Dissolved Organic Matter Biodegradation Research Articles

Degradation mechanism of organic contaminants in complex contaminated groundwater in landfill sites with oxygen micro-nano-bubbles aeration

Complex contaminated groundwater in landfill sites poses a significant health threat. Oxygen micro-nano-bubbles (MNBO2) aeration shows promise for remediation of such complex contaminated groundwater, while its mechanism for degradation contaminants remained unclear. This study investigated the degradation mechanism of organic contaminants in complex contaminated groundwater in landfill sites with MNB-O2 aeration by contaminants degradation process and efficiency, dissolved organic matter (DOM) transformation as well as enzyme activity and microorganism. The test results indicated that after 105th day of MNB-O2 aeration, the removal efficiencies of 5-day biological oxygen demand, chemical oxygen demand and α(254) in the water sample were 92.64 %, 50.56 % and 77.03 %, respectively. Additionally, the refractory DOM increased by 3 % to 11 %, while biodegradable DOM decreased by 57.63 %. MNB-O2 aeration enhanced the activity of aerobic microorganisms but reduced the diversity, evenness and richness of microbial species in the soil sample. Moreover, it altered the microbial community of the soil sample and enhanced oxidation of organic substance in the water sample. MNB-O2 aeration enhanced the biodegradation of the water sample, and its enhanced volatilization and strong chemical oxidation also contributed to organic contaminants degradation in the water sample. Collectively, the finding of this work will provide theoretical support for the practical application of MNB-O2 aeration in remediation of complex contaminated groundwater in landfill sites.

Response of dissolved organic matter and disinfection by-product precursors to algal blooms and thermal stratification in deep reservoirs.

Algal bloom contribute substantially to dissolved organic matter (DOM) and disinfection by-product (DBP) precursors in deep reservoirs, threatening drinking water safety. However, the variations in DOM and DBP precursors in deep-water reservoirs during algal bloom remain unclear. UV and fluorescence spectroscopy and chlorination experiments were used to analyze the variations in DOM and DBP precursors during algal bloom in the Sanhekou Reservoir. Before algal bloom, the DOM and DBP precursors decreased due to biodegradation. After algal bloom, the DOM and DBP precursors increased by 48.3% and 86.9% due to algae producing protein-like compounds. Notably, the algal bloom produced a range of nitrogenous compounds that significantly promote the formation of trichloronitromethane, a major contributor to the mammalian cytotoxicity associated with DBPs. In addition, the heterogeneous matrix led to the stratification of DOM and DBP precursors. The surface water (0-5 m) was more vulnerable to algae, with protein-like components being much higher than in other layers, while humic and fulvic-like components were much lower. However, high temperatures and sufficient oxygen conditions accelerated the biodegradation of DOM and DBP precursors, resulting in significantly lower levels of DOM and DBP precursors in the surface water compared to other layers (p<0.05). This study provides insights into the variations and the drivers in DOM and DBP precursors during algal bloom, essential for developing water intake strategies in similar water reservoirs.

Enhanced removal of biolabile oxygen-depleted dissolved organic matter by coagulation-adsorption process Improves biological stability of reclaimed water

Ensuring biological stability, signifying the maintenance of an unchanged bacterial concentration and composition during water distribution, is essential for mitigating the microbial hazards of reclaimed water. However, the interplay between chlorine-resistant bacteria (CRB) regrowth and molecular transformation of biodegradable organic matter in chlorinated reclaimed water distribution system remain unclear. This work investigated the transformation of dissolved organic matter (DOM) in reclaimed water treated by coagulation and by coagulation-adsorption over a 20-day incubation following chlorination, as well as its interactions with CRB through high-resolution mass spectrometer and high-throughput sequencing. The DOM biotransformation profile and DOM-bacteria interaction network collectively revealed the overall substrate preference and metabolic pattern of CRB (e.g., Methylobacterium-Methylorubrum, Acidovorax, and Sphingomonas), which evinced a propensity for utilizing oxygen-depleted DOM (O/C < 0.5) and producing oxygen-enriched DOM (O/C > 0.4). The incorporation of powdered activated carbon during coagulation markedly decreased the level of biodegradable DOM (4.33 mg C/L) and subsequently retarded the regrowth of CRB by one day compared to coagulation alone, attributable to the selective adsorption of DOM molecules with low O/C onto the activated carbon. This work underscores the critical role of the enhanced removal of oxygen-depleted DOM in ameliorating the biological stability of waters.

