Molecular composition of dissolved organic matter across diverse ecosystems: Preliminary implications for biogeochemical cycling

Simultaneous Elucidation of the Chemodiversity of Dissolved Organic Matter and Quantitation of Trace Organic Contaminant Sucralose by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has become a cutting-edge technique for molecular characterization of dissolved organic matter (DOM) and trace organic contaminants. Nevertheless, FT-ICR MS is rarely applied to simultaneously elucidate DOM chemodiversity and quantify the concentrations of trace organic contaminants. Compared to conventional solid-phase extraction (SPE), pH-dependent fractionation of DOM molecules using a sequential SPE strategy significantly enhanced the recovery of dissolved organic carbon and yielded more chemical formulas for DOM from environmental samples (p < 0.05). The sequential SPE at circumneutral pH provided exclusive isolation of some hydrogen-rich and oxygen-poor molecules, caused a 5-fold reduction in matrix effects, and improved the detection limits for organic contaminants. The chemical structure of a typical wastewater indicator, sucralose, was successfully validated using FT-ICR MS analysis with tandem mass spectrometry and hydrogen/deuterium exchange measurements. With the standard addition method, the limits of detection and quantitation of FT-ICR MS for sucralose in DOM extracts isolated at circumneutral pH were estimated to be 0.26 and 0.87 μg/L, respectively. The concentrations of sucralose in a representative urban lake measured by FT-ICR MS (e.g., 0.9-4.1 μg/L) were comparable to those determined by liquid chromatography coupled to an Orbitrap mass analyzer. The findings of this study expand application of FT-ICR MS in environmental research by highlighting its potential for identifying trace organic contaminants, quantifying their concentrations, and elucidating the chemodiversity of DOM to inform contaminant sources in aquatic systems.

Long-term trends and rising levels of refractory dissolved organic matter in a suburban plateau lake: Impacts of hydrological changes

Dissolved organic matter (DOM) characteristics and concentrations in lakes are strongly associated with terrestrial input, phytoplankton dynamics, and physicochemical environment. Hydrological conditions can affect multiple aspects of the lake environment, thereby interfering with DOM cycling. This study investigates the long-term trends and drivers of DOM accumulation in Lake Erhai, a subtropical plateau lake in southwestern China, focusing on the role of hydrological processes in driving its accumulation and persistence. By analyzing data from 1992 to 2023-including bulk chemical analysis, 3D-EEM fluorescence spectroscopy, degradation experiments and bayesian structural equation modeling (BSEM), it is concluded that a 174 % increase in water residence time (WRT), from 2.8 years to 7.8 years, driven by reduced inflow and outflow volumes, has promoted the accumulation of refractory DOM (RDOM), raising chemical oxygen demand (CODMn) and presenting substantial challenges challenges to water quality management. Degradation experiments revealed limited biodegradability of DOM (15 % over 28 days) and minimal photodegradation (13.5 % over 72 h), with more than 80 % remaining in a refractory state. Spectroscopic analyses revealed compositional shifts in DOM with prolonged WRT, characterized by decreased humic-like substances and increased protein-like compounds, indicating a progressive transition from allochthonous to autochthonous DOM dominance. BSEM analysis identified a significant temporal shift in DOM drivers: during the initial phase (1992-2010), human activity pressure (HAP) and riverine input quality (RIQ) collectively explained 70 % of the variance, with natural drivers contributing less than 20 %; whereas in the subsequent phase (2010-2023), anthropogenic influences diminished as hydrological and climatic factors became predominant, with hydrological regime (HR) and climatic factors (CF) jointly accounting for 87 % of RDOM variance, reflecting a transition from anthropogenic to climate-hydrological driven accumulation patterns. This research underscores the critical role of hydrological residence time in determining DOM composition, sources, and persistence in plateau lakes following partial decoupling of external pollution sources. The findings highlight the dual influence of climate and hydrology on lakes experiencing significant pressures from reduced water resources and increasing water demand, challenging conventional management strategies focused exclusively on external nutrient control. The case of Lake Erhai demonstrates the necessity for integrated management approaches that address both external and internal DOM dynamics to support sustainable water quality and ecosystem integrity.

