we combined high-throughput sequencing and ultra-high resolution mass spectrometry together with a null modeling approach. The advantage of null models is their ability to evaluate the relative influences of stochastic and deterministic assembly processes in both DOM and microbial community assemblages. The present study includes spatiotemporal signatures of DOM and the microbial community in six temperate lakes contrasting continental and Mediterranean climates during the productive season. Different environmental conditions and nutrient sources characterized the studied lakes. Our results have shown high covariance between molecular-level DOM diversity and the diversity of individual microbial communities especially with diversity of microeukaryotes and free-living bacteria indicating their dynamic feedback. We found that the differences between lakes and climatic regions were mainly reflected in the diversity of DOM at the molecular formula-level and the microeukaryota community. Furthermore, using null models the DOM assembly was governed by deterministic variable selection operating consistently and strongly within and among lakes. In contrast, microbial community assembly processes were highly variable across lakes with different trophic status and climatic regions. Difference in the processes governing DOM and microbial composition does not indicate weak coupling between these components, rather it suggests that distinct factors may be influencing microbial communities and DOM assemblages separately. Further understanding of the DOM-microbe coupling (or lack thereof) is key to formulating predictive models of future lake ecology and function.

DOM Associates with Greenhouse Gas Emissions in Chinese Rivers under Diverse Land Uses

Growing evidence indicates that rivers are hotspots of greenhouse gas (GHG) emissions and play multiple roles in the global carbon budget. However, the roles of terrestrial carbon from land use in river GHG emissions remain largely unknown. We studied the microbial composition, dissolved organic matter (DOM) properties, and GHG emission responses to different landcovers in rivers (n = 100). The bacterial community was mainly constrained by land-use intensity, whereas the fungal community was mainly controlled by DOM chemical composition (e.g., terrestrial DOM with high photoreactivity). Anthropogenic stressors (e.g., land-use intensity, gross regional domestic product, and total population) were the main factors affecting chromophoric DOM (CDOM). DOM biodegradability exhibited a positive correlation with CDOM and contributed to microbial activity for DOM transformation. Variations in CO2 and CH4 emissions were governed by the biodegradation or photomineralization of dissolved organic carbon derived from autotrophic DOM and were indirectly affected by land use via changes in DOM properties and water chemistry. Because the GHG emissions of rivers offset some of the climatic benefits of terrestrial carbon (or ocean) sinks, intensified urban land use inevitably alters carbon cycling and changes the regional microclimate.

Biodegradability of algal-derived dissolved organic matter and its influence on methylmercury uptake by phytoplankton

Methylmercury (MeHg) uptake by phytoplankton represents a key step in determining the exposure risks of aquatic organisms and human beings to this potent neurotoxin. Phytoplankton uptake is believed to be negatively related to dissolved organic matter (DOM) concentration in water. However, microorganisms can rapidly change DOM concentration and composition and subsequent impact on MeHg uptake by phytoplankton has rarely been tested. Here, we explored the influences of microbial degradation on the concentrations and molecular compositions of DOM derived from three common algal sources and tested their subsequent impacts on MeHg uptake by the widespread phytoplankton species Microcystis elabens. Our results indicated that dissolved organic carbon was degraded by 64.3‒74.1% within 28 days of incubating water with microbial consortia from a natural meso‑eutrophic river. Protein-like components in DOM were more readily degraded, while the numbers of molecular formula for peptides-like compounds had increased after 28 days' incubation, probably due to the production and release of bacterial metabolites. Microbial degradation made DOM more humic-like which was consistent with the positive correlations between changes in proportions of Peaks A and C and bacterial abundance in bacterial community structures as illustrated by 16S rRNA gene sequencing. Despite rapid losses of the bulk DOM during the incubation, we found that DOM degraded after 28 days still reduced the MeHg uptake by Microcystis elabens by 32.7‒52.7% relative to a control without microbial decomposers. Our findings emphasize that microbial degradation of DOM would not necessarily enhance the MeHg uptakes by phytoplankton and may become more powerful in inhibiting MeHg uptakes by phytoplankton. The potential roles of microbes in degrading DOM and changing the uptakes of MeHg at the base of food webs should now be incorporated into future risk assessments of aquatic Hg cycling.

