Influences of eutrophication on methanogenesis pathways and methanogenic microbial community structures in freshwater lakes

Deposited dead algae influence the microbial communities and functional potentials on the surface sediment in eutrophic shallow lakes

Dead algae deposition will change the nutrient transformation on the sediment-water interface. However, the key factors that drive nutrient turnover, particularly the influence of sediment microbiota, remain poorly understood. As a result, this study conducted an 80-day simulated incubation to investigate the effect of different deposition of death algae on microbial communities and functional potentials in sediments. It was revealed that dead algae deposition changed the microbial communities and interactions. Changes in the bacteria are not only reflected in community composition and diversity but also in the interrelation among bacteria taxa, while changes in the fungi are mainly reflected in the interrelation among fungi taxa. Meanwhile, dead algae deposition increased the abundance of mostly functional genes related to the C, N, P, and S cycle processes and improved the function potentials of microorganisms. Both of them led to the increase of PO43-, NO3-, NH4+, and TOC content in the overlying water, influencing the nutrient cycle processes. Moreover, partial least squares path modeling indicated which key factors are to influence different nutrient cycle processes. Sediment nutrients directly influenced the P cycle process, whereas the C, N, and S cycle processes were directly affected by the changes in biological properties. These results provide a new perspective on the effects of dead algal deposition on the sediment nutrient cycle processes mediated by the sediment microbiota.

Ecological restoration reduces greenhouse gas emissions by altering planktonic and sedimentary microbial communities in a shallow eutrophic lake

Ecological restoration is a promising approach to alleviate eutrophication. However, its impacts on greenhouse gas (GHG) emissions and the underlying microbial mechanisms in different habitats of lakes remain unclear. To address this knowledge gap, we measured carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes at both water-air and sediment-water interfaces of eutrophic (Caohai) and restored area (Dapokou) of Dianchi Lake, a typical eutrophic lake in China. Meanwhile, we investigated the responses of planktonic and sedimentary bacterial and fungal communities by high-throughput sequencing. Our results indicated that 6 years of ecological restoration significantly reduced CO2 and N2O fluxes by 1.0-3.6 and 2.2-2.8 folds respectively, with more pronounced variations at the water-air interface than the sediment-water interface. Ecological restoration also shifted the structures of planktonic bacterial and fungal communities remarkably, leading to a significant reduction in the relative abundances of Actinobacteriota (by 70.94%), Bacteroidota (by 61.65%), Planctomycetota (by 74.18%) and Chytridiomycota (by 95.44%). Correlation analyses further suggested that GHG fluxes at the water-air interface were significantly correlated with planktonic microbial community composition (P < 0.05), and the significant reduction of CO2 and N2O fluxes under ecological restoration could be attributed to the decreased abundances of organic matter decomposers (such as hgcI_clade, Sporichthyaceae and Acidibacter) and increased abundances of autotrophs (such as Hydrogenophaga and Cyanobium_PCC-6307) in water. Collectively, our findings verify the importance of ecological restoration in reducing GHG emissions in inland lake ecosystems, providing new insights for addressing global climate change and advancing carbon neutrality.

Structure and metabolic function of spatiotemporal pit mud microbiome

BACKGROUND

Pit mud (PM) hosts diverse microbial communities, which serve as a medium to impart flavor and quality to Baijiu and exhibit long-term tolerance to ethanol and acids, resulting in a unique ecosystem. However, the ecology and metabolic functions of PM remain poorly understood, as many taxa in PM represent largely novel lineages. In this study, we used a combination of metagenomic analysis and chemical derivatization LC-MS analysis to provide a comprehensive overview of microbial community structure, metabolic function, phylogeny, horizontal gene transfer, and the relationship with carboxyl compounds in spatiotemporal PM samples.

RESULTS

Our findings revealed three distinct stages in the spatiotemporal changes of prokaryotic communities in PM: an initial phase dominated by Lactobacillus, a transitional phase, and a final state of equilibrium. Significant variations in α- and β-diversity were observed across different spatial and temporal PM samples. We identified 178 medium- and high-quality non-redundant metagenome-assembled genomes (MAGs), and constructed their phylogenetic tree, depicting their roles in the carbon, nitrogen, and sulfur cycles. The Wood-Ljungdahl pathway and reverse TCA cycle were identified as the main carbon fixation mechanisms, with both hydrogenotrophic and aceticlastic methanogens playing a major role in methane production, and methylotrophic pathway observed in older PM. Furthermore, we identified relationships between prokaryotes and 29 carboxyl metabolites, including medium- and long-chain fatty acids. Horizontal gene transfer (HGT) was widespread in PM, particularly among clostridia, Bacteroidota, Bacilli, and Euryarchaeota, and was shown to play critical roles in fermentation dynamics, carbon fixation, methane production, and nitrogen and sulfur metabolism.

CONCLUSION

Our study provides new insights into the evolution and function of spatiotemporal PM, as well as its interactions with carboxyl metabolites. Lactobacillus dominated in new PM, while methanogens and clostridia were predominant in older or deeper PM layers. The three distinct stages of prokaryotic community development in PM and HGT played critical roles in metabolic function of spatiotemporal PM. Furthermore, this study highlights the importance of α-diversity, β-diversity, methanogens, and Clostridium as useful indicators for assessing PM quality in the production of high-quality Baijiu.

Fish acting as sinks of methane-derived carbon in Neotropical floodplains

Floodplains function as global hotspots for the natural production of methane. Some of this methane can be oxidized by methanotrophic bacteria and assimilated into their biomass before reaching the atmosphere. Consequently, aquatic invertebrates that feed on methanotrophic bacteria may transfer methane-derived carbon to higher trophic levels in the aquatic food chain. Our objective was to investigate the proportion of methane-derived carbon in the biomass of apex fish across 34 lakes from four major Neotropical floodplains (Amazon, Pantanal, Araguaia, and Paraná) using stable isotopes of carbon (δ13C). We found that methane-derived carbon contributed between 5 % and 16 % to the biomass of 37 apex fish species, providing, for the first time, evidence of the fish's role in the methane cycle in the Neotropics. Consumers in the Amazon and Pantanal floodplains, the largest and most significant regions for methane production, exhibited higher levels of methane-derived carbon in their biomass (11.06 ± 2.87 % and 9.84 ± 3.08 %, respectively). These results underscore the role of aquatic consumers in mitigating methane emissions in floodplains, as methane oxidation and assimilation are linked to reduced emissions. Therefore, conserving fish assemblages in floodplains through strategies that maintain the natural dynamics of these ecosystems is essential for controlling natural methane emissions.