Improving biodegradability of dissolved organic matter (DOM) in old landfill leachate by electrochemical pretreatment: The effect mechanism of polarity

Enhancing the biodegradability of old landfill leachate is vital for the efficient treatment or resource utilization of municipal solid waste. Electrochemical pretreatment emerges as a promising technology for transformation of refractory dissolved organic matter (DOM). However, the specific impact of polarity on improving biodegradability of DOM remains unclear. In this study, a divided electrolyzer was used to explore the changes in the biodegradability of DOM in old landfill leachate during electrolysis. Meanwhile, the correlation mechanism between BOD5 variation and DOM evolution was explored by spectroscopy and Maldi-TOF-MS analysis. Results shown that different polarities all have positive effect on enhancing the biodegradability of DOM, while the structural changes related with BOD5 are depending on the polarity. In the anode chamber, electrochemical oxidation (EO) generates and eliminates carboxyl groups. Additionally, EO concurrently eliminates humic-like substances, which are challenging for microorganisms to degrade, and protein-like substances, which are easily degradable by microorganisms. This creates a competitive mechanism that coexist the promotion and inhibition for biodegradability. In the cathode chamber, electrochemical reduction (ER) transforms DOM components, accumulating easily useable protein components for microorganisms. Kinetic studies show that EO related BOD5 changes are aptly described by a competition model, considering both generation and removal of bioavailable components. ER related BOD5 changes suit a pseudo-first-order kinetic model. These insights into the transformation of old leachate DOM support the development of methods predicting BOD5 evolution, crucial for future process optimization.

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.

Long-term straw return promotes accumulation of stable soil dissolved organic matter by driving molecular-level activity and diversity

Agricultural management practices play pivotal roles in affecting soil organic matter formation pathways, particularly through changes in soil dissolved organic matter (DOM). The potential effects of long-term straw return on the molecular-level activity and transformations of DOM were investigated in this study. Over 9 years, the soil DOM from straw return had high stability, unsaturation degree, oxidability, and strong aromaticity. The relative abundance of biologically refractory DOM components (i.e., lignin-, condensed aromatic-, and tannin-like formulas) increased by 4 %-33 %, whereas the relative abundance of biodegradable DOM components (i.e., lipids and protein/amino sugar-like formulas) decreased by 15 %-23 % under straw return. Furthermore, the positive effects of straw return on the intergroup transformations of biologically refractory compounds favored the persistence of recalcitrant organic matter in the soil. Importantly, intragroup transformations of recalcitrant-active molecules and biologically refractory compounds were critical pathways for stable DOM accumulation under straw return, and these were mainly driven by thermodynamically limited processes. Thus, our results indicated that long-term straw return promoted the persistence of stabilized soil DOM by driving molecular-level activity and diversity.

