Influence Of Initial PH Value On Microbial Iron Reduction In Paddy Soils

Dissimilatory ferric iron reduction is the process in which microorganisms transferelectrons to external ferric iron, reducing it to ferrous iron without assimilating the iron. It isone of the most geochemically significant events that naturally takes place in soils, aquaticsediments, and subsurface environments. Dissimilatory ferric iron reduction has a majorinfluence not only on the iron cycle, but also on the fate of a variety of other trace metals andnutrients in paddy soils, and it plays an important role in degradation of heavy metal andorganic matter. Environmental pH value is one of the most important factors on iron reduction,which not only directly affects ferric iron solubility and the chemical reactions between theredox systems in paddy soils, but also affects microbial growth and morphology, as well as itsenzyme activity of metabolic processes. Furthermore, soil pH is also an important determinanton wordwide bacterial diversity and community structure. Using pure culture system, pastresearch has focused reported neutral environment dissimilatory ferric iron reduction and thegrowth of relative microorganisms. However, there are quite rare reports about relationshipbetween iron reduction and pH value, using slurry and mixed culture systems. In order todetermine the influence of pH value on iron reduction in paddy soils, slurry and mixed culturesystems were used in this paper. In addition, strongly acidic and alkaline paddy soils wereused for isolation, screening and identification of iron reduction microorganisms. The mainconclusions are following:(1) After exchange the pH value between the acidic and alkaline paddy soils, there was acertain similarity under alkaline conditions, and not apparent similarity under acidicconditions. An increase of pH value would promote iron reduction significantly in paddy soils.The similatrity of microbial community iron reduction ability was improved as followingsequence, neutral> acidic> alkaline. Moreover, the extent of iron reduction was promoted aspH value increasing, but the process of iron reduction was inhibited.(2) The effect of initial pH and soil type on iron reduction was significant in paddy soils.There was a significantly positive correlation between the initial pH value and paddy soil ironreduction, and a significantly negative correlation between the initial pH value and microbialiron reduction in paddy soils. But the iron reduction ability of JL microbial communities wasbest at pH value6.00. Therefore, the initial pH has a certain influence on microbialcommunities and soil components. We observed that the iron reducing ability of microbial communities from both paddy soils were similar during the early stages of incubation butbecame different during the later stages. This observation provided support for the idea thatfacultative iron-reducing bacteria contribute to iron reduction in paddy soil.(3) Microorganisms isolated from the strongly alkaline paddy soil grow well in themedium prepared by soil extracts. There were some iron reduction microorganisims, whoseiron reduction rate mostly concentrated in the range of5~10%. Four typical iron reductionmicroorganisms had the same colony characteristics; these were white, round, lustur,applanation, smooth edges and surface, opque, tasteless. Cells of12-15and56-10were shotrod and Gam-positive, and cells of34-4and78-5were rod and Gam-negtive. Strain56-10was most similar with Paenibacillus odorifer TOD45(T), and the similarity is99.09%. Strain78-5was most similar with Bacillus anthracis Aems, and the highest similarity was95.85%.Four iron reduction microorganisms grow and mediated iron reduction well at pH10.51, onthe contrary, weak growth and iron reduction was obversed at pH7.00. There was asignificant difference in the iron reduction rate and the maximum iron reduction rate. Allresults indicated that the strains are alkaliphilic iron-reducing microorganism.(4) Microorganisms isolated from the strongly acidic paddy soil grow well in themedium prepared by soil extracts. There were some iron reduction microorganisims, whoseiron reduction rate mostly>90%. Two typical iron reduction microorganisms had the samecolony characteristics; these were white, round, lustur, applanation, smooth edges and surface,opque, tasteless. Cells of1-25were rod and Gam-negtive, and2-49were shot rod andGam-positive. Strain1-25was most similar with Clostridium beijerinckii NCIMB8052(T),and the similarity is99.66%. Strain2-49was most similar with Sphingomonas hankookensisODN7(T), and the highest similarity was99.02%. Within the selected pH value range in thisresearch, optimum pH value of strain1-25optimum growth and mediated iron reduction was7.00, which suggested that it is not an acidophilic microorganism. Optimum pH value ofstrain2-49for gowth was6.00and for mediating iron reduction was4.84; therefore, it is notan acidophilic microorganism on the strict definition, and only has a certain acid resistance.