Diversity and Distribution of Methane Functional Microorganisms in Sedimentary Columns of Hongfeng Reservoir in Different Seasons

Freshwater ecosystem is a significant natural source of CH4 emission in the atmosphere. To fully understand the dynamics of methane emissions in reservoirs, it is essential to grasp the temporal and vertical distribution patterns, as well as the factors that influence the methanogenic bacterial communities within the sediments. This study investigates the methane dynamics, carbon isotope fractionation (δ13CH4), and abundance of functional microorganisms along the geochemical gradient in the in situ sedimentary column of Hongfeng Reservoir (China). Notably, the methane concentration in sediment in summer ranged in 15.39-127.22 µmol/L, which is twice as high as wintertime concentrations in the surface layer near the sediment-water interface (0-10 cm depth). Illumina sequencing of the sediments identified 11 genera affiliated with methanogenic archaea, with dominant genus Methanosaeta reaching a relative abundance of 34.95% in summer. The total carbon (TOC) content in sedimentary columns in different seasons is positively correlated with Methanosarcina (P < 0.05). In addition, seasonal discrepancies are observed in the sediment profiles for total nitrogen (TN), sulfate (SO42-), and ferrous iron (Fe2+) concentrations. The concentration of total nitrogen (TN) is higher in summer than in winter. In summer, sulfate accumulates in the middle layer of the sedimentary column, while in winter, the maximum concentration of sulfate in the surface layer reaches 0.65 mmol/L. These geochemical gradients drive the biological transformation of nitrogen, sulfur, and iron, may also be linked to the consumption of methane. Thus, it is established that the temporal and spatial dynamics of methanogenic communities in sediments significantly influence the fluctuations in methane release fluxes within reservoirs, highlighting the necessity to account for seasonal biological variations when assessing greenhouse gas emissions from reservoirs.

Vertical profiles of community and activity of methanotrophs in large lake and reservoir of Southwest China

Microbial methane oxidation plays a significant role in regulating methane emissions from lakes and reservoirs. However, the differences in methane oxidation activity and methanotrophic community between lakes and reservoirs remain inadequately characterized. In this study, sediment and water samples were collected from the large shallow lake (Dianchi) and deep reservoirs (Dongfeng and Hongjiadu) located in karst area, Southwest China. The results indicated that the rates of aerobic oxidation of methane (AeOM) in lake sediment ranged from 7.1 to 27.7 μg g-1 d-1, which was higher than that in reservoirs sediment (1.92 to 11.56 μg g-1 d-1). Similarly, the average AeOM in the water column of lake (104.7 μg L-1 d-1) was much higher than that of reservoirs (46 μg L-1 d-1). The content of sediment organic carbon and dissolved inorganic carbon were important factors that influenced the rates of AeOM in sediment and water column, respectively. 16S rRNA genes sequencing revealed a higher relative abundance of methanotrophs in lake sediments compared to reservoir sediments. The dominant methanotrophic taxa in lake was Methylococcaceae (type Ib), while Methylomonadaceae (type Ia) was predominant in reservoirs. Meanwhile, anaerobic methane-oxidizing microorganisms Candidatus Methylomirabilis and Candidatus Methanoperedens were also abundant in sediments of reservoirs. However, metatranscriptomic analysis revealed that the type I methanotrophs, especially Methylobacter, was most active in the sediment of both lake and reservoir. Water depth and conductivity could be the key controlling factors of the structures of methanotrophic communities in sediment and water column, respectively. Metagenome-assembled genomes suggested that type I methanotrophs exhibited greater motility, as evidenced by a higher number of flagellar assembly genes, while type II methanotrophs demonstrated advantages in metabolic processes such as carbon, phosphorus, and methane metabolism.

Unraveling methanogenesis processes and pathways for Quaternary shallow biogenic gas in aquifer systems through geochemical, genomic and transcriptomic analyses

Shallow biogenic gas is crucial in global warming and carbon cycling. Considering the knowledge gap in the understanding of methanogenesis and metabolic mechanisms within shallow groundwater systems, we investigated Quaternary shallow biogenic gas resources from the Hetao Basin in North China, which were previously underexplored. We systematically analyzed the genesis of gas and formation water, microbial communities, methanogenic processes, and pathways using geochemistry, genomics, and transcriptomics. Our findings indicated that active freshwater environments are conducive to microbial activity and the generation of primary microbial gases. A diverse range of microbes with functions, such as hydrolysis (e.g., Caulobacter), acidogenesis, and hydrogen production (e.g., Sediminibacterium), synergistically contributed to the methanogenic process. Methanogens predominantly comprised hydrogenotrophic methanogens (e.g., Methanobacteriales), although H2-dependent methylotrophic methanogens (e.g., Methanofastidiosa) were also prevalent. The metabolic processes of the different methanogenic pathways were revealed based on functional gene analysis and mapping results. Furthermore, the composition of the community structure, functional predictions, metagenomics, and metatranscriptomics underscored the contribution of the hydrogenotrophic pathway, which ranged from 52.22 % to 79.23 %. The aceticlastic pathway exhibited high gene abundance and was primarily associated with methylotrophs and other potential pathways. The H2-dependent methylotrophic methanogenesis pathway was constrained by low metabolic activity. By revealing the methane production mechanism of biogenic gas in shallow aquifer systems, this study provides a new perspective and profound comprehension of its ecological and environmental implications worldwide.

The Effects of Model Insoluble Copper Compounds in a Sedimentary Environment on Denitrifying Anaerobic Methane Oxidation (DAMO) Enrichment

The contribution of denitrifying anaerobic methane oxidation (DAMO) as a methane sink across different habitats, especially those affected by anthropogenic activities, remains unclear. Mining and industrial and domestic use of metals/metal-containing compounds can all cause metal contamination in freshwater ecosystems. Precipitation of metal ions often limits their toxicity to local microorganisms, yet microbial activity may also cause the redissolution of various precipitates. In contrast to most other studies that apply soluble metal compounds, this study investigated the responses of enriched DAMO culture to model insoluble copper compounds, malachite and covellite, in simulated sedimentary environments. Copper ≤ 0.22 µm from covellite appeared to cause immediate inhibition in 10 h. Long-term tests (54 days) showed that apparent methane consumption was less impacted by various levels of malachite and covellite than soluble copper. However, the medium-/high-level malachite and covellite caused a 46.6-77.4% decline in denitrification and also induced significant death of the representative DAMO microorganisms. Some enriched species, such as Methylobacter tundripaludum, may have conducted DAMO or they may have oxidized methane aerobically using oxygen released by DAMO bacteria. Quantitative polymerase chain reaction analysis suggests that Candidatus Methanoperedens spp. were less affected by covellite as compared to malachite while Candidatus Methylomirabilis spp. responded similarly to the two compounds. Under the stress induced by copper, DAMO archaea, Planctomycetes spp. or Phenylobacterium spp. synthesized PHA/PHB-like compounds, rendering incomplete methane oxidation. Overall, the findings suggest that while DAMO activity may persist in ecosystems previously exposed to copper pollution, long-term methane abatement capability may be impaired due to a shift of the microbial community or the inhibition of representative DAMO microorganisms.