Sedimentary organic matter molecular composition reveals the eutrophication of the past 500 years in Lake Daihai, Inner Mongolia

Lake eutrophication seriously threatens water quality and human health. Under continuous global warming and intensified human activity, increasing attention is being paid to how lake trophic status responds to climate change and anthropogenic impacts. Based on the sedimentary organic matter (SOM) molecular composition determined by the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) technology, and combined with the SOM stable nitrogen isotopes (δ¹⁵N_org), we studied how lake trophic status and ecology respond to both climatic changes and anthropogenic impacts of the past 500 yrs at Lake Daihai, Inner Mongolia. The results show that the relative abundance of lipids, proteins, and carbohydrates in lake sediments kept relatively low before AD ∼1850, and increased gradually thereafter, especially after AD ∼1950, suggesting that the lake trophic status was low before AD ∼1850, but obviously increased during the past one more century. On the other hand, the relative abundance of allochthonous condensed aromatics and vascular plant-derived polyphenols compounds gradually decreased after AD ∼1850, which is most likely due to the intensified land-use changes in the catchment. Our results show that the SOM molecular composition is more sensitive to trace the land-use changes than the δ¹⁵N_org ratios, suggesting a potential use of this technique to trace even earlier human land uses (e.g., during the prehistorical times) in a catchment. The results of this study suggest that intensified land-use change, increased discharges of human sewage and industrial wastewater, cropland runoff, and concentrated effects caused by lake level drops may have combinedly increased nutrient concentration and accelerated lake eutrophication at Lake Daihai. Therefore, proper policy is necessary to slow down anthropogenic impacts and limit further eutrophication for lakes like Lake Daihai.

FT-ICR-MS combined with fluorescent spectroscopy reveals the driving mechanism of the spatial variation in molecular composition of DOM in 22 plateau lakes

Dissolved organic matter (DOM) is the largest carbon pool and directly affects the biogeochemistry in lakes. In the current study, fourier transform ion cyclotron mass spectrometry (FT-ICR-MS) combined with fluorescent spectroscopy was used to assess the molecular composition and driving mechanism of DOM in 22 plateau lakes in Mongolia Plateau Lakes Region (MLR), Qinghai Plateau Lakes Region (QLR) and Tibet Plateau Lakes Region (TLR) of China. The limnic dissolved organic carbon (DOC) content ranged from 3.93 to 280.8 mg L^-1 and the values in MLR and TLR were significantly higher than that in QLR. The content of lignin was the highest in each lake and showed a gradually decreasing trend from MLR to TLR. Random forest model and structural equation model implied that altitude played an important role in lignin degradation while the contents of total nitrogen (TN) and chlorophyll a (Chl-a) have a great influence on the increase of DOM Shannon index. Our results also suggested that the inspissation of DOC and the promoted endogenous DOM production caused by the inspissation of nutrient resulted in a positive relationship between limnic DOC content and limnic factors such as salinity, alkalinity and nutrient concentration. From MLR to QLR and TLR, the molecular weight and the number of double bonds gradually decreased but the humification index (HIX) also decreased. In addition, from the MLR to the TLR, the proportion of lignin gradually decreased, while the proportion of lipid gradually increased. Both above results suggested that photodegradation was dominated in lakes of TLR, while microbial degradation was dominated in lakes of MLR.

Thermal power plants pollution assessment based on deep neural networks, remote sensing, and GIS: A real case study in Iran

To investigate the impact of the Bandar Abbas thermal power plant on the waters of the Persian Gulf coast, a combination of satellite images and ground data was utilized to determine the Sea Surface Temperature (SST) as a thermal index, Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD) as biological indices. Additionally, measurements of SO₂, O₃, NO₂, CO₂, CO, and CH₄ values in the atmosphere were taken to determine the plant's impact on air pollution. Temperature values of the water for different months were predicted using Long Short-Term Memory (LSTM), Support Vector Regression (SVR), and Cascade neural networks. The results indicate that the waters near thermal power plants exhibit the highest temperatures in July and September, with temperatures reaching approximately 50 °C. Furthermore, the SST values were found to be strongly correlated with ecological indices. The Multiple Linear Regression (MLR) analysis revealed a strong correlation between the temperature and TOC, COD, and O₂ in water (R_TOC²=0.98), [Formula: see text] , R_COD²=0.87 and O₃, NO₃, CO₂, and CO in the air ( [Formula: see text] ). Finally, the results demonstrate that the LSTM method exhibited high accuracy in predicting the water temperature (R² = 0.98).