Untangling the interplay of dissolved organic matters variation with microbial symbiotic network in sludge anaerobic fermentation triggered by various pretreatments

Various pretreatments are commonly adopted to facilitate dissolved organic matter (DOM) release from waste activated sludge (WAS) for high-valued volatile fatty acids (VFAs) promotion, while the interplay impact of DOM dynamics transformation on microbial population and metabolic function traits is poorly understood. This work constructed “DOM-microorganisms-metabolism-VFAs” symbiotic ecologic networks to disclose how DOM dynamics variation intricately interacts with bacterial community networks, assembly processes, and microbial traits during WAS fermentation. The distribution of DOM was altered by different pretreatments, triggering the release of easily biodegradable compounds (O/C ratio > 0.3) and protein-like substance. This alteration greatly improved the substrates biodegradability (higher biological index) and upregulated microbial metabolism capacity (e.g., hydrolysis and fatty acid synthesis). In turn, microbial activity modifications augment substance metabolism level and expedite the conversion of highly reactive compounds (proteins-like DOM) to VFAs, leading to 1.6–4.2 fold rise in VFAs generation. Strong correlations were found between proteins-like DOM and topological properties of DOM-bacteria associations, suggesting that high DOM availability leads to more intricate ecological networks. A change in the way communities assemble, shifting from stronger uniform selection in pH10 and USp reactors to increased randomness in heat reactor, was linked to DOM composition alterations. The ecologic networks further revealed metabolic synergy between hydrolytic-acidogenic bacteria (e.g., Bacteroidota and Firmicutes) and biodegradable DOM (e.g., proteins and amino sugars) leading to higher VFAs generation. This study provides a deeper knowledge of the inherent connections between DOM and microbial traits for efficient VFAs biosynthesis during WAS anaerobic fermentation, offering valuable insights for effective WAS pretreatment strategies.

Treeline displacement may affect lake dissolved organic matter processing at high latitudes and altitudes

Climate change induced shifts in treeline position, both towards higher altitudes and latitudes induce changes in soil organic matter. Eventually, soil organic matter is transported to alpine and subarctic lakes with yet unknown consequences for dissolved organic matter (DOM) diversity and processing. Here, we experimentally investigate the consequences of treeline shifts by amending subarctic and temperate alpine lake water with soil-derived DOM from above and below the treeline. We use ultra-high resolution mass spectrometry (FT-ICR MS) to track molecular DOM diversity (i.e., chemodiversity), estimate DOM decay and measure bacterial growth efficiency. In both lakes, soil-derived DOM from below the treeline increases lake DOM chemodiversity mainly through the enrichment with polyphenolic and highly unsaturated compounds. These compositional changes are associated with reductions in bulk and compound-level DOM reactivity and reduced bacterial growth efficiency. Our results suggest that treeline advancement has the potential to enrich a large number of lake ecosystems with less biodegradable DOM, affecting bacterial community function and potentially altering the biogeochemical cycling of carbon in lakes at high latitudes and altitudes.

Molecular insights into the microbial degradation of sediment-derived DOM in a macrophyte-dominated lake under aerobic and hypoxic conditions

The mineralization of dissolved organic matter (DOM) in sediments is an important factor leading to the eutrophication of macrophyte-dominated lakes. However, the changes in the molecular characteristics of sediment-derived DOM during microbial degradation in macrophyte-dominated lakes are not well understood. In this study, the microbial degradation process of sediment-derived DOM in Lake Caohai under aerobic and hypoxic conditions was investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and metagenomics. The results revealed that the microbial degradation of sediment-derived DOM in macrophyte-dominated lakes was more intense under aerobic conditions. The microorganisms mainly metabolized the protein-like substances in the macrophyte-dominated lakes, and the carbohydrate-active enzyme genes and protein/lipid-like degradation genes played key roles in sediment-derived DOM degradation. Organic compounds with high H/C ratios such as lipids, carbohydrates, and protein/lipid-like compounds were preferentially removed by microorganisms during microbial degradation. Meanwhile, there was an increase in the abundance of organic molecular formula with a high aromaticity such as tannins and unsaturated hydrocarbons with low molecular weight and low double bond equivalent. In addition, aerobic/hypoxic environments can alter microbial metabolic pathways of sediment-derived DOM by affecting the relative abundance of microbial communities (e.g., Gemmatimonadetes and Acidobacteria) and functional genes (e.g., ABC.PE.P1 and ABC.PE.P) in macrophyte-dominated lakes. The abundances of lipids, unsaturated hydrocarbons, and protein compounds in aerobic environments decreased by 58 %, 50 %, and 44 %, respectively, compared to in hypoxic environments under microbial degradation. The results of this study deepen our understanding of DOM biodegradation in macrophyte-dominated lakes under different redox environments and provide new insights into nutrients releases from sediment and continuing eutrophication in macrophyte-dominated lakes.