Improving water quality and mitigating CH4 and N2O production in urban landscape water simultaneously by optimizing calcium peroxide dosage

Recent studies show that greenhouse gas (GHG) emissions from urban landscape water are significant and cannot be overlooked, underscoring the need to develop effective strategies for mitigating GHG production from global freshwater systems. Calcium peroxide (CaO2) is commonly used as an eco-friendly reagent for controlling eutrophication in water bodies, but whether CaO2 can reduce GHG emissions remains unclear. This study investigated the effects of CaO2 dosage on the production of methane (CH4) and nitrous oxide (N2O) in urban landscape water under anoxic conditions during summer. The findings reveal that CaO2 addition not only improved the physicochemical and organoleptic properties of simulated urban landscape water but also reduced N2O production by inhibiting the activity of denitrifying bacteria across various dosages. Moreover, CaO2 exhibited selective effects on methanogens. Specifically, the abundance of acetoclastic methanogen Methanosaeta and methylotrophic methanogen Candidatus_Methanofastidiosum increased whereas the abundance of the hydrogenotrophic methanogen Methanoregula decreased at low, medium, and high dosages, leading to higher CH4 production at increased CaO2 dosage. A comprehensive multi-objective evaluation indicated that an optimal dosage of 60 g CaO2/m2 achieved 41.21 % and 84.40 % reductions in CH4 and N2O production, respectively, over a 50-day period compared to the control. This paper not only introduces a novel approach for controlling the production of GHGs, such as CH4 and N2O, from urban landscape water but also suggests a methodology for optimizing CaO2 dosage, providing valuable insights for its practical application.

Geographic distribution of bacterial communities of inland waters in China

Microorganisms are integral to freshwater ecological functions and, reciprocally, their activity and diversity are shaped by the ecosystem state. Yet, the diversity of bacterial community and its driving factors at a large scale remain elusive. To bridge this knowledge gap, we delved into an analysis of 16S RNA gene sequences extracted from 929 water samples across China. Our analyses revealed that inland water bacterial communities showed a weak latitudinal diversity gradient. We found 530 bacterial genera with high relative abundance of hgcI clade. Among them, 29 core bacterial genera were identified, that is strongly linked to mean annual temperature and nutrient loadings. We also detected a non-linear response of bacterial network complexity to the increasing of human pressure. Mantel analysis suggested that MAT, HPI and P loading were the major factors driving bacterial communities in inland waters. The map of taxa abundance showed that the abundant CL500-29 marine group in eastern and southern China indicated high eutrophication risk. Our findings enhance our understanding of the diversity and large-scale biogeographic pattern of bacterial communities of inland waters and have important implications for microbial ecology.

Response of microbial communities to exogenous nitrate nitrogen input in black and odorous sediment

Since nitrate nitrogen (NO3--N) input has proved an effective approach for the treatment of black and odorous river waterbody, it was controversial whether the total nitrogen concentration standard should be raised when the effluent from the sewage treatment plant is discharged into the polluted river. To reveal the effect of exogenous nitrate (NO3--N) on black odorous waterbody, sediments with different features from contaminated rivers were collected, and the changes of physical and chemical characteristics and microbial community structure in sediments before and after the addition of exogenous NO3--N were investigated. The results showed that after the input of NO3--N, reducing substances such as acid volatile sulfide (AVS) in the sediment decreased by 80 % on average, ferrous (Fe2+) decreased by 50 %, yet the changing trend of ammonia nitrogen (NH4+-N) in some sediment samples increased while others decreased. High-throughput sequencing results showed that the abundance of Thiobacillus at most sites increased significantly, becoming the dominant genus in the sediment, and the abundance of functional genes in the metabolome increased, such as soxA, soxX, soxY, soxZ. Network analysis showed that sediment microorganisms evolved from a single sulfur oxidation ecological function to diverse ecological functions, such as nitrogen cycle nirB, nirD, nirK, nosZ, and aerobic decomposition. In summary, inputting an appropriate amount of exogenous NO3--N is beneficial for restoring and maintaining the oxidation states of river sediment ecosystems.

Greenhouse gas emissions from Daihai Lake, China: Should eutrophication and salinity promote carbon emission dynamics?

Greenhouse gases (GHGs) emitted or absorbed by lakes are an important component of the global carbon cycle. However, few studies have focused on the GHG dynamics of eutrophic saline lakes, thus preventing a comprehensive understanding of the carbon cycle. Here, we conducted four sampling analyses using a floating chamber in Daihai Lake, a eutrophication saline lake in Inner Mongolia Autonomous Region, China, to explore its carbon dioxide (CO2) and methane (CH4) emissions. The mean CO2 emission flux (FCO2) and CH4 emission flux (FCH4) were 17.54 ± 14.54 mmol/m2/day and 0.50 ± 0.50 mmol/m2/day, respectively. The results indicated that Daihai Lake was a source of CO2 and CH4, and GHG emissions exhibited temporal variability. The mean CO2 partial pressure (pCO2) and CH4 partial pressure (pCH4) were 561.35 ± 109.59 µatm and 17.02 ± 13.45 µatm, which were supersaturated relative to the atmosphere. The regression and correlation analysis showed that the main influencing factors of pCO2 were wind speed, dissolved oxygen (DO), total nitrogen (TN) and Chlorophyll a (Chl.a), whereas the main influencing factors of pCH4 were water temperature (WT), Chl.a, nitrate nitrogen (NO3--N), TN, dissolved organic carbon (DOC) and water depth. Salinity regulated carbon mineralization and organic matter decomposition, and it was an important influencing factor of pCO2 and pCH4. Additionally, the trophic level index (TLI) significantly increased pCH4. Our study elucidated that salinity and eutrophication play an important role in the dynamic changes of GHG emissions. However, research on eutrophic saline lakes needs to be strengthened.

Hydrogen-independent CO2 reduction dominates methanogenesis in five temperate lakes that differ in trophic states

Emissions of microbially produced methane (CH4) from lake sediments are a major source of this potent greenhouse gas to the atmosphere. The rates of CH4 production and emission are believed to be influenced by electron acceptor distributions and organic carbon contents, which in turn are affected by anthropogenic inputs of nutrients leading to eutrophication. Here, we investigate how eutrophication influences the abundance and community structure of CH4 producing Archaea and methanogenesis pathways across time-resolved sedimentary records of five Swiss lakes with well-characterized trophic histories. Despite higher CH4 concentrations which suggest higher methanogenic activity in sediments of eutrophic lakes, abundances of methanogens were highest in oligotrophic lake sediments. Moreover, while the methanogenic community composition differed significantly at the lowest taxonomic levels (OTU), depending on whether sediment layers had been deposited under oligotrophic or eutrophic conditions, it showed no clear trend in relation to in situ distributions of electron acceptors. Remarkably, even though methanogenesis from CO2-reduction was the dominant pathway in all sediments based on carbon isotope fractionation values, taxonomic identities, and genomes of resident methanogens, CO2-reduction with hydrogen (H2) was thermodynamically unfavorable based on measured reactant and product concentrations. Instead, strong correlations between genomic abundances of CO2-reducing methanogens and anaerobic bacteria with potential for extracellular electron transfer suggest that methanogenic CO2-reduction in lake sediments is largely powered by direct electron transfer from syntrophic bacteria without involvement of H2 as an electron shuttle.