Spatial and Temporal Variations in Aquatic Organic Matter Composition in UK Surface Waters

Drinking water is becoming more difficult to treat, especially in the UK, due to the changing concentration and composition of aquatic dissolved organic matter (DOM). The spatial and temporal variations in the DOM composition are not well understood. This study investigated how DOM composition varies along a north/south gradient in the UK, over four years, and between headwaters and reservoirs. There were trends in DOM composition metrics from north to south; carbohydrate and peptide-like compounds were lower in northern sites, while lipid-like compounds were lower further south, suggesting different sources of DOM in north/south catchments. DOM collected in Autumn 2021, after a Summer of low rainfall, was more aromatic, less oxidized, and more diverse than DOM collected in 2018-2020. Decreased lipid content and increased oxy-aromatic content occurred in Autumn, at the end of the plant growing season, when increased rainfall rewets catchments and mobilizes soil OM into surface waters. These seasonal changes in DOM composition coincide with increased DOM concentrations in raw drinking water, leading to more challenges for drinking water treatment, especially as climate change alters rainfall distribution in the UK.

Environmental drivers of dissolved organic matter composition across central European aquatic systems: A novel correlation-based machine learning and FT-ICR MS approach

Dissolved organic matter (DOM) present in surface aquatic systems is a heterogeneous mixture of organic compounds reflecting its allochthonous and autochthonous organic matter (OM) sources. The composition of DOM is determined by environmental factors like land use, water chemistry, and climate, which influence its release, movement, and turnover in the ecosystem. However, studying the impact of these environmental factors on DOM composition is challenging due to the dynamic nature of the system and the complex interactions of multiple environmental factors involved. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enables detailed molecular-level analysis of DOM, allowing the identification of thousands of individual molecular formulas potentially representing unique markers for its "molecular history". The combination of FT-ICR MS with machine-learning techniques is promising to unravel DOM-environment interactions owing to their capacity to capture complex non-linear relationships. We present a novel unsupervised multi-variant machine-learning approach, aiming to model correlation coefficients as robust indicators of how changes in environmental factors (e.g., the concentration of nutrients or the land use) result in changes in the molecular formula descriptors of DOM (i.e., aromaticity index or hydrogen to carbon ratio). We applied this approach to an environmental data set collected from 84 sites across central Europe exhibiting a broad range of water chemistry and land uses. Our model revealed an increase in molecular mass and aromaticity of DOM in densely forested regions as compared to open urban areas, where DOM was characterized by higher concentrations of dissolved ions and increased microbial degradation, leading to smaller and more aliphatic DOM. Our findings highlight the substantial human impact on climate change, as evidenced by the accelerated photochemical and microbial degradation of DOM, which consequently enhances greenhouse gas emissions and exacerbates global warming.

HRMS-Viewer: Software for High Resolution Mass Spectrometry Formula Assignment and Data Visualization

Accurately assigning formulas to thousands of peaks generated by ultrahigh resolution mass spectrometry in a single analysis poses a significant challenge, especially when dealing with diverse molecular compositions across complex mixtures. This difficulty is further compounded by the lack of an established universal mass calibration and formula assignment method. We have developed HRMS-Viewer, a Python-based software tool designed for processing ultrahigh resolution mass spectrometry data specific to petroleum and natural organic matter (NOM). The software employs an efficient, experience-driven approach for small molecule formula assignment, offering a streamlined yet intuitive workflow. Key features include advanced noise reduction, automatic or manual recalibration, real-time visualization of formula assignment results, and options for manual correction. During the workflow, HRMS-Viewer enables the visualization and manual control of critical steps including noise reduction, recalibration, peak identification, and data review.

Revealing the mobilization and age of estuarine dissolved organic matter during floods using radiocarbon and molecular fingerprints

Estuaries significantly affect the transport of dissolved organic matter (DOM) from land to ocean. While the transport and composition of estuarine DOM have been extensively studied, the direct link between DOM chemistry and its age remains unclear, limiting a comprehensive understanding of the dynamics and fate of estuarine DOM under severe conditions (e.g., floods). This study applied radiocarbon and ultrahigh-resolution mass spectrometry analysis to investigate the correlation between DOM chemistry and apparent radiocarbon age of 102 samples collected from the Yangtze River Estuary during both non-flood and flood periods. The results showed that young estuarine DOM are characterized by low-molecular-weight, unsaturated molecules, while aged estuarine DOM are relatively saturated with high-molecular-weight molecules. Phosphorus and nitrogen-containing compounds were key to DOM aging, potentially increasing the lability of aged DOM. Floods significantly impact DOM by introducing more labile aged DOM and young terrestrial DOM. Furthermore, floods enhanced the flux of aged DOM transported to the East China Sea by approximately 1.4 times. Our findings contribute to the study of estuarine DOM and its response during severe floods. Additionally, incorporating apparent radiocarbon age evidence improves the understanding of terrigenous DOM and its fate in large river estuaries before it contributes to the ocean carbon reservoir.

