Methane emissions from aqueous sediments are influenced by complex interactions among microbes and environmental factors: A modeling study

Unexpected increase of sulfate concentrations and potential impact on CH4 budgets in freshwater lakes

The continuous increase in sulfate (SO42-) concentrations discharged by anthropogenic activities lacks insights into their dynamics and potential impact on CH4 budgets in freshwater lakes. Here we conducted a field investigation in the lakes along the highly developed Yangtze River basin, China, additionally, we analyzed long-term data (1950-2020) from Lake Taihu, a typical eutrophic lake worldwide. We observed a gradual increase in SO42- concentrations up to 100 mg/L, which showed a positive correlation with the trophic state of the lakes. The annual variations indicated that eutrophication intensified the fluctuation of SO42- concentrations. A random forest model was applied to assess the impact of SO42- concentrations on CH4 emissions, revealing a significant negative effect. Synchronously, a series of microcosms with added SO42- were established to simulate cyanobacteria decomposition processes and explore the coupling mechanism between sulfate reduction and CH4 production. The results showed a strong negative correlation between CH4 concentrations and initial SO42- levels (R2 = 0.83), indicating that higher initial SO42- concentrations led to lower final CH4 concentrations. This was attributed to the competition for cyanobacteria-supplied substrates between sulfate reduction bacteria (SRB) and methane production archaea (MPA). Our study highlights the importance of considering the unexpectedly increasing SO42- concentrations in eutrophic lakes when estimating global CH4 emission budgets.

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.

Space-for-time substitution leads to carbon emission overestimation in eutrophic lakes

Lacustrine eutrophication is generally considered as an important contributor of carbon emissions to the atmosphere; however, there is still a huge challenge in accuracy estimating carbon emissions from lakes. To test the effect of widely used space-for-time substitution on lake carbon emissions, this study monitored different processes of carbon emissions, including the carbon production potential, dissolved carbon concentrations, and carbon release fluxes in eight lakes along the trophic gradients on a spatial scale and the typical eutrophic Lake Taihu for one year on a temporal scale. Eutrophication promoted carbon production potential, dissolved carbon concentrations, and carbon release fluxes, especially for CH₄. Trophic lake index (TLI) showed positive correlations with the CH₄ production potential, dissolved CH₄ concentrations, and CH₄ release fluxes, and also positive correlations with the CO₂ production potential, dissolved CO₂ concentrations, and CO₂ release fluxes. The space-for-time substitution led to an overestimation for the influence of eutrophication on carbon emissions, especially the further intensification of lake eutrophication. On the spatial scale, the average CH₄ production potential, dissolved CH₄ concentrations and CH₄ release fluxes in eutrophic lakes were 268.6, 0.96 μmol/L, and 587.6 μmol m^-2·h^-1, respectively, while they were 215.8, 0.79 μmol/L, and 548.6 μmol m^-2·h^-1 on the temporal scale. Obviously, CH₄ and CO₂ emissions on the spatial scale were significantly higher than those on the temporal scale in eutrophic lakes. The primary influencing factors were the seasonal changes in the physicochemical environments of lake water, including dissolved oxygen (DO) and temperature. The CH₄ and CO₂ release fluxes showed negative correlations with DO, while temperature displayed positive correlations, respectively. These results suggest that the effects of DO and temperature on lake carbon emissions should be considered, which may be ignored during the accurate assessment of lake carbon budget via space-for-time substitution in eutrophic lakes.

Decomposition process of cefotaxime sodium from antibiotic wastewater by Up-flow Blanket Filter (UBF) reactor: Reactor performance, sludge characteristics and microbial community structure analysis

In this study, a novel Up-flow Blanket Filter (UBF) reactor was applied to the degradation of antibiotic wastewater. The experiments showed that when the hydraulic retention time (HRT) was 24 h and the ratio of volatile fatty acids (VFA) to alkalinity (ALK) was 0.3, the best removal efficiency was achieved in the combined packing UBF reactor, and the COD removal efficiency reached 80.1%-84.6%, exhibiting a significant difference in reaction performance from the other two reactors (P < 0.05) and a good efficiency of cefotaxime sodium removal. Moreover, the microstructure and surface characteristics of the reactor fillers were studied through scanning electron microscope (SEM) analysis, which showed that three fillers all had biofilm adhesion, but the combined packing gave best performance. Energy dispersive spectrometer (EDS) tests indicated abundant element components in the combined packing. The particle size distribution of sludge was also considered in the experiment, and the result showed the particle size of sludge increased with the operation of the reactor. In addition, microbial community structures of sludge and biofilm with the combined packing were analyzed. High-throughput sequencing confirmed the existence of Pseudomonas, which had good adaptability to antibiotic wastewater and became the dominant bacteria. Decomposition process of cefotaxime sodium after hydrolysis and anaerobic treatment was analyzed through Fourier transform infrared spectroscopy (FTIR). The reactor, which is economical, exhibited favorable performance in degrading the pollutions in the antibiotic wastewater.

Nonlinear response of methane release to increased trophic state levels coupled with microbial processes in shallow lakes

Shallow lakes are a crucial source of methane (CH₄), a potent greenhouse gas, to the atmosphere. However, large uncertainties still exist regarding the response of CH₄ emissions to the increasing trophic levels of lakes as well as the underlying mechanisms. Here, we investigate the CH₄ emission flux from lakes with different trophic states in the middle and lower reaches of the Yangtze River basin, China to evaluate the effect of the trophic lake index (TLI) on CH₄ emissions. The mean CH₄ emission fluxes from mesotrophic, eutrophic, middle-eutrophic, and hyper-eutrophic lakes were 0.1, 4.4, 12.0, and 130.4 mg m^-2 h^-1, respectively. Thus, the CH₄ emission flux ranged widely and was positively correlated with the degree of eutrophication. The relative abundance of methanogens with respect to the total population for the mesotrophic, eutrophic, mid-eutrophic, and hyper-eutrophic states was 0.03%, 0.35%, 0.94%, and 1.17%, respectively. The biogeographic-scale pattern of lakes classified as each of these four trophic states indicated that CH₄ emissions could be well-predicted by the NH₄⁺-N concentration in the water column, as both NH₄⁺-N and CH₄ were produced during mineralisation of labile organic matter in lake sediment. In addition, the shift from clear to turbid water, which is an unhealthy evolution for lakes, was associated with a nonlinear increase in the CH₄ emissions from the studied lakes. In particular, the hypereutrophic lakes functioned as CH₄ emission hotspots. Our findings highlight that nutrient levels, as a potential facilitator of CH₄ emissions, should be considered in future research to accurately evaluate the greenhouse gas emissions from shallow lakes.