Bacterial and Microeukaryotic Community Compositions and Their Assembly Processes in Lakes on the Eastern Qinghai-Tibet Plateau

Bacterial and microeukaryotic community compositions and their assembly processes have remained challenging and remained unclear in lake ecosystems on the Qinghai-Tibet Plateau (QTP). We revealed the diversity and community compositions, driving factors, ecological assembly processes, and co-occurrence networks of bacterial and microeukaryotic communities in water bodies of the eight lake ecosystems across the Eastern QTP. The results demonstrated that the predominant bacteria in most samples were Proteobacteria, with an average relative abundance of 41.78%, whereas the most abundant of microeukaryotes differed among the sample sites. The redundancy analysis revealed that latitude and pH were the most important driving factors in shaping the bacterial and microeukaryotic community compositions. Homogeneous selection (56.40%) was the dominant process in assembling the bacterial communities, whereas dispersal limitation (67.24%) was the major process in governing the microeukaryotic communities. Furthermore, dissolved organic carbon and salinity were the major factors mediating the balance of deterministic and stochastic assembly processes in the bacterial and microeukaryotic communities. Both the bacterial and microeukaryotic community co-occurrence networks exhibited topological features of modularity and non-random topological features. The results offer insights into the mechanisms underpinning bacterial and microeukaryotic diversities and communities in the lake ecosystems on the QTP.

Untapped talents: insight into the ecological significance of methanotrophs and its prospects

The deep ocean is a rich reservoir of unique organisms with great potential for bioprospecting, ecosystem services, and the discovery of novel materials. These organisms thrive in harsh environments characterized by high hydrostatic pressure, low temperature, and limited nutrients. Hydrothermal vents and cold seeps, prominent features of the deep ocean, provide a habitat for microorganisms involved in the production and filtration of methane, a potent greenhouse gas. Methanotrophs, comprising archaea and bacteria, play a crucial role in these processes. This review examines the intricate relationship between the roles, responses, and niche specialization of methanotrophs in the deep ocean ecosystem. Our findings reveal that different types of methanotrophs dominate specific zones depending on prevailing conditions. Type I methanotrophs thrive in oxygen-rich zones, while Type II methanotrophs display adaptability to diverse conditions. Verrumicrobiota and NC10 flourish in hypoxic and extreme environments. In addition to their essential role in methane regulation, methanotrophs contribute to various ecosystem functions. They participate in the degradation of foreign compounds and play a crucial role in cycling biogeochemical elements like metals, sulfur, and nitrogen. Methanotrophs also serve as a significant energy source for the oceanic food chain and drive chemosynthesis in the deep ocean. Moreover, their presence offers promising prospects for biotechnological applications, including the production of valuable compounds such as polyhydroxyalkanoates, methanobactin, exopolysaccharides, ecotines, methanol, putrescine, and biofuels. In conclusion, this review highlights the multifaceted roles of methanotrophs in the deep ocean ecosystem, underscoring their ecological significance and their potential for advancements in biotechnology. A comprehensive understanding of their niche specialization and responses will contribute to harnessing their full potential in various domains.

Significant methane ebullition from large shallow eutrophic lakes of the semi-arid region of northern China

Eutrophic lakes are a major source of the atmospheric greenhouse gas methane (CH4), and CH4 ebullition emissions from inland lakes have important implications for the carbon cycle. However, the spatio-temporal heterogeneity of CH4 ebullition emission and its influencing factors in shallow eutrophic lakes of arid and semi-arid regions remain unclear. This study aimed to determine the mechanism of CH4 emission via eutrophication in Lake Ulansuhai, a large shallow eutrophic lake in a semi-arid region of China.To this end, monthly field surveys were conducted from May to October 2021, and gas chromatography was applied using the headspace equilibrium technique with an inverted funnel arrangement. The total CH4 fluxes ranged from 0.102 mmol m-2 d-1 to 59.296 mmol m-2 d-1 with an average value of 4.984 ± 1.82 mmol m-2 d-1. CH4 ebullition emissions showed significant temporal and spatial variations. The highest CH4 ebullition emission was observed in July with a grand mean of 9.299 mmol m-2 d-1, and the lowest CH4 ebullition emissions occurred in October with an average of 0.235 mmol m-2 d-1. Among seven sites (S1-S7), the maximum (3.657 mmol m-2 d-1) and minimum (1.297 mmol m-2 d-1). CH4 ebullition emissions were observed at S2 and S7, respectively. As the main route of CH4 emission to the atmosphere in Lake Ulansuhai, the CH4 ebullition flux during May to October accounted for 69% of the total CH4 flux. Statistical analysis showed that CH4 ebullition was positively correlated with temperature (R = 0.391, P < 0.01) and negatively correlated with air pressure (R = 0.286, P < 0.00). Temperature and air pressure were found to strongly regulate the production and oxidation of CH4. Moreover, nutritional status indicators such as TP and NH4+-N significantly affect CH4 ebullition emissions (R = 0.232, P < 0.01; R = -0.241, P < 0.01). This study reveals the influencing factors of CH4 ebullition emission in Lake Ulansuhai, and provides theoretical reference and data support for carbon emission from eutrophic lakes. Nevertheless, research on eutrophic shallow lakes needs to be further strengthened. Future research should incorporate improved flux measurement techniques with process-based models to improve the accuracy from regional to large-scale estimation of CH4 emissions and clarify the carbon budget of aquatic ecosystems. In this manner, the understanding and predictability of CH4 ebullition emission from shallow lakes can be improved.

Organic matter lability modifies the vertical structure of methane-related microbial communities in lake sediments

Eutrophication increases the input of labile, algae-derived, organic matter (OM) into lake sediments. This potentially increases methane (CH4) emissions from sediment to water through increased methane production rates and decreased methane oxidation efficiency in sediments. However, the effect of OM lability on the structure of methane oxidizing (methanotrophic) and methane producing (methanogenic) microbial communities in lake sediments is still understudied. We studied the vertical profiles of the sediment and porewater geochemistry and the microbial communities (16S rRNA gene amplicon sequencing) at five profundal stations of an oligo-mesotrophic, boreal lake (Lake Pääjärvi, Finland), varying in surface sediment OM sources (assessed via sediment C:N ratio). Porewater profiles of methane, dissolved inorganic carbon (DIC), acetate, iron, and sulfur suggested that sites with more autochthonous OM showed higher overall OM lability, which increased remineralization rates, leading to increased electron acceptor (EA) consumption and methane emissions from sediment to water. When OM lability increased, the abundance of anaerobic nitrite-reducing methanotrophs (Candidatus Methylomirabilis) relative to aerobic methanotrophs (Methylococcales) in the methane oxidation layer of sediment surface decreased, suggesting that Methylococcales were more competitive than Ca. Methylomirabilis under decreasing redox conditions and increasing methane availability due to their more diverse metabolism (fermentation and anaerobic respiration) and lower affinity for methane. Furthermore, when OM lability increased, the abundance of methanotrophic community in the sediment surface layer, especially Ca. Methylomirabilis, relative to the methanogenic community decreased. We conclude that increasing input of labile OM, subsequently affecting the redox zonation of sediments, significantly modifies the methane producing and consuming microbial community of lake sediments. IMPORTANCE Lakes are important natural emitters of the greenhouse gas methane (CH4). It has been shown that eutrophication, via increasing the input of labile organic matter (OM) into lake sediments and subsequently affecting the redox conditions, increases methane emissions from lake sediments through increased sediment methane production rates and decreased methane oxidation efficiency. However, the effect of organic matter lability on the structure of the methane-related microbial communities of lake sediments is not known. In this study, we show that, besides the activity, also the structure of lake sediment methane producing and consuming microbial community is significantly affected by changes in the sediment organic matter lability.

