In situ anoxic bioremediation is an easy-to-use technology to remediate polycyclic aromatic hydrocarbon (PAH)-contaminated soil. Degradation of PAHs mediated by soil bacteria and archaea using CO₂ as the electron acceptor is an important process for eliminating PAHs under methanogenic conditions; however, knowledge of the performance and mechanisms involved is poorly unveiled. In this study, the effectiveness and efficiency of NaHCO₃ (CO₂) as an electron acceptor to stimulate the degradation of PAHs by bacteria and archaea in highly contaminated soil were investigated. The results showed that CO₂ addition (EC2000) promoted PAH degradation compared to soil without added CO₂ (EC0), with 4.18%, 9.01%–8.05%, and 6.19%–12.45% increases for 2-, 3- and 4-ring PAHs after 250 days of incubation, respectively. Soil bacterial abundances increased with increasing incubation time, especially for EC2000 (2.90 × 10⁸ g⁻¹ soil higher than EC0, p < 0.05). Different succession patterns of the soil bacterial and archaeal communities during PAH degradation were observed. According to the PCoA and ANOSIM results, the soil bacterial communities were greatly (ANOSIM: R = 0.7232, P = 0.001) impacted by electron acceptors, whereas significant differences in the archaeal communities were not observed (ANOSIM: R = 0.553, P = 0.001). Soil bacterial and archaeal co-occurrence network analyses showed that positive correlations outnumbered the negative correlations throughout the incubation period for both treatments (e.g., EC0 and EC2000), suggesting the prevalence of coexistence/cooperation within and between these two domains rather than competition. The higher complexity, connectance, edge, and node numbers in EC2000 revealed stronger linkage and a more stable co-occurrence network compared to EC0. The results of this study could improve the knowledge on the removal of PAHs and the responses of soil bacteria and archaea to CO₂ application, as well as a scientific basis for the in situ anoxic bioremediation of PAH-contaminated industrial sites.

Human activities such as dams disturb the structure and function of wetlands, triggering large soil CO₂ and CH₄ emissions. However, controls over field CO₂ and CH₄ emissions and their carbon isotopic signatures in reservoir wetlands are not yet fully understood. We investigated in situ CO₂ and CH₄ emissions, the δ¹³C values of CO₂ and CH₄, and associated environments in the saturated and drained states under four elevations (i.e., the water column, <147 m, permanent inundation area without plants; the low, 145–160 m, frequently flooded area with revegetation; the high, 160–175 m, rarely flooded area with revegetation; and the upland area as the control, >175 m, nonflooded area with original plants) in the Three Gorges Reservoir area. The CO₂ emissions was significantly higher in high elevation, and they also significantly differed between the saturated and drained states. In contrast, the CH₄ emissions on average (41.97 μg CH₄ m⁻² h⁻¹) were higher at high elevations than at low elevations (22.73 μg CH₄ m⁻² h⁻¹) during the whole observation period. CH₄ emissions decreased by 90% at low elevations and increased by 153% at high elevations from the saturated to drained states. The δ¹³C of CH₄ was more enriched at high elevations than in the low and upland areas, with a more depleted level under the saturated state than under the drained state. We found that soil CO₂ and CH₄ emissions were closely related to soil substrate quality (e.g., C: N ratio) and enzyme activities, whereas the δ¹³C values of CO₂ and CH₄ were primarily associated with root respiration and methanogenic bacteria, respectively. Specifically, the effects of the saturated and drained states on soil CO₂ and CH₄ emissions were stronger than the effect of reservoir elevation, thereby providing an important basis for assessing carbon neutrality in response to anthropogenic activities.

