Coupled Aerobic Methane Oxidation and Arsenate Reduction Contributes to Soil-Arsenic Mobilization in Agricultural Fields

Anaerobic oxidation of methane driven by different electron acceptors: A review

Methane, the most significant reduced form of carbon on Earth, acts as a crucial fuel and greenhouse gas. Globally, microbial methane sinks encompass both aerobic oxidation of methane (AeOM), conducted by oxygen-utilizing methanotrophs, and anaerobic oxidation of methane (AOM), performed by anaerobic methanotrophs employing various alternative electron acceptors. These electron acceptors involved in AOM include sulfate, nitrate/nitrite, humic substances, and diverse metal oxides. The known anaerobic methanotrophic pathways comprise the internal aerobic oxidation pathway found in NC10 bacteria and the reverse methanogenesis pathway utilized by anaerobic methanotrophic archaea (ANME). Diverse anaerobic methanotrophs can perform AOM independently or in cooperation with symbiotic partners through several extracellular electron transfer (EET) pathways. AOM has been documented in various environments, including seafloor methane seepages, coastal wetlands, freshwater lakes, soils, and even extreme environments like hydrothermal vents. The environmental activities of AOM processes, driven by different electron acceptors, primarily depend on the energy yields, availability of electron acceptors, and environmental adaptability of methanotrophs. It has been suggested that different electron acceptors driving AOM may occur across a wider range of habitats than previously recognized. Additionally, it is proposed that methanotrophs have evolved flexible metabolic strategies to adapt to complex environmental conditions. This review primarily focuses on AOM, driven by different electron acceptors, discussing the associated reaction mechanisms and the habitats where these processes are active. Furthermore, it emphasizes the pivotal role of AOM in mitigating methane emissions.

Key energy metabolisms in modern living microbialites from hypersaline Andean lagoons of the Salar de Atacama, Chile

Microbialites are organosedimentary structures formed mainly due to the precipitation of carbonate minerals, although they can also incorporate siliceous, phosphate, ferric, and sulfate minerals. The minerals' precipitation occurs because of local chemical changes triggered by changes in pH and redox transformations catalyzed by the microbial energy metabolisms. Here, geochemistry, metagenomics, and bioinformatics tools reveal the key energy metabolisms of microbial mats, stromatolites and an endoevaporite distributed across four hypersaline lagoons from the Salar de Atacama. Chemoautotrophic and chemoheterotrophic microorganisms seem to coexist and influence microbialite formation. The microbialite types of each lagoon host unique microbial communities and metabolisms that influence their geochemistry. Among them, photosynthetic, carbon- and nitrogen- fixing and sulfate-reducing microorganisms appear to control the main biogeochemical cycles. Genes associated with non-conventional energy pathways identified in MAGs, such as hydrogen production/consumption, arsenic oxidation/reduction, manganese oxidation and selenium reduction, also contribute to support life in microbialites. The presence of genes encoding for enzymes associated with ureolytic processes in the Cyanobacteria phylum and Gammaproteobacteria class might induce carbonate precipitation in hypersaline environments, contributing to the microbialites formation. To the best of our knowledge, this is the first study characterizing metagenomically microbialites enriched in manganese and identifying metabolic pathways associated with manganese oxidation, selenium reduction, and ureolysis in this ecosystem, which suggests that the geochemistry and bioavailability of energy sources (As, Mn and Se) shapes the microbial metabolisms in the microbialites.

Identification of methane cycling pathways in Quaternary alluvial-lacustrine aquifers using multiple isotope and microbial indicators

Groundwater rich in dissolved methane is often overlooked in the global or regional carbon cycle. Considering the knowledge gap in understanding the biogeochemical behavior of methane in shallow aquifers, particularly those in humid alluvial-lacustrine plains with high organic carbon content, we investigated methane sources and cycling pathways in groundwater systems at the central Yangtze River basins. Composition of multiple stable isotopes (2H/18O in water, 13C in dissolved inorganic carbon, 13C/2H in methane, and 13C in carbon dioxide) was combined with the characteristics of microbes and dissolved organic matter (DOM) in the study. The results revealed significant concentrations of biogenic methane reaching up to 13.05 mg/L in anaerobic groundwater environments with abundant organic matter. Different pathways for methane cycling (methanogenic CO2-reduction and acetate-fermentation, and methane oxidation) were identified. CO2-reduction dominated acetate-fermentation in the two methanogenic pathways primarily associated with humic DOM, while methane oxidation was more closely associated with microbially derived DOM. The abundance of obligate CO2-reduction microorganisms (Methanobacterium and Methanoregula) was higher in samples with substantial CO2-reduction, as indicated by isotopic composition. The obligate acetate-fermentation microorganism (Methanosaeta) was more abundant in samples exhibiting evident acetate-fermentation. Additionally, a high abundance of Candidatus Methanoperedens was identified in samples with apparent methane oxidation. Comparing our findings with those in other areas, we found that various factors, such as groundwater temperature, DOM abundance and types, and hydrogeological conditions, may lead to differences in groundwater methane cycling. This study offered a new perspective and understanding of methane cycling in worldwide shallow alluvial-lacustrine aquifer systems without geothermal disturbance.