Evidence for nontraditional mcr-containing archaea contributing to biological methanogenesis in geothermal springs

Recent discoveries of methyl-coenzyme M reductase-encoding genes (mcr) in uncultured archaea beyond traditional euryarchaeotal methanogens have reshaped our view of methanogenesis. However, whether any of these nontraditional archaea perform methanogenesis remains elusive. Here, we report field and microcosm experiments based on ¹³C-tracer labeling and genome-resolved metagenomics and metatranscriptomics, revealing that nontraditional archaea are predominant active methane producers in two geothermal springs. Archaeoglobales performed methanogenesis from methanol and may exhibit adaptability in using methylotrophic and hydrogenotrophic pathways based on temperature/substrate availability. A five-year field survey found Candidatus Nezhaarchaeota to be the predominant mcr-containing archaea inhabiting the springs; genomic inference and mcr expression under methanogenic conditions strongly suggested that this lineage mediated hydrogenotrophic methanogenesis in situ. Methanogenesis was temperature-sensitive , with a preference for methylotrophic over hydrogenotrophic pathways when incubation temperatures increased from 65° to 75°C. This study demonstrates an anoxic ecosystem wherein methanogenesis is primarily driven by archaea beyond known methanogens, highlighting diverse nontraditional mcr-containing archaea as previously unrecognized methane sources.

Neglected methane production and toxicity risk in low-frequency ultrasound for controlling harmful algal blooms

Algal blooms are regarded as a significant source of CH₄ emissions. Ultrasound has been gradually employed as a fast and efficient algae removal technology in recent years. However, the changes in water environment and potential ecological effects caused by ultrasonic algae removal are not fully clear. Here, a 40-day microcosm study was performed to simulate the collapse of Microcystis aeruginosa blooms after ultrasonic treatment. The results showed that low-frequency ultrasound at 29.4 kHz for 15 min removed 33.49% of M. aeruginosa and contributed to the destruction of cell structure, but it intensified the leakage of intracellular algal organic matter and microcystins. The accelerated collapse of M. aeruginosa blooms after ultrasonication promoted the rapid formation of anaerobic and reductive methanogenesis conditions, and elevated dissolved organic carbon content. Moreover, the release of labile organics, including tyrosine, tryptophan, protein-like compositions, and aromatic proteins, was facilitated by the collapse of M. aeruginosa blooms after ultrasonic treatment, and they supported the growth of anaerobic fermentation bacteria and hydrogenotrophic Methanobacteriales. This was also demonstrated by the increase in methyl-coenzyme M reductase (mcrA) genes in sonicated algae added treatments at the end of incubation. Finally, the CH₄ production in sonicated algae added treatments was 1.43-fold higher than that in non-sonicated algae added treatments. These observations suggested that ultrasound for algal bloom control potentially increased the toxicity of treated water and its greenhouse gas emissions. This study can provide new insights and guidance to evaluate environmental effects of ultrasonic algae removal.

MeHg production in eutrophic lakes: Focusing on the roles of algal organic matter and iron-sulfur-phosphorus dynamics

The mechanisms by which eutrophication affects methylmercury (MeHg) production have not been comprehensively summarized, which hinders accurately predicting the MeHg risk in eutrophic lakes. In this review, we first discussed the effects of eutrophication on biogeochemical cycle of mercury (Hg). Special attentions were paid to the roles of algal organic matter (AOM) and iron (Fe)-sulfur (S)-phosphorus (P) dynamics in MeHg production. Finally, the suggestions for risk control of MeHg in eutrophic lakes were proposed. AOM can affect in situ Hg methylation by stimulating the abundance and activities of Hg methylating microorganisms and regulating Hg bioavailability, which are dependent on bacteria-strain and algae species, the molecular weight and composition of AOM as well as environmental conditions (e.g., light). Fe-S-P dynamics under eutrophication including sulfate reduction, FeS formation and P release could also play crucial but complicated roles in MeHg production, in which AOM may participate through influencing the dissolution and aggregation processes, structural order and surface properties of HgS nanoparticles (HgS_NP). Future studies should pay more attention to the dynamics of AOM in responses to the changing environmental conditions (e.g., light penetration and redox fluctuations) and how such variations will subsequently affect MeHg production. The effects of Fe-S-P dynamics on MeHg production under eutrophication also deserve further investigations, especially the interactions between AOM and HgS_NP. Remediation strategies with lower disturbance, greater stability and less cost like the technology of interfacial O₂ nanobubbles are urgent to be explored. This review will deepen our understanding of the mechanisms of MeHg production in eutrophic lakes and provide theoretical guidance for its risk control.

Hydrogenotrophic pathway dominates methanogenesis along the river-estuary continuum of the Yangtze River

Rivers are considered as an important source of methane (CH₄) to the atmosphere, but our understanding for the methanogenic pathway in rivers and its linkage with CH₄ emission is very limited. Here, we investigated the diffusive flux of CH₄ and its stable carbon isotope signature (δ¹³C-CH₄) along the river-estuary continuum of the Yangtze River. The diffusive CH₄ flux was estimated to 27.9 ± 11.4 μmol/m²/d and 36.5 ± 24.4 μmol/m²/d in wet season and dry season, respectively. The δ¹³C-CH₄ values were generally lower than -60‰, with the fractionation factor (α_c) higher than 1.055 and the isotope separation factor (ε_c) ranged from 55 to 100. In situ microbial composition showed that hydrogenotrophic methanogens accounts for over 70% of the total reads. Moreover, the incubation test showed that the headspace CH₄ concentration by adding CO₂/H₂ to the sediment was orders of magnitude higher than that by adding trimethylamine and sodium acetate. These results jointly verified the river-estuary continuum is a minor CH₄ source and dominated by hydrogenotrophic pathway. Based on the methanogenic pathway here and previous reported data in the same region, the historical variation of diffusive CH₄ flux was calculated and results showed that CH₄ emission has reduced 82.5% since the construction of Three Gorges Dam (TGD). Our study verified the dominant methanogenic pathway in river ecosystems and clarified the effect and mechanism of dam construction on riverine CH₄ emission.