Seasonal hydrological variation impacts nitrogen speciation and enhances bioavailability in plateau lake sediments

Global warming has intensified the distinction between dry and wet seasons in monsoonal climates. The synergistic effect of high temperatures and rainfall during the wet season promotes the release of endogenous nitrogen (N) and eutrophication within lake ecosystems. However, the seasonal variations in sediments N speciation and bioavailability, and their intrinsic connection to release potential, remain unclear. This study employed sequential extraction method and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) to characterize extractable N (Ex-N) in Erhai Lake sediments during dry and wet seasons. The results indicated that ion-exchangeable organic form N (IEF-ON) serves as a substrate for microbial mineralization, with the highest proportion of protein-like substances (26.2%). The influx of N-containing polysaccharides and poly-N glycoproteins during the wet season further increased its bioavailability. Algal-derived N exists as the weak acid-extractable organic form N (WAEF-ON), which has the highest lipid proportion (11.7%) and the lowest double bond equivalent (DBE) values. Overall, elevated water temperatures and N input during the wet season accelerate both the mineralization rate of organic N (ON) and the content of labile N components. This potentially triggers a "priming effect" that could further activate the refractory N components in the sediments. Additionally, the wet season reduces sediment pH and redox potential, making WAEF-ON and strong alkali-extractable form N (SAEF-N) more labile and susceptible to release. This study reveals the adverse effects of seasonal variations on N sequestration in lake sediments, complicating the control of endogenous pollution release under the backdrop of climate change.

Marine Recalcitrant Dissolved Organic Matter Gained by Processing at Sandy Subterranean Estuaries

The sandy subterranean estuary (STE) connecting fresh groundwater to saline sea water is characterized by strong geochemical (salinity, redox, and pH) gradients, with evidence emerging for its role as a hot spot for consumption of labile substrates. This inspired us to conduct a study to evaluate whether this holds true for dissolved organic matter (DOM), especially given the still mysterious origin of marine recalcitrant DOM. Here, characterization of DOM of 21 groundwater samples (depth 1-13 m, salinity 3.9‰ to 32.4‰) across a 65 m transect of an STE located in coastal Guangdong, China, has found systematic biotransformation toward "recalcitrant" carboxyl-rich alicyclic molecules (CRAM). The fraction of CRAM (%CRAM) increases from 33.1% to 76.7% with an increasing degree of DOM degradation and increasing salinity. Further, processing of DOM, including the more "biolabile" DOM with lower %CRAM released from aquitard, is more active under oxic conditions than under reducing conditions. Given the large quantities of sea water that recirculates through the sandy STEs globally, the amount of "recalcitrant" DOM (RDOM) entering the ocean after processing is likely to be considerable. While more studies are needed, the ocean can gain "recalcitrant" CRAM-like compounds in this way.

Unveiling the global dynamics of dissolved organic carbon in aquatic ecosystems: Climatic and anthropogenic impact, and future predictions

