甲烷氧化菌群介导的Fe(III)还原和生物固氮及其耦合机制

Fe(III) reduction and biological nitrogen fixation mediated by a methane-oxidizing consortium and their coupling mechanism

摘要:

目的 铁还原依赖的甲烷厌氧氧化(Fe-AOM)是厌氧环境中甲烷减排的重要途径，然而在缺氮条件下甲烷氧化微生物如何进行Fe-AOM仍不清楚。方法 选取甲烷氧化培养物和水铁矿为研究对象，通过氮同位素示踪、三维荧光光谱分析、电化学分析和高通量测序等方法，探究缺氮条件下Fe-AOM的效率及其耦合生物固氮的可能性。结果 在缺氮条件下，甲烷氧化培养物能够催化Fe-AOM，将水铁矿还原转化为菱铁矿等矿物。当添加甲烷时，甲烷氧化培养物的固氮酶活性和¹⁵N同化量显著高于无甲烷组，证明甲烷氧化培养物能够耦合Fe-AOM和生物固氮过程。三维荧光光谱分析和电化学分析表明，Fe-AOM促进了溶解态蛋白质类物质的产生，增加了甲烷氧化培养物的氧化还原活性，并且以直接电子传递的方式进行水铁矿的还原。微生物群落结构分析显示，甲烷杆菌属(Methanobacterium，19.32%)，地杆菌属(Geobacter，6.14%)、脱硫弧菌属(Desulfovibrio，17.52%)等铁还原菌及固氮弯曲菌属(Azoarcus，1.69%)、固氮螺菌属(Azospirillum，0.43%)等固氮菌在Fe-AOM过程中显著富集。DNA-SIP分析发现，Azoarcus在标记同位素组的重层显著富集，证实其固定了同位素氮。结论 因此推测在该Fe-AOM耦合生物固氮过程主要由Methanobacterium进行甲烷氧化，而Geobacter、Desulfovibrio等铁还原菌负责水铁矿的还原，Azoarcus则催化了生物固氮。此外，甲烷氧化细菌[甲基胞囊菌属(Methylocystis)]与铁还原菌和固氮菌之间呈现正相关关系，暗示其可能对该过程具有一定的贡献。这些结果为理解厌氧环境中铁依赖型甲烷氧化耦合固氮过程提供了新视角。

Abstract:

Objective Iron reduction-dependent anaerobic oxidation of methane (Fe-AOM) is an important pathway for methane emission reduction in anaerobic environments. However, it remains unclear how methane-oxidizing microbes perform Fe-AOM under nitrogen-limiting conditions.Methods Focusing on a methane-oxidizing consortium and ferrihydrite, this study employed nitrogen isotope tracing, three-dimensional fluorescence spectroscopy, electrochemical analysis, and high-throughput sequencing to investigate the Fe-AOM efficiency and the possibility of coupling Fe-AOM with biological nitrogen fixation under nitrogen-limiting conditions.Results The methane-oxidizing consortium was able to catalyze Fe-AOM under nitrogen-limiting conditions, reducing ferrihydrite to minerals such as siderite. The nitrogenase activity and ¹⁵N assimilation of the methane-oxidizing consortium in the presence of methane were significantly higher than those in the absence of methane, which demonstrated that the consortium could couple Fe-AOM with biological nitrogen fixation. Three-dimensional fluorescence spectroscopy and electrochemical analysis revealed that Fe-AOM promoted the production of dissolved protein-like substances, enhanced the redox activity of the methane-oxidizing consortium, and reduced ferrihydrite via direct electron transfer. Microbial community structure analysis showed significant enrichment of Methanobacterium (19.32%), iron-reducing bacteria such as Geobacter (6.14%) and Desulfovibrio (17.52%), as well as nitrogen-fixing bacteria like Azoarcus (1.69%) and Azospirillum (0.43%) during the Fe-AOM process. DNA-SIP analysis found that Azoarcus was significantly enriched in the heavy fraction of the labeled isotope group, confirming that it fixed isotope nitrogen.Conclusion It is thus hypothesized that the coupling of Fe-AOM with biological nitrogen fixation was primarily carried out by Methanobacterium which oxidized methane, Geobacter and Desulfovibrio responsible for the reduction of ferrihydrite, and Azoarcus catalyzing biological nitrogen fixation. Additionally, the positive correlations of the methane-oxidizing bacterium Methylocystis with iron-reducing bacteria and nitrogen-fixing bacteria suggested a certain contribution of Methylocystis to this process. These results provide new insights into understanding iron-dependent methane oxidation and nitrogen fixation in anaerobic environments.