Freshwater trophic status mediates microbial community assembly and interdomain network complexity

Freshwater microorganisms and their interactions are important drivers of nutrient cycling that are in turn affected by nutrient status, causing shifts in microbial community diversity, composition, and interactions. However, the impact of water trophic status on bacterial-archaeal interdomain interactions remains poorly understood. This study focused on the impact of trophic status, as characterized by trophic state index (TSI), on the interdomain interactions of freshwater microbial communities from 45 ponds in Hangzhou. Our results showed that the mesotrophic wetland bordering on lightly eutrophic (Hemu: TSI of 49; lightly eutrophic is defined as 50 ≤ TSI <60) harbored a much more complex bacterial-archaeal interdomain network, which showed significantly (P < 0.05) higher connectivity than the wetlands with lower (TSI of 38) or higher (TSI of 57) trophic levels. Notably, light eutrophication strengthened the network modules' negative associations with organic carbon through some network hubs, which could trigger carbon loss in wetlands. We also detected a non-linear response of interdomain network complexity to the increasing of nutrients with a turning point of approximately TSI 50. Quantitative estimates of community assembly processes and structural equation modelling analysis indicated that chlorophyll-a, total nitrogen, and total phosphorus could regulate interdomain network complexity (50% of the variation explanation rate) by driving microbial community assembly. This study demonstrates that microbial interdomain network complexity could be used as a bioindicator for ecological changes, which would helpful for improving ecological assessment of the freshwater eutrophication.

Impacts of cyanobacterial biomass and nitrate nitrogen on methanogens in eutrophic lakes

Methanogenesis is a key process in carbon cycling in lacustrine ecosystems. Knowledge of the methanogenic pathway is important for creating mechanistic models as well as predicting methane emissions. Due to low concentrations of methyl substrates in freshwater lakes, the proportion of methylotrophic methanogenesis is believed to be negligible in such environments. However, the high abundance of methylotrophic methanogens previously detected in Dianchi Lake suggests that methylotrophic methanogenesis may be underestimated in eutrophic lakes, whereas their influencing factors and mechanisms are not yet clear. In this study, the effects of cyanobacteria biomass (CB) or/and nitrate nitrogen on methanogenesis, especially methylotrophic pathway, in eutrophic lakes were investigated using microcosm simulation experiments combined with chemical analysis and high-throughput sequencing techniques. The results showed that either CB or nitrate nitrogen had significant effects on methane flux, the archaeal diversity and community structure of methanogens. Functional prediction, together with the result of chemical analysis, revealed that CB could promote methylotrophic methanogenesis by providing methyl organic substrates, while nitrate nitrogen increased the relative abundance of obligate methylotrophic methanogens by competitively inhibiting the other two methanogenic pathways. In eutrophic lake where both CB and nitrate present at a high concentration, methylotrophic methanogenesis could play a much more important role than previously believed.

Frequent algal blooms dramatically increase methane while decrease carbon dioxide in a shallow lake bay

Freshwater ecosystems play a key role in global greenhouse gas estimations and carbon budgets, and algal blooms are widespread owing to intensified anthropological activities. However, little is known about greenhouse gas dynamics in freshwater experiencing frequent algal blooms. Therefore, to explore the spatial and temporal variations in methane (CH₄) and carbon dioxide (CO₂), seasonal field investigations were performed in the Northwest Bay of Lake Chaohu (China), where there are frequent algal blooms. From the highest site in the nearshore to the pelagic zones, the CH₄ concentration in water decreased by at least 80%, and this dynamic was most obvious in warm seasons when algal blooms occurred. CH₄ was 2-3 orders of magnitude higher than the saturated concentration, with the highest in spring, which makes this bay a constant source of CH₄. However, unlike CH₄, CO₂ did not change substantially, and river mouths acted as hotspots for CO₂ in most situations. The highest CO₂ concentration appeared in winter and was saturated, whereas at other times, CO₂ was unsaturated and acted as a sink. The intensive photosynthesis of rich algae decreased the CO₂ in the water and increased dissolved oxygen and pH. The increase in CH₄ in the bay was attributed to the mineralization of autochthonous organic carbon. These findings suggest that frequent algal blooms will greatly absorb more CO₂ from atmosphere and increasingly release CH₄, therefore, the contribution of the bay to the lake's CH₄ emissions and carbon budget will be major even though it is small. The results of this study will be the same to other shallow lakes with frequent algal bloom, making lakes a more important part of the carbon budget and greenhouse gases emission.

Changes in sediment methanogenic archaea community structure and methane production potential following conversion of coastal marsh to aquaculture ponds

Widespread conversion of coastal wetlands into aquaculture ponds in coastal region often results in degradation of the wetland ecosystems, but its effects on sediment's potential to produce greenhouse gases remain unclear. Using field sampling, incubation experiments and molecular analysis, we studied the sediment CH₄ production potential and the relevant microbial communities in a brackish marsh and the nearby aquaculture ponds in the Min River Estuary in southeastern China. Sediment CH₄ production potential was higher in the summer and autumn months than in spring and winter months, and it was significantly correlated with sediment carbon content among all environmental variables. The mean sediment CH₄ production potential in the aquaculture ponds (20.1 ng g^-1 d^-1) was significantly lower than that in the marsh (45.2 ng g^-1 d^-1). While Methanobacterium dominated in both habitats (41-59%), the overall composition of sediment methanogenic archaea communities differed significantly between the two habitats (p < 0.05) and methanogenic archaea alpha diversity was lower in the aquaculture ponds (p < 0.01). Network analysis revealed that interactions between sediment methanogenic archaea were much weaker in the ponds than in the marsh. Overall, these findings suggest that conversion of marsh land to aquaculture ponds significantly altered the sediment methanogenic archaea community structure and diversity and lowered the sediment's capacity to produce CH₄.

Pollution alters methanogenic and methanotrophic communities and increases dissolved methane in small ponds

Small ponds have become a hotspot of greenhouse gas emissions, but our understanding of methane (CH₄) cycling and its biological regulation in small polluted ponds remains limited. To assess how pollution affects CH₄ content, we investigated dissolved CH₄ concentrations, water and sediments properties, methanogenic and methanotrophic communities in two types of small polluted ponds. Compared with low pollution (LP) ponds, high pollution (HP) ponds showed significantly (P < 0.05) higher dissolved CH₄ in water. Sequencing of methyl coenzyme M reductase (mcrA) and particulate methane monooxygenase (pmoA) genes showed that HP led to significant (P < 0.05) shifts of CH₄-cycling microbial communities, with increased Shannon index of sediment methanogenic communities and water methanotrophic communities. There were also strong negative associations (P < 0.05) between dissolved CH₄ concentrations and interdomain methanogen-methanotroph network connectivity in water and sediments, respectively. The partial least squares path modeling indicated that dissolved oxygen, total organic carbon, ammonium nitrogen and nitrate nitrogen of water, and total nitrogen and total carbon of sediment, and CH₄-cycling microbes could regulate the CH₄ content. This study clarified the effects of environmental deterioration on CH₄ cycling in small ponds, highlighting the use of methanogen-methanotroph network connectivity to assess the CH₄ production.