Dissolved organic carbon (DOC) and its biodegradability (BDOC%) in aquatic ecosystems significantly impact the global carbon cycle, varying greatly across rivers, lakes, and estuaries due to environmental and anthropogenic factors. However, a thorough understanding of these variations is still lacking. This study investigated the interactions between climate, hydrology, physiography, soil, land cover, and human activity on DOC dynamics in rivers, lakes, and estuaries. Utilizing a robust dataset comprising 744 global data points for DOC concentrations (0.18-29.33 mg/L) and 341 samples for BDOC% (0.44 %-81.12 %), spanning a wide range of geographic and climatic gradients across six continents, machine learning techniques were employed to elucidate the relationships between DOC and BDOC% and environmental and anthropogenic factors and to develop predictive models for global DOC and BDOC storage. Results showed that climate primarily affected DOC and BDOC% levels, with other factors varying by ecosystem type. In rivers, soil and human activity had positive influences, while in lakes, hydrology had a positive effect and human activity had a negative one. In estuaries, soil positively impacted the levels of DOC and BDOC%, whereas human activity had a negative effect. Furthermore, we created separate random forest models for DOC and BDOC% based on different factors in each aquatic ecosystem (R2 = 0.50-0.89), and applied to data of environmental and anthropogenic factors worldwide, predicting DOC and BDOC storage for 181 countries. Notably, large countries like Canada, Russia, the United States (U.S.), Brazil, and China accounted for 76.07 % and 51.56 % of the total global DOC and BDOC storage, respectively. Storage prediction models under future climate scenarios indicated significant impacts in Europe under the high fossil fuel use scenario. Thus, prioritizing high-storage, climate-vulnerable areas is essential for effective climate change strategies, aiding in the protection of aquatic ecosystems, maintaining the global carbon balance, and promoting sustainable development.

Evaluating the role of recalcitrant dissolved organic matter in bacterial community dynamics in urbanized freshwater ecosystems

Dissolved organic matter (DOM) and recalcitrant dissolved organic matter (RDOM) play distinct roles in shaping microbial communities. However, characterizing these roles is difficult, especially in ecosystems subjected to varying degrees of anthropogenic influence. This study investigated the molecular compositions and ecological impacts of DOM and RDOM in the Fen River, Shanxi Taiyuan, comparing pristine upstream regions with highly urbanized downstream areas. Using 16S rRNA gene sequencing and LC-MS-based metabolomics, we observed significant shifts in microbial community composition, diversity, and metabolic functions. Upstream communities, characterized by higher diversity, were dominated by Bacteroidota, Proteobacteria, and Cyanobacteria, while downstream communities, influenced by pollution, exhibited increased expression of genes related to amino acid metabolism. Fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that upstream DOM contained higher proportions of complex, high molecular weight compounds, including significant proportions of carboxyl-rich alicyclic molecules (CRAM) and island of stability (IOS) compounds, which play key roles in long-term carbon storage and microbial carbon sequestration. In contrast, downstream DOM was characterized as having lower aromaticity and more saturated compounds, with reduced proportions of CRAM and IOS, reflecting the impact of anthropogenic activities. These findings underscored the critical roles of CRAM and IOS in regulating DOM stability and microbial communities, further highlighting the need for targeted pollution control strategies to preserve ecosystem function in urbanized water bodies.

Characteristics of organic matter driven by Eichhornia crassipes during co-contamination with per(poly)fluoroalkyl substances (PFASs) and microplastics (MPs)

Co-contamination with MPs and PFASs has been recorded, particularly in surface-water environments. Floating macrophyte microcosms are an important part of the surface water ecosystem, and dissolved organic matter (DOM) driven by floating macrophytes (FMDDOM) is critical for maintaining material circulation. However, knowledge gaps remain regarding the impact of MPs and PFASs co-pollution on FMDDOM. An greenhouse simulation experiment was conducted in this study to investigate the effects of four PFASs, perfluorooctanoic acid (PFOA), perfluoro-octane-sulfonic acid (PFOS), perfluoro-2-methyl-3-oxahexanoic acid (Gen X), and potassium 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B), on FMDDOM sourced from Eichhornia crassipes (E. crassipes), a typical floating macrophyte, in the presence and absence of polystyrene (PS) MPs. Four PFASs increased FMDDOM release from E. crassipes, leading to a 32.52-77.49 % increase in dissolved organic carbon (DOC) levels. PS MPs further increased this, with results ranging from -21.28 % to 26.49 %. Based on the parallel factor analysis (PARAFAC), FMDDOM was classified into three types of fluorescent components: tryptophan-like, humic-like, and tyrosine-like compounds. Contaminants of MPs and PFASs modified the relative abundance of these three components. Protein secondary structure analysis showed that fluorocarbon bonds tended to accumulate on the α-helix of proteins in FMDDOM. The relative abundance of fluorescent and chromophorous FMDDOMs varied from 0.648 ± 0.044 to 0.964 ± 0.173, indicating that the photochemical structures of the FMDDOM were modified. FMDDOM exhibits decreased humification and increased aromaticity when contaminated with MPs and PFASs, which may be detrimental to the geochemical cycling of carbon. This study offers a theoretical basis for assessing the combined ecological risks of MPs and PFASs in floating macrophyte ecosystems.

