Wallenius Anna J, Dalcin Martins Paula, Slomp Caroline P, Jetten Mike S M
Department of Microbiology, Institute for Water and Wetland Research, Radboud University Nijmegen, Nijmegen, Netherlands.
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, Netherlands.
Front Microbiol. 2021 Feb 18;12:631621. doi: 10.3389/fmicb.2021.631621. eCollection 2021.
Large amounts of methane, a potent greenhouse gas, are produced in anoxic sediments by methanogenic archaea. Nonetheless, over 90% of the produced methane is oxidized via sulfate-dependent anaerobic oxidation of methane (S-AOM) in the sulfate-methane transition zone (SMTZ) by consortia of anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). Coastal systems account for the majority of total marine methane emissions and typically have lower sulfate concentrations, hence S-AOM is less significant. However, alternative electron acceptors such as metal oxides or nitrate could be used for AOM instead of sulfate. The availability of electron acceptors is determined by the redox zonation in the sediment, which may vary due to changes in oxygen availability and the type and rate of organic matter inputs. Additionally, eutrophication and climate change can affect the microbiome, biogeochemical zonation, and methane cycling in coastal sediments. This review summarizes the current knowledge on the processes and microorganisms involved in methane cycling in coastal sediments and the factors influencing methane emissions from these systems. In eutrophic coastal areas, organic matter inputs are a key driver of bottom water hypoxia. Global warming can reduce the solubility of oxygen in surface waters, enhancing water column stratification, increasing primary production, and favoring methanogenesis. ANME are notoriously slow growers and may not be able to effectively oxidize methane upon rapid sedimentation and shoaling of the SMTZ. In such settings, ANME-2d () and ANME-2a may couple iron- and/or manganese reduction to AOM, while ANME-2d and NC10 bacteria () could couple AOM to nitrate or nitrite reduction. Ultimately, methane may be oxidized by aerobic methanotrophs in the upper millimeters of the sediment or in the water column. The role of these processes in mitigating methane emissions from eutrophic coastal sediments, including the exact pathways and microorganisms involved, are still underexplored, and factors controlling these processes are unclear. Further studies are needed in order to understand the factors driving methane-cycling pathways and to identify the responsible microorganisms. Integration of the knowledge on microbial pathways and geochemical processes is expected to lead to more accurate predictions of methane emissions from coastal zones in the future.
产甲烷古菌在缺氧沉积物中会产生大量强效温室气体甲烷。尽管如此，超过90%产生的甲烷会在硫酸盐 - 甲烷过渡带（SMTZ）通过厌氧甲烷氧化古菌（ANME）和硫酸盐还原细菌（SRB）的聚集体，经由依赖硫酸盐的甲烷厌氧氧化（S - AOM）作用被氧化。沿海系统占海洋甲烷总排放量的大部分，且通常硫酸盐浓度较低，因此S - AOM的作用较小。然而，诸如金属氧化物或硝酸盐等替代电子受体可用于甲烷厌氧氧化而非硫酸盐。电子受体的可用性由沉积物中的氧化还原分区决定，其可能因氧气可用性变化以及有机物质输入的类型和速率而有所不同。此外，富营养化和气候变化会影响沿海沉积物中的微生物群落、生物地球化学分区和甲烷循环。本综述总结了当前关于沿海沉积物中甲烷循环所涉及的过程和微生物以及影响这些系统甲烷排放的因素的知识。在富营养化的沿海地区，有机物质输入是底层水缺氧的关键驱动因素。全球变暖会降低氧气在表层水中的溶解度，增强水柱分层，增加初级生产力，并有利于甲烷生成。众所周知，ANME生长缓慢，在SMTZ快速沉积和变浅时可能无法有效氧化甲烷。在这种情况下，ANME - 2d（）和ANME - 2a可能将铁和/或锰还原与甲烷厌氧氧化耦合，而ANME - 2d和NC10细菌（）可能将甲烷厌氧氧化与硝酸盐或亚硝酸盐还原耦合。最终，甲烷可能在沉积物上部几毫米或水柱中被好氧甲烷氧化菌氧化。这些过程在减轻富营养化沿海沉积物甲烷排放中的作用，包括确切途径和涉及的微生物，仍未得到充分探索，且控制这些过程的因素尚不清楚。需要进一步研究以了解驱动甲烷循环途径的因素并确定相关微生物。将微生物途径和地球化学过程的知识整合起来，有望在未来更准确地预测沿海地区的甲烷排放。