Chemodiversity of Dissolved Organic Matter Is Governed by Microbial Biogeography in Inland Waters

Dissolved organic matter (DOM) is crucial for the carbon biogeochemical cycle and has a close link with microbiome in aquatic ecosystems; however, the causal relationship between DOM and microbial diversity in inland waters is not very clear so far. Therefore, a national survey of China's inland waters was conducted, and the DOM chemical composition and microbial community composition were determined by Fourier transform ion cyclotron resonance mass spectrometry and high-throughput sequencing to clarify the abovementioned question. Here, we found that DOM chemodiversity was governed by microbial community assembly in inland waters, not vice versa. Under the control of microbial biogeography, DOM chemodiversity showed a clear geographical distribution difference. Water DOM chemodiversity was mainly constrained by bacterial and archaeal community composition, whereas sediment DOM chemodiversity was mainly controlled by eukaryotic and fungal community composition. In addition, the sediment DOM chemical composition was also affected by the interaction of different microbial groups between waters and sediments. The study is the first to clarify the causal relationship and proposes a microbial regulatory mechanism on the geographical distribution pattern of DOM chemodiversity, thus further deepening the understanding of the DOM biogeochemical cycle.

How does anthropogenic activity influence the spatial distribution of dissolved organic matter in rivers of a typical basin located in the Loess Plateau, China

River ecosystems interact strongly with adjacent terrestrial environments and receive dissolved organic matter (DOM) from a variety of sources, all of which are vulnerable to human activities and natural processes. However, it is unclear how and to what extent human and natural factors drive DOM quantity and quality changes in river ecosystems. Here, three fluorescence components were identified via optical techniques, including two humic-like substances and one protein-like component. The protein-like DOM was mainly accumulated in anthropogenically impacted regions, while humic-like components exhibit the opposite trend. Furthermore, the driving mechanisms of both natural and anthropogenic factors on the variations in DOM composition were investigated using partial least squares structural equation modelling (PLS-SEM). Human activities, especially agriculture, positively influence the protein-like DOM directly by enhancing anthropogenic discharge with protein signals and also indirectly by affecting water quality. Water quality directly influences the DOM composition by stimulating in-situ production through a high nutrient load from anthropogenic discharge and inhibiting the microbial humification processes of DOM due to higher salinity levels. The microbial humification processes can also be restricted directly by a shorter water residence time during the DOM transport processes. Furthermore, protein-like DOM was more sensitive to direct anthropogenic discharge than indirect in-situ production (0.34 vs. 0.25), especially from non-point source input (39.1%), implying that agricultural industry optimization may be an efficient way to improve water quality and reduce protein-like DOM accumulation.

The pH-sensitve oxygenation of FeS: Mineral transformation and immobilization of Cr(VI)

Iron sulfide (FeS) has been widely used to reduce toxic Cr(VI) into Cr(III) in anoxic aquatic environments, where pH could strongly influence Cr(VI) removal. However, it remains unclear how pH regulates the fate and transformation of FeS under oxic conditions and the immobilization of Cr(VI). The results of this study showed that typical pH conditions of natural aquatic environment significantly affected the mineral transformation of FeS. Under acidic conditions, FeS was principally transformed to goethite, amarantite, and elemental sulfur with minor lepidocrocite through proton-promoted dissolution and oxidation. Instead, under basic conditions, the main products were lepidocrocite and elemental sulfur via surface-mediated oxidation. In typical acidic or basic aquatic environment, the pronounced pathway for the oxygenation of FeS solids may alter their ability to remove Cr(VI). Longer oxygenation impeded Cr(VI) removal at acidic pH, and a decreasing ability to reduce Cr(VI) caused a drop in Cr(VI) removal performance. Cr(VI) removal decreased from 733.16 to 36.82 mg g^-1 with the duration of FeS oxygenation increasing to 5760 min at pH 5.0. In contrast, newly generated pyrite from brief oxygenation of FeS improved Cr(VI) reduction at basic pH, followed by a drop in Cr(VI) removal performance due to the impaired reduction capacity with increasing to the complete oxygenation. Cr(VI) removal increased from 669.58 to 804.83 mg g^-1 with increasing oxygenation time to 5 min and then decreased to 26.27 mg g^-1 after the full oxygenation for 5760 min at pH 9.0. These findings provide insight into the dynamic transformation of FeS in oxic aquatic environments with various pHs and the impact on Cr(VI) immobilization.

