Molecular analysis of diversity dynamics and activity of total and free living nitrogen fixing bacterial communities in arsenic contaminated agricultural soil

Abstract

Arsenic (As) contamination in agricultural land is a serious issue as it increases the risk of Astranslocation newlineto crop plants as well as reduces soil fertility. The reduction of soil fertility might be newlinedue to perturbation of soil microbial community as a consequence of As contamination. Through newlinemicrobial community survey and couple of in vivo and in vitro experiments, the effect of such Ascontamination newlineon diversity, distribution of total and free-living diazotrophs and their activity in newlineterms of nifH gene expression in agricultural soil was investigated. Study was conducted at three newlinedifferent levels; comparison of microbial community in agricultural fields having different newlineconcentration of As, setting up laboratory microcosms amending different level of As and newlinebacterial isolates exposed to different concentration of As. Culture-independent molecular method newlinewas used to compare microbial communities. Both DNA and RNA from samples with different newlinedegrees of As level and from As exposed and unexposed bacterial isolates were extracted. newlineExtracted RNA was converted to cDNA. Microbial community of two different rice fields was newlineassessed during a cultivation period that includes tillage, tillering and post-harvesting by 16S newlinerRNA and nifH genes based clone library analyses and DNA sequencing. The data of clone library newlineanalysis indicated the abundance of copiotrophic Proteobacterial lineages in As contaminated soil newlinewhile Acidobacteria were prevalent in the soil without As contamination. This data also indicated newlinediverse nifH gene bearing unidentified bacteria in arsenic-rich soil but this does not imply all nifH newlinegenes bearing bacteria actually express dinitrogenase. Real time PCR analyses based on 16S newlinerRNA and nifH gene revealed elevated levels of total and nitrogen fixers in the agricultural land newlinewithout As contamination history through the cultivation period than As-contaminated land. newline