Methylmercury (MeHg) produced by anaerobic bacteria in lakes and reservoirs, poses a threat to ecosystem and human health due to its ability to bioaccumulate in aquatic food webs. This study used 48-hr microcosm incubations of profundal sediment and bottom water from a sulfate-rich, hypereutrophic reservoir to assess seasonal patterns of MeHg cycling under various treatments. Treatments included addition of air, Hg(II), organic carbon, and microbial inhibitors. Both aeration and sodium molybdate, a sulfate-reducing bacteria (SRB) inhibitor, generally decreased MeHg concentration in microcosm water, likely by inhibiting SRB activity. The methanogenic inhibitor bromoethanesulfonate increased MeHg concentration 2- to 4- fold, suggesting that methanogens were potent demethylators. Pyruvate increased MeHg concentration under moderately reduced conditions, likely by stimulating SRB, but decreased it under highly reduced conditions, likely by stimulating methanogens. Acetate increased MeHg concentration, likely due to the stimulation of acetotrophic SRB. Results suggest that iron-reducing bacteria (IRB) were not especially prominent methylators and MeHg production at the sediment-water interface is elevated under moderately reduced conditions corresponding with SRB activity. In contrast, it is suppressed under oxic conditions due to low SRB activity, and under highly reduced conditions (<-100 mV) due to enhanced demethylation by methanogens.

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.

Bacteria and archaea (prokaryotes) are vital components for maintaining healthy function of groundwater ecosystems. The prokaryotic community composition and associated putative functional processes were examined in a shallow sandy aquifer in a wet-dry tropical environment. The aquifer had a contaminated gradient of saline mine-water, which primarily consisted of elevated magnesium (Mg²⁺) and sulfate (SO₄²⁻), although other major ions and trace metals were also present. Groundwaters were sampled from piezometers, approximately 2 m in depth, located in the creek channel upstream and downstream of the mine-water influence. Sampling occurred during the dry-season when only subsurface water flow was present. Next generation sequencing was used to analyse the prokaryote assemblages using 16S rDNA and metabolic functions were predicted with FAPROTAX. Significant changes in community composition and functional processes were observed with exposure to mine-waters. Communities in the exposed sites had significantly lower relative abundance of methanotrophs such as Methylococcaceae and methanogens (Methanobacteriaceae), but higher abundance in Nitrososphaeraceae, associated with nitrification, indicating potentially important changes in the biogeochemistry of the exposed sites. The changes were most strongly correlated with concentrations of SO₄²⁻, Mg²⁺ and Na⁺. This knowledge allows an assessment of the risk of mine-water contamination to groundwater ecosystem function and aids mine-water management.

In this study, the YH consortium, an ethene-producing culture, was used to evaluate the effect of vitamin B₁₂ (VB₁₂) on trichloroethene (TCE) dechlorination by transferring the original TCE-reducing culture with or without adding exogenous VB₁₂. Ultra-high performance liquid chromatography - tandem mass spectrometry (UPLC-MS/MS) was applied to detect the concentrations of VB₁₂ and its lower ligand 5,6-dimethylbenzimidazole (DMB) in the cultures. After three successive VB₁₂ starvation cycles, the dechlorination of TCE stopped mostly at cis-dichloroethene (cDCE), and no ethene was found; methane production increased significantly, and no VB₁₂ was detected. Results suggest that the co-cultured microbes may not be able to provide enough VB₁₂ as a cofactor for the growth of Dehalococcoides in the YH culture, possibly due to the competition for corrinoids between Dehalococcoides and methanogens. The relative abundances of 16 S rRNA gene of Dehalococcoides and reductive dehalogenase genes tceA or vcrA were lower in the cultures without VB₁₂ compared with the cultures with VB₁₂. VB₁₂ limitation changed the microbial community structures of the consortia. In the absence of VB₁₂, the microbial community shifted from dominance of Chloroflexi to Proteobacteria after three consecutive VB₁₂ starvation cycles, and the dechlorinating genus Dehalococcoides declined from 42.9% to 13.5%. In addition, Geobacter, Clostridium, and Desulfovibrio were also present in the cultures without VB₁₂. Furthermore, the abundance of archaea increased under VB₁₂ limited conditions. Methanobacterium and Methanosarcina were the predominant archaea in the culture without VB₁₂.

