Anaerobic enrichment of As5+ reducing bacteria in the presence and/or absence of organic carbon (OC) and As5+ from As contaminated soil of Brahmaputra river basin (BRB) (Jorhat, Assam) was performed. Denaturing gradient gel electrophoresis of the 16SrRNA gene sequences amplified from the enriched microbial community indicated occurrence of maximum diversity under conditions receiving no OC (MSM) followed by moderate OC (LB). However, higher OC or As showed antagonistic effect on bacterial enrichment whereas together (BB + As) they showed a synergistic effect. Phylogenetic analysis of the prominent bands revealed an overall abundance of Lachnoanaerobaculum (39%), Clostridium (39%), Bacillus, Peptostreptococcaceae, Anaerostipes (13%), and Desulfotomaculum (8.7%). Moderate OC (LB) led to maximum As mobilization i.e. 27.42 µg/L, whereas presence of added As together with high OC (BB + As) enhanced the mobilization process. Mineralogical analyses of the sediments after incubation showed prominent weathering and loss of crystallinity in MSM and LB. Appearance of a new peak corresponding to arsenolamprite (As) in LB and LB + As indicated opening up of secondary phases of the minerals harboring As due to microbial leaching under moderate OC. This is the first study reporting Lachnoanaerobaculumas a potent As5+ dissimilating bacterium isolated from As contaminated subsurface sediment of BRB.
Keywords: Anaerobic enrichment for As dissimilating bacteria; DGGE; Firmicutes; scanning electron microscopy; sediment mineralogy.