Molecular and taxonomic characterization of arsenic (As) transforming Bacillus sp. strain IIIJ3-1 isolated from As-contaminated groundwater of Brahmaputra river basin, India

Abstract

Background: Microbe-mediated redox transformation of arsenic (As) leading to its mobilization has become a serious environmental concern in various subsurface ecosystems especially within the alluvial aquifers. However, detailed taxonomic and eco-physiological attributes of indigenous bacteria from As impacted aquifer of Brahmaputra river basin has remained under-studied. Results: A newly isolated As-resistant and -transforming facultative anaerobic bacterium IIIJ3-1 from As-contaminated groundwater of Jorhat, Assam was characterized. Near complete 16S rRNA gene sequence affiliated the strain IIIJ3-1 to the genus Bacillus and phylogenetically placed within members of B. cereus sensu lato group with B. cereus ATCC 14579(T) as its closest relative with a low DNA-DNA relatedness (49.9%). Presence of iC17:0, iC15:0 fatty acids and menaquinone 7 corroborated its affiliation with B. cereus group, but differential hydroxy-fatty acids, C18:2 and menaquinones 5 & 6 marked its distinctiveness. High As resistance [Maximum Tolerable Concentration = 10 mM As3+, 350 mM As5+], aerobic As3+ (5 mM) oxidation, and near complete dissimilatory reduction of As 5+ (1 mM) within 15 h of growth designated its physiological novelty. Besides O2, cells were found to reduce As5+, Fe3+, SO42-, NO3-, and Se6+ as alternate terminal electron acceptors (TEAs), sustaining its anaerobic growth. Lactate was the preferred carbon source for anaerobic growth of the bacterium with As5+ as TEA. Genes encoding As5+ respiratory reductase (arr A), As3+ oxidase (aioB), and As3+ efflux systems (ars B, acr3) were detected. All these As homeostasis genes showed their close phylogenetic lineages to Bacillus spp. Reduction in cell size following As exposure exhibited the strain's morphological response to toxic As, while the formation of As-rich electron opaque dots as evident from SEM-EDX possibly indicated a sequestration based As resistance strategy of strain IIIJ3-1. Conclusion: This is the first report on molecular, taxonomic, and ecophysiological characterization of a highly As resistant, As3+ oxidizing, and dissimilatory As5+ reducing Bacillus sp. IIIJ3-1 from As contaminated sites of Brahmaputra river basin. The strain's ability to resist and transform As along with its capability to sequester As within the cells demonstrate its potential in designing bioremediation strategies for As contaminated groundwater and other ecosystems.