Non-additive effects on biodegradation of moso bamboo litter- and broadleaf tree litter-leached dissolved organic matter mixtures in a subtropical forest of southern China

Phyllostachys pubescens (moso bamboo) has extensively expanded to subtropical broadleaf forests. However, how moso bamboo expansion influences litter-leached dissolved organic matter (DOM) biodegradation is unclear. In this study, we collected fresh leaf litter of moso bamboo and 10 broadleaf tree species from a subtropical forest in southern China and extracted litter-leached dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP). Then, using a 42-day incubation experiment, we measured litter-leached DOM biodegradation of the selected 11 species and assessed the relative mixing effects on biodegradation of bamboo litter- and broadleaf tree litter-leached DOM mixtures with volume mixing ratios of 1:3, 1:1, and 3:1. In the litter leachates, bamboo had lower DOC:DTN ratio, DOC:DTP ratio, and DOM aromaticity (i.e., lower SUVA254 and SUVA350 values) than most broadleaf tree species. Litter-leached DOM biodegradation did not differ among bamboo, Liquidambar formosana, Vernicia fordii, and Cyclobalanopsis glauca, but was greater for bamboo than for the other seven broadleaf tree species. Leaf litter-leached DOM biodegradation correlated negatively with DOC:DTN and DOC:DTP ratios, but exhibited no significant relationship with DOM aromaticity. Regardless of volume mixing ratios, antagonistic effects were observed when bamboo litter-leached DOM was mixed with broadleaf tree litter-leached DOM with comparable biodegradation, whereas synergistic effects occurred when bamboo litter-leached DOM was mixed with broadleaf tree litter-leached DOM with lower biodegradation. The relative mixing effects on DOM biodegradation increased linearly with elevated interspecific difference in litter-leached DOM biodegradation between bamboo and broadleaf tree species across the incubation periods. These findings indicate that moso bamboo expansion will substantially alter litter-leached DOM biodegradation by improving substrate quality and changing species interactions, and the magnitudes of such changing trends are dependent on the native tree litter-leached DOM biodegradation in subtropical broadleaf forests.

Divergent Fate and Roles of Dissolved Organic Matter from Spatially Varied Grassland Soils in China During Long-Term Biogeochemical Processes.

Terrestrial dissolved organic matter (DOM) is critical to global carbon and nutrient cycling, climate change, and human health. However, how the spatial and compositional differences of soil DOM affect its dynamics and fate in water during the carbon cycle is largely unclear. Herein, the biodegradation of DOM from 14 spatially distributed grassland soils in China with diverse organic composition was investigated by 165 days of incubation experiments. The results showed that although the high humified fraction (high-HS) regions were featured by high humic-like fractions of 4-25 kDa molecular weight, especially the abundant condensed aromatics and tannins, they unexpectedly displayed greater DOM degradation during 45-165 days. In contrast, the unique proteinaceous and 25-100 kDa fractions enriched in the low humified fraction (low-HS) regions were drastically depleted and improved the decay of bulk DOM but only during 0-45 days. Together, DOM from the high-HS regions would cause lower CO2 outgassing to the atmosphere but higher organic loads for drinking water production in the short term than that from the low-HS regions. However, this would be reversed for the two regions during the long-term transformation processes. These findings highlight the importance of spatial and temporal variability of DOM biogeochemistry to mitigate the negative impacts of grassland soil DOM on climate, waters, and humans.

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.