Thermodynamics and explainable machine learning assist in interpreting biodegradability of dissolved organic matter in sludge anaerobic digestion with thermal hydrolysis

Dissolved organic matter (DOM) is essential in biological treatment, yet its specific roles remain incompletely understood. This study introduces a machine learning (ML) framework to interpret DOM biodegradability in the anaerobic digestion (AD) of sludge, incorporating a thermodynamic indicator (λ). Ensemble models such as Xgboost and LightGBM achieved high accuracy (training: 0.90-0.98; testing: 0.75-0.85). The explainability of the ML models revealed that the features λ, measured m/z, nitrogen to carbon ratio (N/C), hydrogen to carbon ratio (H/C), and nominal oxidation state of carbon (NOSC) were significant formula features determining biodegradability. Shapley values further indicated that the biodegradable DOM were mostly formulas with λ lower than 0.03, measured m/z value higher than 600 Da, and N/C ratios higher than 0.2. This study suggests that a strategy based on ML and its explainability, considering formula features, particularly thermodynamic indicators, provides a novel approach for understanding and estimating the biodegradation of DOM.

Comparing molecular signatures of dissolved organic matter (DOM) in four large freshwater lakes differing in hydrological connectivity to the Changjiang River

Freshwater lakes serve as active conduits for processing terrestrial dissolved organic matter (DOM), playing a crucial role in global carbon cycle. Little attention has been paid to how hydrological connectivity to a large river would affect the molecular signatures of DOM in lakes. Here, we systematically characterized and compared the molecular signatures of DOM in surface waters of four large freshwater lakes in the middle and lower Changjiang River basin that are directly connected to the river (Lake Dongting and Lake Poyang, referred to as Lakeconnected) or indirectly connected to the river (Lake Chao and Lake Tai, referred to as Lakenonconnected). The DOM in Lakeconnected was found to have similar total organic carbon (TOC)-normalized contents and characteristics of lignin phenols to the DOM in surface waters from the upstream Changjiang river, indicating allochthonous/terrestrial sources from riverine inputs. As indicated by the UV-vis and fluorescence analyses, the DOM in Lakeconnected overall had higher aromaticity and larger average molecular weight as well as stronger allochthonous feature compared to the DOM in Lakenonconnected. Consistently, the FT-ICR MS analysis revealed that the DOM in Lakeconnected had higher molecular diversity, higher unsaturation degree, and larger proportions of highly aromatic compounds. In contrast, the DOM in Lakenonconnected had larger proportions of lipids and peptide-like structures, but lower proportions of aromatic compounds, which could be ascribed to the enhanced autochthonous production and photodegradation due to pollution and eutrophication as well as longer water residence time. The results highlight the strong impacts of the hydrological connectivity to a large river on the molecular signatures of lake DOM. CAPSULE: The hydrological connectivity of the lakes to the Changjiang River has strong impacts on the molecular signatures of lake DOM.

Long-term effects of nitrogen and phosphorus fertilization on profile distribution and characteristics of dissolved organic matter in fluvo-aquic soil

Dissolved organic matter (DOM) drives numerous biogeochemical processes (e.g. carbon cycling) in agro-ecosystems and is sensitive to fertilization management. Nevertheless, changes in the quantity and quality of DOM in the vertical soil profile following long-term continuous nitrogen (N) and phosphorus (P) inputs remain unclear. In this study, the contents and optical characteristics of DOM along a 2-m soil profile were investigated using a 40-year wheat/maize rotation combined with experiments using different N and P fertilization rates in the North China Plain. The results revealed that the dissolved organic carbon (DOC) content decreased with an increase in soil depths. Compared with that in the control (no fertilization), 40-year N, P, and N + P additions increased the soil DOC content by 26%-69%, except for 270-kg N, and 67.5-kg P treatments. N + P application resulted in higher DOC contents than N-alone and P-alone applications. N, P, and N + P inputs increased or did not affect the aromaticity and hydrophobicity of DOM at 0-40 cm but reduced them from 40 to 200 cm. Compared with that in the control, N, P, and N + P inputs enhanced the content of humic acid-like substances (C1+C2+C3+C4) and decreased the content of protein-like substance (C5). C1 was the dominant component among the five DOM, representing the microbial humic component. Optical indices also indicated that soil DOM primarily originated from microbial sources. Nutrient addition accelerated transformation between complex C1 and simple C5 via promoting microbial activities. These results imply that N and P fertilizers increased the DOM content and altered its composition, thereby potentially affecting the stability of soil organic matter in the agroe-cosystems.