Changes of Soil Dissolved Organic Matter and Its Relationship with Microbial Community along the Hailuogou Glacier Forefield Chronosequence

Glacier-retreated areas are ideal areas to study soil biogeochemical processes during vegetation succession, because of the limited effect of other environmental and climatic factors. In this study, the changes of soil dissolved organic matter (DOM) and its relationship with microbial communities along the Hailuogou Glacier forefield chronosequence were investigated. Both microbial diversity and DOM molecular chemodiversity recovered rapidly at the initial stage, indicating the pioneering role of microorganisms in soil formation and development. The chemical stability of soil organic matter enhanced with vegetation succession due to the retaining of compounds with high oxidation state and aromaticity. The molecular composition of DOM affected microbial communities, while microorganisms tended to utilize labile components to form refractory components. This complex relationship network between microorganisms and DOM components played an important role in the development of soil organic matter as well as the formation of stable soil carbon pool in glacier-retreated areas.

Sediment organic carbon dynamics response to land use change in diverse watershed anthropogenic activities

Sediment organic carbon (SOC) is a precious archive that synthesizes anthropogenic processes that remove geochemical fluxes from watersheds. However, the scarcity of inspection about the dynamic mechanisms of anthropogenic activities on SOC limits understanding into how key human factors drive carbon dynamics. Here, four typical basins with similar natural but significantly diverse human contexts (high-moderate-low disturbance: XJ-ZS and YJ-LS) were selected to reconstruct sedimentation rates (SR) and SOC dynamics nearly a century based on 200-cm corers. A partial least squares path model (PLS-PM) was used to establish successive (70 years) and multiple anthropogenic data (population, agriculture, land use, etc.) quantification methods for SOC. Intensified anthropogenic disturbances shifted all SR from pre-stable to post-1960s fluctuating increases (total coefficient: high: 0.63 < low: 0.47 < medium: 0.45). Although land use change was co-critical driver of SOC variations, their trend and extent differed under the dams and other disturbances (SOC mutated in high-moderate but stable in low). For high basin, land use changes increased (0.12) but dams reduced (-0.10) the downstream SOC. Furthermore, SOC mutation corresponded to soil erosion due to urbanization in both periods A and B. For moderate, SOC was reversed with the increase in afforestation and cropland (-0.19) due to the forest excitation effect and deep ploughing, which corresponded to the drought in phase B and the anthropogenic ecological project in A. For low, the increase in SOC corresponded to the Great Leap Forward deforestation in period B and the reed sweep in A, which suggested the minor land change substantially affected (0.16) SOC in fragile environments. Overall, SOC dynamics revealed that anthropogenic activities affected terrestrial and aquatic ecosystems for near the centenary, especially land use. This is constructive for agroforestry management and reservoir construction, consistent with expectations like upstream carbon sequestration and downstream carbon stabilization.

Regulation of the Nutrient Cycle Pathway and the Microbial Loop Structure by Different Types of Dissolved Organic Matter Decomposition in Lakes

To explore the effect of different types of dissolved organic matter (DOM) decomposition on nutrient cycling pathways and the microbial loop, four lakes with different DOM sources were investigated monthly. In Lake Tangxun, Dolichospermum decay released highly labile dissolved organic nitrogen into the water column. This induced bacterial organic nitrogen decomposition, as indicated by the increased abundance of gltB, gltD, gdh, and glnA as well as aminopeptidase activity. Genes associated with dissimilatory nitrate reduction to ammonium further fueled ammonium accumulation, driving Microcystis blooms in the summer. In Lake Zhiyin, fish bait deposits (high nitrogen, similar to Dolichospermum detritus) also caused Microcystis blooms. Detritus from Microcystis decomposition then produced high levels of labile dissolved organic phosphorus, inducing phosphatase activity and increasing soluble reactive phosphorus concentrations from September to April in Lakes Tangxun and Zhiyin. The high refractory DOM from macrophytes in Lake Houguan led to insufficient nutrient availability, leading to nutrient mutualism between algae and bacteria. The high levels of labile dissolved organic carbon from terrestrial detritus in Lake Yandong increased bacterial biomass and production, resulting in low chlorophyll content due to the competitive relationship between algal and bacterial nutrient requirements. Therefore, different DOM compositions induce unique connections among available nutrients, algae, and bacteria in the microbial loop.

