Electrochemical Activity Of Iron Reducing Bacteria And Diversity Of Methanogens In Sediments Of Bohai Sea And Its Coastal Rivers

Methane is an important greenhouse gas,which is produced by methanogens,and the coastal area sediments are important potential sources.In conventional opinion,the iron oxides can compete with methanogens for electrons and thereby inhibiting methane production.Our previous research has showed that it would be different for ironreducing bacteria with the ability of extracellular electron transfer.Therefore,how to reveal the diversity of iron-reducing bacteria with electronchemical activity and methanogens with different metabolic pathways in this region,as well the effect of iron oxide transformation processes on the methanogenesis,which are of crucial scientific significance for assessing the greenhouse gas emissions in the river-sea interaction region and elucidating the biogeochemical carbon and iron coupling cycle.In this study,the sediments from the Bohai Sea and its inlets to the sea were studied.Firstly,the diversity of iron-reducing bacteria in different regions of the coastal river was analyzed and a new iron-reducing bacterium with electroactivity was isolated.Secondly,the iron oxides transformation under anaerobic methanogenesis condition was revealed.Lastly,the spatial distribution of methanogens in sediments of different sections in Bohai Sea,with the diversity and production of methanogens under different gradients,including spatial and salinity,across river to sea were analyzed.The main conclusions obtained are as follows:With the study of electrochemically active iron(III)-reducing bacteria in coastal riverine sediments,the results showed that the Jh2 and Jh3 samples of the JieHe river sediments had iron reducibility.And the clone library showed that the two sediments had the characteristics of the iron reducing bacteria community structure.The iron reducing bacteria in the Jh2 samples were mainly dominated by Geobacter sp.,while the Jh3 samples was Clostridium sp.And the electroactivity bacteria(JhA)was isolated from coastal river sediments.These results expand the understanding of the electrical activity of the new strain of Anaerosinus sp.At the same time,the electroactive isolates from the environment will provide the source of strain for the bioremediation of heavy metal environment with important application value.With the study of the production and transformation of magnetite in the methanogenic consortia from coastal riverine sediments,it has been found that ironreducing bacterial and methanogens were contained in it.In the presence of amorphous iron(III)oxides,the iron-reducing bacteria would reduce the ferric iron that compete the electrons with methanogens,thereby inhibiting the methan production.But with the conversion of amorphous iron to magnetite,the extracellular electron transfer occurred mediated by magnetite,so the methane production was promoted.Beyond our consideration,the magnetite itself could also act as an electron acceptor.As a result,the magnetite was reduced by iron-reducing bacteria with competing electrons and inhibiting methane production.These results revealed that the effect of magnetite production and conversion in methanogens consortia on the electron flow between ironreducing bacteriaand methanogens.Studies on microbial community diversity and methanogenic potential in sediments from different areas of the Bohai Sea showed that there were similar main microbial T-RFs in the in situ samples with spatially different.Total iron extracted from sediments has significant effects on bacterial community structures,but not for archaeal community.Desulfovibrio and Thiobacillus were the dominanted bacteria in the sample,and Methanobacterium and Methanosarcina were the main dominanted archaea.The methane production of regional sediment samples varies in the enrichment process,showing a higher methane yield in near-shore samples.It is speculated that bacteria in the sediments of the Bohai Sea,especially those related to the iron cycle,might be an important factor affecting the potential for methanogenesis.It was deserved to mention that human activities might increase the potential for methane emission in the offshore regions.In order to explore more detail information about the changes of methanogens across Yellow River to Bohai Sea gradient,it has been found that the diversity of methanogens in the sediments varied from multi-trophic communities to specific methylotrophic communities.Methylotrophic methanogens were in both in situ sediments.Methanococcoides was the main methanogens in the Bohai Sea,while that of the Yellow River samples were dominated by Methanosarcina.After seawater enrichment culture,it was revealed that active Methanococcoides dominated in the BS enrichment cultures with trimethylamine as the substrate,methylotrophic Methanolobus dominated in the YR enrichment cultures,which was limitedly detected in in situ sediment samples.Methanosarcina were also detected in this gradient samples as well.These results indicated that the methanogens in Bohai Sea were not mainly from the Yellow River.The same species of Methanosarcina mazei were isolated from the area across a river to sea by the culture-dependent method,which were wide distributed.Our results showed the distribution of diverse methanogens across a river to sea gradient may shed light on adaption strategies and survival mechanisms of methanogens.