Preparation of Simple Bicyclic Carboxylate-Rich Alicyclic Molecules for the Investigation of Dissolved Organic Matter

Dissolved organic matter (DOM) is a vast and complex chemical mixture that plays a key role in the mediation of the global carbon cycle. Fundamental understanding of the source and fate of oceanic organic matter is obscured due to poor definition of the key molecular contributors to DOM, which limits accurate sample analysis and prediction of the Earth's carbon cycle. Previous work has attempted to define the components of the DOM through a variety of chromatographic and spectral techniques. However, modern preparative and analytical methods have not isolated or unambiguously identified molecules from DOM. Therefore, previously proposed structures are based solely on the mixture's aggregate properties and do not accurately describe any true individual molecular component. In addition to this, there is a lack of appropriate analogues of the individual chemical classes within DOM, limiting the scope of experiments that probe the physical, chemical, and biological contributions from each class. To address these problems, we synthesized a series of analogues of carboxylate-rich alicyclic molecules (CRAM), a molecular class hypothesized to exist as a major contributor to DOM. Key analytical features of the synthetic CRAMs were consistent with marine DOM, supporting their suitability as chemical substitutes for CRAM. This new approach provides access to a molecular toolkit that will enable previously inaccessible experiments to test many unproven hypotheses surrounding the ever-enigmatic DOM.

Spatial variations of DOM in a diverse range of lakes across various frozen ground zones in China: Insights into molecular composition

Dissolved organic matter (DOM) plays a significant role in aquatic biogeochemical processes and the carbon cycle. As global climate warming continues, it is anticipated that the composition of DOM in lakes will be altered. This could have significant ecological and environmental implications, particularly in frozen ground zones. However, there is limited knowledge regarding the spatial variations and molecular composition of DOM in lakes within various frozen ground zones. In this study, we examined the spatial variations of in-lake DOM both quantitatively, focusing on dissolved organic carbon (DOC), and qualitatively, by evaluating optical properties and conducting molecular characterization using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Lakes in cold regions retained more organic carbon compared to those in warmer regions, the comparison of the mean value of DOC concentration of all sampling sites in the same frozen ground zone showed that the highest mean lake DOC concentration found in the permafrost zone at 21.4 ± 19.3 mg/L. We observed decreasing trends in E2:E3 and MLBL, along with increasing trends in SUVA254 and AImod, along the gradually warming ground. These trends suggest lower molecular weight, reduced aromaticity, and increased molecular lability of in-lake DOM in the permafrost zone compared to other frozen ground zones. Further FT-ICR MS characterization revealed significant molecular-level heterogeneity of DOM, with the lowest abundance of assigned DOM molecular formulas found in lakes within permafrost zones. In all studied zones, the predominant molecular formulas in-lake DOM were compounds consisted by CHO elements, accounting for 40.1 % to 63.1 % of the total. Interestingly, the percentage of CHO exhibited a gradual decline along the warming ground, while there was an increasing trend in nitrogen-containing compounds (CHON%). Meanwhile, a substantial number of polyphenols were identified, likely due to the higher rates of DOM mineralization and the transport of terrestrial DOM derived from vascular plants under the elevated temperature and precipitation conditions in the warming region. In addition, sulfur-containing compounds (CHOS and CHNOS) associated with synthetic surfactants and agal derivatives were consistently detected, and their relative abundances exhibited higher values in seasonal and short-frozen ground zones. This aligns with the increased anthropogenic disturbances to the lake's ecological environment in these two zones. This study reported the first description of in-lake DOM at the molecular level in different frozen ground zones. These findings underline that lakes in the permafrost zone serve as significant hubs for carbon processing. Investigating them may expand our understanding of carbon cycling in inland waters.