Response Of Abundance And Community Structure Of Iron-reducing Bacteria To Flooding Time In Paddy Soil

Fe( Ⅲ) is the most abundant electron acceptor in anaerobic soil. Process of iron-oxidation and reduction has a strong influence on circulation of carbon and nitrogen, movement of nutrition elements and especially on the degradation, transformation and fix of organic and inorganic pollutants. Paddy soil is a complex and typical ecosystem. Investigating response of abundance and community structure of iron-reducing bacteria in paddy soil is of great importance to study iron reduction, nutrient transformations, the suppression of methanogeneses and bioremediation. The response of abundance and community structure of bacteria and its Geobacter, Clostridium, Bacillus, Pseudomonas and Anaeromyxobacter to flooding time in Sichuan paddy soil and their relationship with the process of reduction of Fe(Ⅲ) were investigated by Real-time PCR, PCR-DGGE and Construction of Clone Libarary, which can provide theory basis for future research on contributions of iron-reducing bacteria in iron reduction and other ecological function. The main results and conclusions are as follows:(1) Content of Fe(Ⅱ) in flooded paddy soil increased fastly in early stage(1 h-10 d), while tended to be a kind of dynamic stability. The max iron reduction potential was 5.929 mg/g and the Vmax was 0.989 mg/(g.d) at the corresponding time(TVmax) of 2.853 d.(2) The highest bacterial abundance was observed at 1 d with the second highest value at 40 d, indicating that bacterial abundance fluctuated over the flooding period. Succession of bacterial community structure was observed along the entire flooding period: r-strategists were only present in the early flooding stage; k-strategists emerged and were dominant in the late flooding stage; r-k-strategists symbiotic organisms were present in the entire flooding period. The diversity of the bacterial community during the flooding period rose initially and then tended to drop as succession took place. PCA indicated that the changes to bacterial community structure slowed during the mid- to late flooding stages. Sequencing results showed that Firmicutes, Chloroflexi, Bacteroidetes, Proteobacteria and Acidobacteria, which have close phylogenetic relationships with the groups from paddy soils and other soils of different regions, activated sludge systems and lake sediments, were the dominant habitants of the study site.(3) Analysis of abundance of 5 iron-reducing bacteria showed that: Through the entire flooding period, abundance of Bacillus keeped the highest, while abundance of Pseudomonas keeped the lowest. In the early stage of flooding incubation, abundance of Clostridium and Bacillus both rose rapidly and maximized in 1 d, while abundance of Geobacter, Pseudomonas and Anaeromyxobacter reached the minimum in 1 d, then presented rising trend and maximized in 40 d, 30 d and 60 d respectively. It’s possible that Bacillus had the largest contribution to iron reduction and Pseudomonas contributes least. From the view of time, the contribution of Clostridium and Bacillus was larger in earlier stage, while Geobacter, Pseudomonas and Anaeromyxobacter played more important roles in later period.(4) Analysis of community structure of 5 iron-reducing bacteria indicated that: Succession of community structure was observed along the entire flooding period. Analysis of α diversity indices showed that Shannon-wiener Index(H’) of iron-reducing bacteria changed significantly, especially Bacillus, ranging from 1.674 of 1 h to 4.129 to 40 d. PCA indicated a result which is similar to bacteria that 5 iron-reducing bacteria were divided into different habitats, and the late stage treatments were in the independent habitats, also indicating that the changes to community structure of 5 iron-reducing bacteria tended to be stable during the late flooding period.(5) Phylogenetic analysis of predominant bacterium of 5 kinds of iron-reducing bacteria showed that Geobacter, Clostridium, Bacillus, Pseudomonas and Anaeromyxobacter in paddy soil have a wide distribution and get close to the bacteria which have the function of pollutants degradation and heavy metal reduction.(6) CCA indicated that content of Fe(Ⅱ), pH, abundance and Shannon-wiener Index(H’) all contributed to the similarity of habitat during the flooding period.Overall, abundance and community structure of bacteria and its 5 kinds of iron-reducing bacteria had responses to the process of iron reduction respectively. It is supposed that Bacillus had the largest contribution to iron reduction, and Geobacter, and Anaeromyxobacter also played important roles in the late flooding period.