Effect Of Iron Redox Process On Iron-Reducing Microbial Community Structure In Paddy Soils

The distribution of Fe(?)-reducing bacterial(Fe RB)that was significance for soil pollution remediation and suppression of greenhouse gas emissions in paddy soil was extremely widespread.The periodic flooding and drying of paddy soil caused the soil to be in the state of reduction and oxidation,in which the types and abundance of iron oxide and oxygen content in the soil changed significantly.The impact of changes in iron oxide and oxygen on the community structure of Fe RB in paddy soil and their contribution to iron reduction needed further discussion.At the same time,the inherent iron oxide in paddy soils in different planting areas also effect the community structure of Fe RB.Therefore,this study included two parts: flooding and drowning,and aimd to systematically study the community structure and succession of Fe RB during anaerobic incubation with different types of iron oxide and oxygen content in in different planting areas,and clarified the contribution of ironreducing microbial communities throughout the iron cycleThe paddy soils collected from Hunan Province(HN)and Sichuan Province(SC)were used as the test soil.Ferrihydrite(amorphous iron oxide)and goethite(crystalline iron oxide)were added to anaerobic paddy soils.Paddy soils without iron oxide were control.The differences in abundance and community structure of four typical Fe BR of Geobacter,Anaeromyxobacter,Clostridium and Bacillus in different treatments were explored.After strictly anaerobic incubation of paddy soil for 15 days,2% oxygen was introduced above the water layer and the inside of the mud was treated with oxygen and the anaerobic paddy soil was used as a control.The effect of different oxygen content on the community structure of four typical Fe BR during oxygenation was explored.The changes of different types of iron oxide and oxygen content on the iron reduction process in paddy soil were analyzed,and the biological mechanism of this process change was discussed from the perspective of Fe BR community structure changes.(1)The difference in abundance of typical Fe BR caused by different types of iron oxide changes with the cultivation time.For HN,ferrihydrite and goethite significantly promoted abundance of four Fe BR;for SC,only ferrihydrite promoted abundance of Fe BR,while the goethite promoting effect is not significant.This indicated Fe BR in different planting areas had different utilization of crystalline iron oxide.The facultative Fe BR represented by Clostridium and Bacillus are less sensitive to ferrihydrite than fertile Fe BR represented by Geobacter and Anaeromyxobacter,and the sensitivity to goethite is just right in contrast.(2)Firmicutes and Proteobacteria were the dominant phylum in anaerobic paddy soil.For HN,the relative abundances of Geobacter and Clostridium were significantly increased in treatments of ferrihydrite and goethite;the relative abundance of Anaeromyxobacter was significantly reduced in ferrihydrite treatment,increased in goethite treatment;the relative abundance of Bacillus did not respond significantly to the two iron oxides.For SC,Geobacter,Anaeromyxobacter and Clostridium were significantly increased in treatment of ferrihydrite,but no significant changed in treatment of goethite;The relative abundance of Bacillus did not respond to the two iron oxides.The effect of culture time on the community structure of the four typical Fe BR was stronger than iron oxide.The effect of ferrihydrite was generally stronger than that of goethite.Phylogenetic development indicated that there are many types of OTUs with iron reduction potential during the anaerobic enrichment with iron oxide and they were widely distributed d in different genera.(3)The relative abundance of Firmicutes increased with the increase of oxygen content,while Proteobacteria was just the opposite during aerobic incubaction.The relative abundance of some OTUs in Geobacter increased significantly under high oxygen content,while the relative abundance of some OTUs also decreased significantly;the relative abundance of Anaeromyxobacter increased with increasing oxygen content;The relative abundance of Clostridium decreased with the increase of oxygen;the relative abundance of Bacillus was less than 1% when cultured for 20-40 days,and its relative abundance changed obviously under different oxygen concentrations.Some OTUs of the four typical Fe BR have high homology with the reported microorganisms with iron-reducing function.They may participate in the iron-reducing process in an aerobic environment.(4)The addition of ferrihydrite significantly promoted the iron reduction process in paddy soil,while goethite did not significantly promote the iron reduction process.As the oxygen content increased,the inhibitory effect of oxygen on the iron reduction process gradually increased.The relative abundance and diversity index of the four typical Fe BR during anaerobic cultivation were closely related to the iron-reducing process.Ferrihydrite mainly promoted the increase of the number of Geobacter and Anaeromyxobacter during 5 days of cultivation,which promoted the increase of iron reduction rate in the early stage of cultivation.In aerobic environment,some oxygen-tolerant Fe BR used the iron oxide produced by oxidation to conducted the microbial iron reduction process in an aerobic environment.