Enrichment Of Iron-dependent Anaerobic Methane-oxidizing Microorganisms And Its Influencing Factors

Iron-dependent anaerobic oxidation of methane(Fe-AOM)is a microbial process of anaerobic oxidation of methane with ferric iron as electron acceptor.It has a potenial contribution on reducing the release of greenhouse gas methane into the atmosphere;Meanwhile,it promotes the earth's biochemical cycle of iron.Ecosystems are usually very complicated,and there are many factors possibly affecting the Fe-AOM process,but little is known.In this study,Fe-AOM microorganisms were cultivated from paddy soil under different substrate conditions of dissolved Fe(III)(EDTA-Fe(III))and insoluble Fe(III)(ferrihydrite).Moreover,the effects of iron minerals,metal chelating agents and humic acids on Fe-AOM were also investigated.The results are as follows:(1)Using iron compounds with different solubility as the matrix,through 420 d cultivation,Fe-AOM enrichment was successfully obtained,and it was found that the insoluble ferrihydrite enriches the soil Fe-AOM microorganism better than the soluble EDTA-Fe(III).Although Fe(III)and EDTA-Fe(III)can stimulate Fe-AOM activity in the short term,they cannot significantly increase the Fe-AOM activity of the enrichment in the long term.According to the results of community analysis,it was speculated that the two possible mechanisms for the microorganisms in the enrichment to achieve the Fe-AOM process were as follows: First,the archaeal Methanomassiliicoccaceae or archaeal p Grf C26 completes methane oxidation,and the electrons were transferred to the iron-reducing bacteria such as Geobacter and Geothrix.Complete iron reduction;second,archaeal Methanomassiliicoccaceae or archaeal p Grf C26 alone to achieve methane oxidation and iron reduction process,without the need for bacteria to participate.(2)Using different crystal morphology iron minerals as the matrix,the effects of four environmentally relevant iron minerals on Fe-AOM microbial enrichment were investigated.The microbial specific activity was: goethite> ferrihydrite> hematite> magnetite.Goethite was abundant in natural soil,suggesting that the goethite-induced Fe-AOM process had important environmental significance.The microbial community structure of the ferrihydrite experimental group had a large gap with the results of other iron mineral groups,and the difference between the inoculum was the largest.(3)Metal chelating agents(EDTA)and humic acids(HA)had a certain degree of promotion effect on the Fe-AOM process,and EDTA was better than HA,and the methane oxidation activity was 1.2 times that of the latter.Quantitative PCR results show that HA could stimulate the growth of archaea and bacteria at the same time,and the number of mcr A copies increased greatly under the stimulation of EDTA,and its Fe-AOM response may be mediated by archaea.