Accelerated organic matter decomposition in thermokarst lakes upon carbon and phosphorus inputs.

Mineralization of dissolved organic matter (DOM) in thermokarst lakes plays a non-negligible role in the permafrost carbon (C) cycle, but remains poorly understood due to its complex interactions with external C and nutrient inputs (i.e., aquatic priming and nutrient effects). Based on large-scale lake sampling and laboratory incubations, in combination with 13 C-stable-isotope labeling, optical spectroscopy, and high-throughput sequencing, we examined large-scale patterns and dominant drivers of priming and nutrient effects of DOM biodegradation across 30 thermokarst lakes along a 1100-km transect on the Tibetan Plateau. We observed that labile C and phosphorus (P) rather than nitrogen (N) inputs stimulated DOM biodegradation, with the priming and P effects being 172% and 451% over unamended control, respectively. We also detected significant interactive effects of labile C and nutrient supply on DOM biodegradation, with the combined labile C and nutrient additions inducing stronger microbial mineralization than C or nutrient treatment alone, illustrating that microbial activity in alpine thermokarst lakes is co-limited by both C and nutrients. We further found that the aquatic priming was mainly driven by DOM quality, with the priming intensity increasing with DOM recalcitrance, reflecting the limitation of external C as energy sources for microbial activity. Greater priming intensity was also associated with higher community-level ribosomal RNA gene operon (rrn) copy number and bacterial diversity as well as increased background soluble reactive P concentration. In contrast, the P effect decreased with DOM recalcitrance as well as with background soluble reactive P and ammonium concentrations, revealing the declining importance of P availability in mediating DOM biodegradation with enhanced C limitation but reduced nutrient limitation. Overall, the stimulation of external C and P inputs on DOM biodegradation in thermokarst lakes would amplify C-climate feedback in this alpine permafrost region.

Rethinking terrestrial dissolved organic matter in dam reservoirs before mixing: Linking photodegradation and biodegradation and the phenanthrene binding behavior

With the increased construction of dam reservoirs and the demand for water security, terrestrial dissolved organic matter (DOM) has received attention because of its role in regulating water quality, ecological functions, and the fate and transport of pollutants in dam reservoirs. This study investigated the transformations of soil DOM and vegetation DOM of dam reservoirs following photodegradation and biodegradation before conservative mixing, as well as the resultant effects on phenanthrene binding. Based on the results, terrestrial DOM could undergo transformation via photodegradation and biodegradation before conservative mixing in dam reservoirs. Although both processes resulted in substantial decreases in DOM concentrations, the changes in chromophoric DOM and fluorescent DOM depended on the original DOM sources. Furthermore, the photodegradation of terrestrial DOM resulted in more pronounced photobleaching than photomineralization. In addition, photodegradation of terrestrial DOM resulted in the generation of DOM-derived by-products with low molecular weight and low aromaticity, whereas the biodegradation of terrestrial DOM resulted in DOM-derived by-products with low molecular weight and high aromaticity. Subsequently, the photodegradation and biodegradation of terrestrial DOM substantially enhanced the binding affinity of phenanthrene. Soil DOM is prior to vegetation DOM when predicting the ecological risk of HOCs. These results indicate that the terrestrial DOM in dam reservoirs should be reconsidered before conservative mixing. Further studies on the coupling effects of both biogeochemical processes, as well as on the relative contributions of soil DOM and vegetation DOM after transformation to the aquatic DOM in dam reservoirs, are required. This study provides information on the environmental effects of dam construction from the perspective of biogeochemical processes.

