Abstract:

Syntrophic fatty acid oxidation is a rate-limiting step during the methanogenic degradation of organic matter, and arsenic(As) resistance strategies are important for the activities of syntrophic methanogenic communities under As stress. However, their As-transforming abilities and the underlying mechanisms remain unknown. Butyrate is a key intermediate in the degradation of organic matter to methane. The As-methylating ability of butyrate-degrading methanogenic communities and their As resistance mechanisms are essential for understanding the anaerobic microbial As transformation processes. In this study, butyrate-degrading methanogenic enrichment was obtained from paddy soils using butyrate as the sole carbon source and As(III) or MMAs(III) as the As substrate. It was found that the enrichments could methylate As(III) to MMAs, DMAs, and TMAsO, and methylate MMAs(III) to DMAs and TMAsO. Syntrophomonadaceae, known as the key butyrate oxidizer, was the main bacterial taxon in the enrichment. Methanosarcina and Methanobacterium are the main methanogenic archaea, which may be in syntrophic association with butyrate oxidizers, thereby driving the methanogenesis under As stress. A comparison of key microbial groups under different As stresses revealed that Syntrophomonadaceae (55.2%‒61.0%) and Methanobacterium (49.0%‒55.7%) were enriched under MMAs(III) conditions. Potential As-methylating microorganisms were further identified in these two taxa, indicating that their ability to methylate As may confer resistance to As toxicity in the environment. Therefore, syntrophic bacteria and methanogenic archaea may perform As methylation to detoxify highly toxic trivalent inorganic or methylated As species in anoxic environments, which may play a role in their survival strategies under As stress. This study provides new insights into metalloid resistance mechanisms adopted by anaerobic microorganisms and the coupled process of carbon and arsenic cycling.

Key words: paddy soil, short-chain fatty acid, syntrophic methanogenesis, arsenic-methylating microorganisms, arsenic resistance

摘要：

短链脂肪酸互营产甲烷是厌氧有机质分解过程中的关键限速步骤，互营产甲烷菌群砷抗性机制是其适应砷胁迫的重要途径，然而它们的砷转化特征和抗性策略尚不清楚。丁酸是有机质分解产甲烷过程中生成的重要中间产物，丁酸产甲烷菌群的砷甲基化能力及对砷胁迫的耐受机制对于理解厌氧微生物砷转化过程至关重要。利用丁酸为唯一碳源和无机As(III)或有机三价砷MMAs(III)为砷底物分别富集稻田土壤源产甲烷菌群，发现丁酸产甲烷富集物能够将As(III)转化为一甲基砷（MMAs）、二甲基砷（DMAs）和三甲基砷（TMAsO），也能将MMAs(III)转化为DMAs和TMAsO。通过细菌、古菌和功能微生物群落分子解析技术，结果表明共养单胞菌科Syntrophomonadaceae作为丁酸互营氧化功能菌是富集物中主要的细菌类群，甲烷八叠球菌属Methanosarcina和甲烷杆菌属Methanobacterium是主要的产甲烷古菌，它们协同驱动了砷胁迫下的丁酸氧化产甲烷过程。不同砷胁迫条件下的关键微生物类群对比结果显示，MMAs(Ⅲ)胁迫条件进一步富集了Syntrophomonadaceae（55.2%－61.0%）和Methanobacterium（49.0%－55.7%），并且这两种类群中存在潜在砷甲基化微生物，它们可能通过砷甲基化来耐受环境中的砷胁迫。所以，短链脂肪酸互营氧化产甲烷菌群中互营细菌和产甲烷古菌成员可能利用砷甲基化进行解毒，以适应厌氧环境中的高毒性三价无机砷或甲基砷，是其耐受砷胁迫的重要生存策略。研究结果可为理解厌氧微生物类金属耐受机制及其参与的碳砷元素耦合转化过程提供新视角。

关键词: 水稻土, 短链脂肪酸, 互营产甲烷, 砷甲基化微生物, 砷抗性