Variability of the structure of winter microbial communities in Chelyabinsk lakes

Microorganisms form complex and dynamic communities that play a key role in the biogeochemical cycles of lakes. A high level of urbanization is currently a serious threat to bacterial communities and the ecosystem of freshwater bodies. To assess the contribution of anthropogenic load to variations in the structure of winter microbial communities in lakes, microorganisms of four water bodies of Chelyabinsk region were studied for the first time. We used cultural, chromatography-mass spectrometric, and modern methods of statistical data processing (particularly, multivariate exploratory analysis and canonical analysis of correspondences). The research showed that the composition of winter microbial communities in lakes Chebarkul’, Smolino, Pervoye, and Shershenevskoye Reservoir did not differ significantly between the main phyla of microorganisms. The dominant microorganisms were found to be of the Firmicutes phylum and Actinobacteria phylum. The structure of bacterial communities had special features depending on the characteristics of the water body and the sampling depths. Thus, in the lakes Smolino, Pervoye, and Shershenevskoye Reservoir, an important role was played by associations between microorganisms – indicators of fecal contamination: coliform bacteria and Enterococcus. On the contrary, in Chebarkul’ Lake, members of the genus Bacillus, which are natural bioremediators, formed stable winter associations. However, the differences between water bodies and sampling depths reflected 28.1% and 9.8% of the variability of the winter microbial communities, respectively. The largest contribution (about 60%) to the variability of the structure was made by intra-water processes, which determined the high heterogeneity of samples from different water areas. We assume that an important role in this variability was played by the high anthropogenic impact in a large industrial metropolis. In our opinion, this line of research is very promising for addressing key environmental issues.

Variability of the structure of winter microbial communities in Chelyabinsk lakes

Microorganisms form complex and dynamic communities that play a key role in the biogeochemical cycles of lakes. A high level of urbanization is currently a serious threat to bacterial communities and the ecosystem of freshwater bodies. To assess the contribution of anthropogenic load to variations in the structure of winter microbial communities in lakes, microorganisms of four water bodies of Chelyabinsk region were studied for the first time. We used cultural, chromatography-mass spectrometric, and modern methods of statistical data processing (particularly, multivariate exploratory analysis and canonical analysis of correspondences). The research showed that the composition of winter microbial communities in lakes Chebarkul’, Smolino, Pervoye, and Shershenevskoye Reservoir did not differ significantly between the main phyla of microorganisms. The dominant microorganisms were found to be of the Firmicutes phylum and Actinobacteria phylum. The structure of bacterial communities had special features depending on the characteristics of the water body and the sampling depths. Thus, in the lakes Smolino, Pervoye, and Shershenevskoye Reservoir, an important role was played by associations between microorganisms – indicators of fecal contamination: coliform bacteria and Enterococcus. On the contrary, in Chebarkul’ Lake, members of the genus Bacillus, which are natural bioremediators, formed stable winter associations. However, the differences between water bodies and sampling depths reflected 28.1% and 9.8% of the variability of the winter microbial communities, respectively. The largest contribution (about 60%) to the variability of the structure was made by intra-water processes, which determined the high heterogeneity of samples from different water areas. We assume that an important role in this variability was played by the high anthropogenic impact in a large industrial metropolis. In our opinion, this line of research is very promising for addressing key environmental issues.

Seasonal Dynamics of Abundance, Structure, and Diversity of Methanogens and Methanotrophs in Lake Sediments

Understanding temporal and spatial microbial community abundance and diversity variations is necessary to assess the functional roles played by microbial actors in the environment. In this study, we investigated spatial variability and temporal dynamics of two functional microbial sediment communities, methanogenic Archaea and methanotrophic bacteria, in Lake Bourget, France. Microbial communities were studied from 3 sites sampled 4 times over a year, with one core sampled at each site and date, and 5 sediment layers per core were considered. Microbial abundance in the sediment were determined using flow cytometry. Methanogens and methanotrophs community structures, diversity, and abundance were assessed using T-RFLP, sequencing, and real-time PCR targeting mcrA and pmoA genes, respectively. Changes both in structure and abundance were detected mainly at the water-sediment interface in relation to the lake seasonal oxygenation dynamics. Methanogen diversity was dominated by Methanomicrobiales (mainly Methanoregula) members, followed by Methanosarcinales and Methanobacteriales. For methanotrophs, diversity was dominated by Methylobacter in the deeper area and by Methylococcus in the shallow area. Organic matter appeared to be the main environmental parameter controlling methanogens, while oxygen availability influenced both the structure and abundance of the methanotrophic community.

Cyanobacterial bloom induces structural and functional succession of microbial communities in eutrophic lake sediments

Cyanobacterial blooms have considerable effects on lacustrine microbial communities. The current study explored the temporal pattern of sedimentary archaea and bacteria during cyanobacterial bloom in a eutrophic lake. With the sampling period divided into bloom phase, interval phase and end phase according to the variation of physicochemical parameters, the structures and functions of both kingdoms presented a significant difference among phases. Bloom phases could be characterized with the lowest diversity and up-regulated functions in biodegradation of cyanobacterial metabolites driven by bacteria. Archaeal community showed an increased metabolic function during interval phases, including active methanogenesis sensitive to carbon input. The highest diversity and an enrichment of hub genera in microbial network were both observed in end phase, allowing for closer cooperation among groups involved in cyanobacteria-derived organic matter transformation. Although the archaeal community was less variable or diverse than bacteria, methanogenic functions dramatically fluctuated with cyanobacterial dynamics. And microbial groups related to methane cycling played an important role in microbial network. The results provided new insights into temporal dynamics of lacustrine microbial communities and microbial co-occurrence, and highlighted the significant ecological role of methane cycling-related microbes in lake sediments under the influence of cyanobacterial blooms.

Algal Organic Matter Drives Methanogen-Mediated Methylmercury Production in Water from Eutrophic Shallow Lakes

Algal blooms bring massive amounts of algal organic matter (AOM) into eutrophic lakes, which influences microbial methylmercury (MeHg) production. However, because of the complexity of AOM and its dynamic changes during algal decomposition, the relationship between AOM and microbial Hg methylators remains poorly understood, which hinders predicting MeHg production and its bioaccumulation in eutrophic shallow lakes. To address that, we explored the impacts of AOM on microbial Hg methylators and MeHg production by characterizing dissolved organic matter with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy and quantifying the microbial Hg methylation gene hgcA. We first reveal that the predominance of methanogens, facilitated by eutrophication-induced carbon input, could drive MeHg production in lake water. Specifically, bioavailable components of AOM (i.e., CHONs such as aromatic proteins and soluble microbial byproduct-like materials) increased the abundances (Archaea-hgcA gene: 438-2240% higher) and activities (net CH₄ production: 16.0-44.4% higher) of Archaea (e.g., methanogens). These in turn led to enhanced dissolved MeHg levels (24.3-15,918% higher) for three major eutrophic shallow lakes in China. Nevertheless, our model results indicate that AOM-facilitated MeHg production could be offset by AOM-induced MeHg biodilution under eutrophication. Our study would help reduce uncertainties in predicting MeHg production, providing a basis for mitigating the MeHg risk in eutrophic lakes.