Applying fluorescence spectroscopy and DNA pyrosequencing with 2D-COS and co-occurrence network to deconstruct dynamical DOM degradation of air-land-water sources in an urban river

In an urban river, comprehending the interplay between dissolved organic matter (DOM) and atmospheric, terrestrial, and aquatic sources is crucial. This encompassed investigating temporal variations in DOM and its association with the bacterioplankton community to gain profound insights into the biogeochemical dynamics and biodegradability of DOM. DOM was extracted from PM2.5, soil, sediment, bait, and terrestrial/aquatic plant residuals collected along the Wenyuhe River in Beijing, China - a region predominantly supplied with reclaimed water. Subsequently, mixed microbial communities from the river were introduced into DOM samples originating from each source and incubated for 10 days. Principal component analysis (PCA) applied to reassembled excitation-emission matrix (EEM) data revealed two distinct clusters: cluster 1 comprising soil, sediment, and PM2.5 samples; and cluster 2 consisting of bait as well as terrestrial/aquatic plant residuals. According to parallel factor analysis, C1 (microbial humic-like) and C2-C3 (fulvic-like) dominated the DOM from soil, sediment, and PM2.5. These components were continuously degraded during incubation, except for PM2.5. DOM from bait and terrestrial/aquatic plants contained representative components of C6 (phenolic-like) and C7 (tryptophan-like), which underwent extensive decomposition. Interestingly, DOM in PM2.5 contained aliphatic compounds and polycyclic aromatic hydrocarbons (PAHs) but exhibited weak degradation with the complete disappearance of C6 and C7. Rhodococcus was a unique species capable of degrading PAHs, which might be particularly important considering the specificity of PM2.5 pollution. Based on two-dimensional correlation spectroscopy (2D-COS), variations in DOM components such as C6, and C7 were significantly larger compared to those of C1, C2, C3, and C5 (terrestrial humic-like) from bait samples, sediments, and residual terrestrial plants. MW-2D-COS analysis revealed that DOM from bait samples and terrestrial/aquatic plants experienced substantial degradation by the second day while DOM from soil or sediment decomposed mainly on the fourth day. Notably, the decomposition of DOM fractions in PM2.5 occurred throughout the entire four-day period. Co-occurrence network analysis classified sources of DOM into two clusters similar to PCA results: cluster 1 showed significant microbial degradation of fulvic-like compounds while cluster 2 demonstrated deep microbial decomposition of tyrosine-like and phenolic compounds. Therefore, the artificial loading of DOM into rivers not only expands the chemical diversity within DOM but also perturbs bacterioplankton diversities.

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

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

Microplastics Reshape the Fate of Aqueous Carbon by Inducing Dynamic Changes in Biodiversity and Chemodiversity.

The interactions among dissolved organic matter (DOM), microplastics (MPs) and microbes influence the fate of aqueous carbon and greenhouse gas emissions. However, the related processes and mechanisms remain unclear. Here, we found that MPs determined the fate of aqueous carbon by influencing biodiversity and chemodiversity. MPs release chemical additives such as diethylhexyl phthalate (DEHP) and bisphenol A (BPA) into the aqueous phase. The microbial community, especially autotrophic bacteria such as Cyanobacteria, showed a negative correlation with the additives released from MPs. The inhibition of autotrophs promoted CO2 emissions. Meanwhile, MPs stimulated microbial metabolic pathways such as the tricarboxylic acid (TCA) cycle to accelerate the DOM biodegradation process, and then the transformed DOM presented low bioavailability, high stability, and aromaticity. Our findings highlight an urgent need for chemodiversity and biodiversity surveys to assess ecological risks from MP pollution and the impact of MPs on the carbon cycle.

Deciphering riverine dissolved organic matter biodegradation: Evidence from three-dimensional fluorescence

Biological degradation contributes significantly to aquatic dissolved organic matter (DOM) turnover. Yet to date, the specific biological pathways involved in DOM degradation are still obscure. Here, we show how DOM changes in response to 28-d incubation in a karst river, via the combined fluorescent excitation-emission matrix (EEM) with parallel factor analysis (PARAFAC) and peak picking. Tyrosine-, tryptophan- and UV humic-like components were identified as the primary DOM in original waters. Notable decreases of protein-like components were observed after 15 ℃ incubation, suggesting preferential consumption of biogenic DOM. Elevated temperature (30 ℃), however, caused significant decreases in visible humic-like and terrestrial fulvic-like components, but an increase in UV humic-like components in comparison to 15 ℃ incubation (p < 0.05). Humification index (HIX) decreased dramatically after 15 ℃ incubation, indicating that biological processes regulated river DOM properties. Our results highlight the significant biogenic signals in karst rivers, and evidence that fluorescence measurement can decipher riverine DOM biodegradation as a useful approach.

