Isolation And Identification Of Iron/Manganese-oxidizing Bacteria From Paddy Soil And Cadmium Adsorption By Microbially Produced Iron Oxides

Biological iron and manganese oxides can enrich and prevent heavy metals from entering the roots of rice to a certain extent,which of important ecological significance in soil pollution remediation and reducing the risk of rice pollution.The contribution of microorganisms to the formation of iron and manganese oxides has been well proved.Isolating iron and manganese oxidizing bacteria from paddy field,studying their growth conditions and influencing factors,and finding out their role in oxide formation and heavy metal enrichment in paddy field,will help to increase the ecological effect of microbial iron oxide.The current situation of heavy metal pollution in Chinse arable land is severe.There is a huge potential of iron-manganese oxidizing bacteria application,worth further investigation.In this paper,the paddy soil with relatively high iron content in Zhenjiang was used to study the bacterial community structure and the abundance of iron-manganese oxidizing bacteria in the sample.High-throughput sequencing results of 16S rRNA gene of rice field samples revealed that all samples contained iron-oxidizing bacteria,and the Gallionellaceae family accounted for abundance major porition.The optimized semi-solid gradient tube method and single colony dilution transfer method with were used to separate and purify the iron/manganese oxidizing bacteria(Fe(Ⅱ)/Manganese(Ⅱ)-oxidizing bacteria,FeOB,MnOB),individually.As a result,five strains able to oxidize Mn(Ⅱ)were isolated by single colony dilution transfer method.The strains were characterized by electron microscope scanning(SEM),energy spectrum analysis(EDS)and 16S rRNA gene sequencing.The strain isolated by the semi-solid gradient tube method produces a flaming iron film and has an oxidizing function for Fe(Ⅱ),and belongs to the typical iron oxidizing bacterium Gallionella.On this basis,the basic physiological and biochemical characteristics of the strain were studied,and the iron oxidation rate and cadmium adsorption efficiency were analyzed.The isolate was inoculated close to rice roots growinghydroponic nutrient solution,to study the oxidation of iron and its effect on cadmium adsorption.The main results are as follows:(1)Proteobacteria,Chloroflexi and Acidobacteria are the dominant phyla in paddy soil.Gallionella and Geobacter,which are the major bacterial groups of Proteobacteria,have iron oxidation and reduction functions respectively.Gallionella,microaerophilic bacteria,had a significant negative correlation with Fe3+ and water content(p<0.01).There was a very significant negative correlation between Acidobateria abundance,organic matter,and Fe3+content(p<0.01).This kind of bacteria is very likely to be autotrophic bacteria with iron oxidation function.There was a very significant negative correlation between the relative abundance of Actinobacteria and the content of Fe3+ in each treatment(p<0.001),and there might exsit a large number of iron-reducing bacteria in this phylum.(2)The 5 strains of bacteria obtained by plate culture belonged to Acinetobacter,and all had the function of iron/manganese oxiding.The closest relative species is Acinetobacter guillouiae.(3)The microaerophilic c iron oxidizing strain Fw,isolated from gradient tubes,belong to Gallionella.The optimum growth pH is 6.5 and the optimum growth temperature is 25℃.The strain uses Fe2+ as the only electron donorwhich could accelerate the process of iron oxidation,with the increase in the Fe2+ oxidation rate of by more than 60%.(4)Strain Fw can grow under the condition of 1-10%oxygen,and maintain active oxidation of iron,but the oxidation rate of iron decreased at the concentration of 10%oxygen.It is sensitive to CO2 supply.When CO2 decreased to atmospheric level,the total amount of iron oxidation of strain Fw decreased with a slow growth of the strain.(5)Strain Fw can promote the oxidation of iron in rice rhizosphere,facilitate the formation of iron plaque on the root surface,and significantly reduce the concentration of dissolved cadmium in nutrient solution.The growth of rice in the nutrient solution until the second tillering period shows no significant difference in plant height and weight between the inoculation of iron bacteria and the control group,indicating that strain Fw had no significant effect on the growth of rice during this growth period.