Unified Modeling Approach for Quantifying the Proton and Metal Binding Ability of Soil Dissolved Organic Matter

Soil dissolved organic matter (DOM) is composed of a mass of complex organic compounds in soil solutions and significantly affects a range of (bio)geochemical processes in soil environment. However, how the chemical complexity (i.e., heterogeneity and chemodiversity) of soil DOM molecules affects their proton and metal binding ability remains unclear, which limits our ability for predicting the environmental behavior of DOM and metals. In this study, we developed a unified modeling approach for quantifying the proton and metal binding ability of soil DOM based on Cu titration experiments, Fourier transform ion cyclotron resonance mass spectrometry data, and molecular modeling method. Although soil DOM samples from different regions have enormously heterogeneous and diverse properties, we found that the molecules of soil DOM can be divided into three representative groups according to their Cu binding capacity. Based on the molecular models for individual molecular groups and the relative contributions of each group in each soil DOM, we were able to further develop molecular models for all soil DOM to predict their molecular properties and proton and metal binding ability. Our results will help to develop mechanistic models for predicting the reactivity of soil DOM from various sources.

Brownification of freshwater promotes nitrogen-cycling microorganism growth following terrestrial material increase and ultraviolet radiation reduction

Brownification is an increasingly concerning phenomenon faced by aquatic ecosystems in the changing environments, and the microbiome plays an irreplaceable role in material circulation and food web construction. Insight into the influence of brownification on microbial communities is crucial from an ecological standpoint. In this study, we simulated brownification using a the mesocosm system and explored the relationship between the characteristics of microbial communities and brownification using excitation-emission matrix (EEM) fluorescence spectroscopy and ultraviolet (UV) spectroscopy combined with high-throughput amplicon sequencing techniques. The results showed that brownification reduced the richness of the microbial community and selectively promoted the growth of nitrogen-cycling microorganisms, including hgcI_clade, Microbacteriaceae, and Limnohabitans. Brownification affected microbial communities by altering the carbon source composition and underwater spectrum intensity; UV, blue, violet, and cyan light were significantly (p < 0.05) correlated with microbial community richness, and random forest analysis revealed that UV, C1 (microbial humic-like), and C3 (terrestrial humic-like) were the major factors significantly influencing microbiome variation. We found that brownification affected microorganisms in shallow lakes, especially nitrogen cycling microorganisms, and propose that controlling terrestrial material export is an effective strategy for managing freshwater brownification.

Inhibition of methylmercury uptake by freshwater phytoplankton in presence of algae-derived organic matter

As the first step of methylmercury (MeHg) entry into the aquatic food webs, MeHg uptake by phytoplankton is crucial in determining the final human MeHg exposure risks. MeHg availability to plankton is regulated by dissolved organic matter (DOM) in the water, while the extent of the impacts can vary largely based on the sources of DOM. Here, we investigated impacts of DOM sources on MeHg bioconcentration by three freshwater phytoplankton species (i.e. S. quadricauda, Chlorella sp., Microcystis elabens) in the laboratory system. We found that algae-derived DOM would prohibited the cellular MeHg bioconcentration by a percent up to 77-93%, while the soil-derived DOM didn't show similar inhibition effects. DOM characterization by the excitation‒emission matrices, Fourier transform infrared spectrum, ultra‒high performance liquid chromatography‒tandem quadrupole time of flight mass spectrometry shown that the molecular size of S-containing compound, rather than thiol concentration, has played a crucial role in regulating the MeHg uptake by phytoplankton. Climate change and increasing nutrient loadings from human activities may affect plankton growth in the freshwater, ultimately changing the DOM compositions. Impacts of these changes on cellular MeHg uptakes by phytoplankton should be emphasized when exploring the aquatic Hg cycling and evaluating their risks to human beings and wild life.