Reduced arsenic availability in paddy soil through Fe-organic ligand complexation mediated by bamboo biochar

The reuse of arsenic (As)-contaminated paddy fields is a global challenge because long-term flooding would result in As release due to the reductive dissolution of iron minerals. Biochar amendment is a common and effective remediation technique for As-contaminated paddy soil. However, the literature is still lacking in systematic research on the function of biochar in controlling the complexation of released dissolved organic matter (DOM) and iron oxides and its synergistic impact on the availability of As in flooded paddy soil. In the present study, bamboo biochar was prepared at different pyrolysis temperatures (300, 450 and 600 °C), as BB300, BB450 and BB600. Four paddy soil treatments including BB300, BB450, BB600 applications (1% ratio, m/m, respectively) and control (CK, no biochar application) were set and incubated for 60 d in flooding condition. The results showed that As availability represented by adsorbed As species (A-As) was mitigated by BB450 amendment compared with CK. The amendment of BB450 in paddy soil facilitated the complexation of HCl extractable Fe(III)/(II) and DOM and formation of amorphous iron oxides (e.g. complexed Fe species). Moreover, the abundance of Geobacteraceae and Xanthomonadaceae, as common electroactive bacteria, was promoted in the BB450 treated paddy soil in comparison to CK, which assisted to form amorphous iron oxides. The formed amorphous iron oxides then facilitated the formation of ternary complex (As-Fe-DOM) with highly stability, which could be considered as a mechanism for As immobilization after biochar was applied to the flooding paddy soil. Thus, the synergistic effect between amorphous iron oxides and electroactive stains could make main contribution to the passivation of released As in paddy soil under long-term flooding condition. This study provided a new insight for As immobilization via regulating iron-organic ligand complexation amendment with biochar in flooding paddy soil.

Biogas utilization without desulfurization pretreatment in a bioelectrochemical system

Utilizing biogas as a fuel for heating and power generation usually requires desulfurization pretreatment. In this study, the biogas utilization without desulfurization pretreatment in a bioelectrochemical system (BES) was explored. The results showed that the biogas-fueled BES was successfully started up within 36 d and the presence of hydrogen sulfide promoted both methane consumption and electricity generation. The optimal performance (i.e., a methane consumption of 0.523 ± 0.004 mmol/d, a peak voltage of 577 ± 1 mV, a coulomb production of 37.86 ± 0.43C/d, a coulombic efficiency of 9.37 ± 0.06 % and the maximum power density of 2.070 W/m³) was obtained under bicarbonate buffer solution and 40 °C conditions. The addition of 1 mg/L sulfide and 5 mg/L L-cysteine facilitated methane consumption and electricity generation. In the anode biofilm, the dominant bacteria were Sulfurivermis, unclassified_o__Ignavibacteriales and Lentimicrobium, while Methanobacterium, Methanosarcina and Methanothrix were the predominant archaea. Besides, the metagenomics profiles reveal that anaerobic methane oxidation and electricity generation were closely related to sulfur metabolism. These findings provide a novel approach for utilizing biogas without desulfurization pretreatment.

Unexpected genetic and microbial diversity for arsenic cycling in deep sea cold seep sediments

Cold seeps, where cold hydrocarbon-rich fluid escapes from the seafloor, show strong enrichment of toxic metalloid arsenic (As). The toxicity and mobility of As can be greatly altered by microbial processes that play an important role in global As biogeochemical cycling. However, a global overview of genes and microbes involved in As transformation at seeps remains to be fully unveiled. Using 87 sediment metagenomes and 33 metatranscriptomes derived from 13 globally distributed cold seeps, we show that As detoxification genes (arsM, arsP, arsC1/arsC2, acr3) were prevalent at seeps and more phylogenetically diverse than previously expected. Asgardarchaeota and a variety of unidentified bacterial phyla (e.g. 4484-113, AABM5-125-24 and RBG-13-66-14) may also function as the key players in As transformation. The abundances of As cycling genes and the compositions of As-associated microbiome shifted across different sediment depths or types of cold seep. The energy-conserving arsenate reduction or arsenite oxidation could impact biogeochemical cycling of carbon and nitrogen, via supporting carbon fixation, hydrocarbon degradation and nitrogen fixation. Overall, this study provides a comprehensive overview of As cycling genes and microbes at As-enriched cold seeps, laying a solid foundation for further studies of As cycling in deep sea microbiome at the enzymatic and processual levels.