Characteristics and metabolic patterns of soil methanogenic archaea communities in the high-latitude natural forested wetlands of China

Soil methanogenic microorganisms are one of the primary methane-producing microbes in wetlands. However, we still poorly understand the community characteristic and metabolic patterns of these microorganisms according to vegetation type and seasonal changes. Therefore, to better elucidate the effects of the vegetation type and seasonal factors on the methanogenic community structure and metabolic patterns, we detected the characteristics of the soil methanogenic mcrA gene from three types of natural wetlands in different seasons in the Xiaoxing'an Mountain region, China. The results indicated that the distribution of Methanobacteriaceae (hydrogenotrophic methanogens) was higher in winter, while Methanosarcinaceae and Methanosaetaceae accounted for a higher proportion in summer. Hydrogenotrophic methanogenesis was the dominant trophic pattern in each wetland. The results of principal coordinate analysis and cluster analysis showed that the vegetation type considerably influenced the methanogenic community composition. The methanogenic community structure in the Betula platyphylla-Larix gmelinii wetland was relatively different from the structure of the other two wetland types. Indicator species analysis further demonstrated that the corresponding species of indicator operational taxonomic units from the Alnus sibirica wetland and the Betula ovalifolia wetland were similar. Network analysis showed that cooperative and competitive relationships exist both within and between the same or different trophic methanogens. The core methanogens with higher abundance in each wetland were conducive to the adaptation to environmental disturbances. This information is crucial for the assessment of metabolic patterns of soil methanogenic archaea and future fluxes in the wetlands of the Xiaoxing'an Mountain region given their vulnerability.

Dramatic temporal variations in methane levels in black bloom prone areas of a shallow eutrophic lake

Global lakes serve as a key natural source of methane (CH₄) and suffer from increasing hypoxia due to unprecedented anthropogenic activities and climate change. A black bloom is a temporary hypoxia triggered by a longstanding algal bloom, which facilitates CH₄ production by creating reducing conditions and abundant algae-sourced organic carbon. One-year investigations were conducted to examine temporal CH₄ dynamics in the water and sediment pore water in black bloom prone areas (BBPAs) in Lake Taihu, China, where there had been at least two recorded black bloom events. The CH₄ in the water changed significantly with time (p < 0.001), with the highest concentrations appearing in warm months when an abnormal lower dissolved oxygen content was observed at different sites, which were one to two orders of magnitude higher than other months. Compared with the control site, there were significantly higher CH₄ concentrations in BBPA waters (p < 0.001), which was consistent with the higher CH₄ in the sediment pore water. Methane dynamics in the water showed significant positive correlations with temperature, total phosphorus, total nitrogen, ammonia-N, and soluble reactive phosphorus (p < 0.05), but showed a significant inverse correlation with dissolved oxygen (p < 0.01). Redundancy analysis indicated dissolved oxygen made the largest contribution to CH₄ dynamics in the BBPAs. A significant increase in the CH₄ in water will turn BBPAs into temporary hot spots with substantial CH₄ emissions with the appearance of black blooms. The results provide new insights into understanding future CH₄ dynamics under globally prevailing algal blooms and climate change.

Spatial variations in diffusive methane fluxes and the role of eutrophication in a subtropical shallow lake

Shallow lakes account for most of the diffusive CH₄ emissions from global lakes, and they also suffer from eutrophication worldwide. Determining the effect of eutrophication on diffusive CH₄ fluxes is fundamental to understanding CH₄ emissions in shallow lakes. This study aimed to investigate the spatial variations in diffusive CH₄ fluxes and explore the role of eutrophication in Lake Chaohu, a large and shallow eutrophic lake in the lower reaches of the Yangtze River. A one-year field observation was carried out to examine CH₄ concentrations in the sediment and water and the diffusive fluxes of CH₄ across the sediment-water interface (F_s-w) and water-air interface (F_w-a). Both F_s-w (0.306-1.56 mmol m^-2 d^-1) and F_w-a (0.097-0.529 mmol m^-2 d^-1) were upward and showed significant spatial heterogeneity and were significantly positively correlated. Parameters related to eutrophication had significant positive relationships with F_w-a, and the total phosphorus distribution in the water explained the greatest proportion of the spatial variation in F_w-a. Distance to shore and water depth were inversely correlated with F_w-a and modified the effects of eutrophication. Overall, the results provide direct evidence of the key role of eutrophication in shaping the spatial distribution of diffusive CH₄ fluxes and a scientific basis for predicting changes in CH₄ emissions with future eutrophication changes in shallow lakes in subtropical zones.

Expanding ecological assessment by integrating microorganisms into routine freshwater biomonitoring

Bioindication has become an indispensable part of water quality monitoring in most countries of the world, with the presence and abundance of bioindicator taxa, mostly multicellular eukaryotes, used for biotic indices. In contrast, microbes (bacteria, archaea and protists) are seldom used as bioindicators in routine assessments, although they have been recognized for their importance in environmental processes. Recently, the use of molecular methods has revealed unexpected diversity within known functional groups and novel metabolic pathways that are particularly important in energy and nutrient cycling. In various habitats, microbial communities respond to eutrophication, metals, and natural or anthropogenic organic pollutants through changes in diversity and function. In this review, we evaluated the common trends in these changes, documenting that they have value as bioindicators and can be used not only for monitoring but also for improving our understanding of the major processes in lotic and lentic environments. Current knowledge provides a solid foundation for exploiting microbial taxa, community structures and diversity, as well as functional genes, in novel monitoring programs. These microbial community measures can also be combined into biotic indices, improving the resolution of individual bioindicators. Here, we assess particular molecular approaches complemented by advanced bioinformatic analysis, as these are the most promising with respect to detailed bioindication value. We conclude that microbial community dynamics are a missing link important for our understanding of rapid changes in the structure and function of aquatic ecosystems, and should be addressed in the future environmental monitoring of freshwater ecosystems.

Carbon cloth facilitates semi-continuous anaerobic digestion of organic wastewater rich in volatile fatty acids from dark fermentation

The anaerobic digestion of wastewater rich in volatile fatty acids (VFAs) provides a sustainable approach for methane production whilst reducing environmental pollution. However, the anaerobic digestion of VFAs may not be stable during long-term operation under a short hydraulic retention time. In this study, conductive carbon cloth was supplemented to investigate the impacts on the anaerobic digestion of VFAs in wastewater sourced from dark fermentation. The results demonstrated that the failure of anaerobic digestion could be avoided when carbon cloth was supplemented. In the stable stage, the methane production rate with carbon cloth supplementation was improved by 200-260%, and the chemical oxygen demand (COD) removal efficiency was significantly enhanced compared with that in the control without carbon cloth. The relative abundance of potential exoelectrogens on the carbon cloth was increased by up to 8-fold compared with that in the suspension. Electrotrophic methanogens on the carbon cloth were enriched by 4.2-17.2% compared with those in the suspension. The genera Ercella and Petrimonas along with the methanogenic archaea Methanosaeta and Methanosarcina on the carbon cloth may facilitate direct interspecies electron transfer, thereby enhancing methane production.