Responses of molecular composition and biodegradation of dissolved organic matter to erosion in topsoil versus subsoil in a Mollisol agricultural ecosystem

Although dissolved organic matter (DOM) has a substantial influence on biogeochemical processes relevant to erosion in agricultural ecosystems, the mechanisms underlying the dynamics and biodegradation of DOM in eroding landscapes are yet to clarified. In this study, we examined the changes in dissolved organic carbon (DOC), profile distribution, biodegradation, and chemical characteristics associated with erosion in a Mollisol agricultural landscape. The results showed that DOC content in the mid-slope was significantly higher than that in the down-slope, particularly at depths of 40–100 cm. The higher humification index and low biological index indicated that the DOM in the depositional position was characterized by a higher humic content compared to the up-slope positions. The up-slope samples were found to contain larger amounts of protein-like substances, whereas the mid- and down-slope soils comprised a larger proportion of anthropogenic humic-like materials, indicating that DOM in the up-slope soil was primarily microbial-derived. For the topsoils, the activities of β-glucosidase and cellulase were highest in the down-slope both before and after incubation, thereby suggesting that the deposition was conducive to the secretion of these enzymes by soil microbes. Overall, our observations indicated that erosion and deposition had a significant effect on the chemical composition and biodegradation pathways of DOM in soil profiles. These findings provided detailed information on the heterogeneous DOM characteristics at the molecular level, as well as DOC effects on environmental behavior in agricultural eroding environments.

Distinct seasonal variations of dissolved organic matter across two large freshwater lakes in China: Lability profiles and predictive modeling

Biological lability of dissolved organic matter (DOM) is a crucial indicator of carbon cycle and contaminant attenuation in freshwater lakes. In this study, we employed a multi-stage plug-flow bioreactor and spectrofluorometric indices to characterize the seasonal variations in DOM composition and lability across Poyang Lake (PY) and Lake Taihu (TH), two large freshwater lakes in China with distinct hydrological seasonality. Our findings showed that the export of floodplain-derived organics and river-lake interaction led to a remarkable increase in terrestrial aromatic and humic-like DOM with high molecular weights and long turnover times in PY. Consequently, the labile fraction was extremely low (average LDOC% of 3%) during the rising-to-flood season (spring and summer). Conversely, autochthonous production in TH considerably enriched semi-labile (average SDOC% of 26%) and biodegradable DOM (average BDOC% of 34%) during the phytoplankton bloom to post-bloom season (summer and autumn). This was reflected by the accumulation of low-light-absorbing and protein-like components with high biological and fluorescence indices. In the dry and non-bloom season (winter), the better preservation of humic substances maintained the high molecular weight and humic degree of DOM in PY, while the decay of aquatic plants strengthened autochthonous production, resulting in a similar BDOC% of PY samples (23%–34%) to TH samples (18%–33%). We further applied partial least squares regression using DOM optical indices as predictive proxies, which generated a greater prediction strength for BDOC% (R2 = 0.80) compared to SDOC% (R2 = 0.57) and LDOC% (R2 = 0.28). The regression model identified aromaticity (SUVA254) as the most effective and negative predictor and low molecular weight (A250/A365) as the highly and positively influential factor. Our study provides new evidence that the seasonality of DOM lability profiles is regulated by the trade-off between flow-related variation and phytoplankton production, and presents an approach to describe and predict DOM lability across freshwater lakes.