In this work, anthraquinone-2-sulfonate (AQS) was covalently immobilized onto activated carbon cloth (ACC), to be used as redox mediator for the reductive decolorization of reactive red 2 (RR2) by an anaerobic consortium. The immobilization of AQS improved the capacity of ACC to transfer electrons, evidenced by an increment of 3.29-fold in the extent of RR2 decolorization in absence of inhibitors, compared to incubations lacking AQS. Experiments conducted in the presence of vancomycin, an inhibitor of acidogenic bacteria, and with 2-bromoethane sulfonic acid (BES), an inhibitor of methanogenic archaea, revealed that acidogenic bacteria are the main responsible for RR2 biotransformation mediated by immobilized AQS. Nonetheless, the results also suggest that some methanogens are able to maintain their capacity to use immobilized AQS as an electron acceptor to sustain the decolorization process, even in the presence of BES.

Arsenic (As) contamination is a global problem. Straw incorporation is widely performed in As contaminated paddy fields. To understand how straw and straw biochar incorporation affect As transformation and translocation in the soil-microbe-rice system, a pot experiment was carried out with different dosages of rice straw and straw biochar application. Results showed that both straw biochar and straw application significantly increased As mobility. Straw biochar mobilized As mainly through increasing soil pH and DOM content. Straw incorporation mainly through enhancing As release from iron (Fe) minerals and arsenate (As(V)) reduction to arsenite (As(III)). Straw biochar didn't significantly affect As methylation, while straw incorporation significantly enhanced As methylation, elevated dimethylarsenate (DMA) concentration in soil porewater and increased As volatilization. Straw biochar didn't significantly change total As accumulation in rice grains, but decreased As(III) accumulation by silicon (Si) inhibition. Straw incorporation significantly increased DMA, but decreased As(III) concentration in rice grains. After biochar application, dissolved As was significantly positively correlated with the abundance of Bacillus, indicating that Bacillus might be involved in As release, and As(III) concentration in polished grains was negatively correlated with Si concentration. The significant positive correlation between dissolved As with Fe and the abundance of iron-reducing bacteria suggested the coupling of As and Fe reduction mediated by iron-reducing bacteria. The significant positive correlation between DMA in rice grains and the abundance of methanogenic bacteria indicated that methanogenic bacteria could be involved in As methylation after straw application. The results of this study would advance the understanding how rice straw incorporation affects As fate in soil-microbe-rice system, and provide some guidance to straw incorporation in As contaminated paddy soil. This study also revealed a wealth of microorganisms in the soil environment that dominate As mobility and transformation after straw incorporation.

Biochar application into paddy is an improved strategy for addressing methane (CH₄) stimulation of straw biomass incorporation. Whereas, the differentiative patterns and mechanisms on CH₄ emission of straw biomass and biochar after long years still need to be disentangled. Considering economic feasibility, a seven-year of field experiment was conducted to explore the long-term CH₄ mitigation effect of annual low-rate biochar incorporation (RSC, 2.8 t ha⁻¹), with annual rice straw incorporation (RS, 8 t ha⁻¹) and control (CK, with no biochar or rice straw amendment incorporation) as a comparation. Results showed that RSC mitigated CH₄ emission while RS stimulated CH₄ significantly (p < 0.05) and stably over 7 experimental years compared with CK. RSC mitigated 14.8–46.7% of CH₄ emission compared with CK. In comparison to RSC, RS increased 111–950.5% of CH₄ emission during 7 field experimental years. On the 7th field experimental year, pH was significantly increased both in RS and RSC treatment (p < 0.05). RSC significantly (p < 0.05) increased soil nitrate (NO₃⁻-N) compared with RS while RS significantly (p < 0.05) increased dissolved carbon (DOC) compared to RSC. Soil NO₃⁻-N inhibition on methanogens and promotion on methanotrophs activities were verified by laboratory experiment, while soil pH and DOC mainly promoted methanogens abundance. Significantly (p < 0.05) increased DOC and soil pH enhanced methanogens growth and stimulated CH₄ emission in RS treatment. Higher soil NO₃⁻-N content in RSC than CK and RS contributed to CH₄ mitigation. Soil NO₃⁻-N and DOC were identified as the key factors differentiating CH₄ emission patterns of RS and RSC in 2019. Collectively, soil NO₃⁻-N impacts on CH₄ flux provide new ideas for prolonged effect of biochar amendment on CH₄ mitigation after years.

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.