Abstract
Sulfate- and iron-reducing heterotrophic bacteria represented minor proportion of the indigenous microbial community of highly acidic, oligotrophic acid mine drainage (AMD), but they can be successfully stimulated for in situ bioremediation of an AMD impacted soil (AIS). These anaerobic microorganisms although played central role in sulfate- and metal-removal, they remained inactive in the AIS due to the paucity of organic carbon and extreme acidity of the local environment. The present study investigated the scope for increasing the abundance and activity of inhabitant sulfate- and iron-reducing bacterial populations of an AIS from Malanjkhand Copper Project. An AIS of pH 3.5, high soluble SO42− (7838 mg/l) and Fe (179 mg/l) content was amended with nutrients (cysteine and lactate). Thorough geochemical analysis, 16S rRNA gene amplicon sequencing and qPCR highlighted the intrinsic metabolic abilities of native bacteria in AMD bioremediation. Following 180 days incubation, the nutrient amended AIS showed marked increase in pH (to 6.6) and reduction in soluble -SO42− (95%), -Fe (50%) and other heavy metals. Concomitant to physicochemical changes a vivid shift in microbial community composition was observed. Members of the Firmicutes present as a minor group (1.5% of total community) in AIS emerged as the single most abundant taxon (∼56%) following nutrient amendments. Organisms affiliated to Clostridiaceae, Peptococcaceae, Veillonellaceae, Christensenellaceae, Lachnospiraceae, Bacillaceae, etc. known for their fermentative, iron and sulfate reducing abilities were prevailed in the amended samples. qPCR data corroborated with this change and further revealed an increase in abundance of dissimilatory sulfite reductase gene (dsrB) and specific bacterial taxa. Involvement of these enhanced populations in reductive processes was validated by further enrichments and growth in sulfate- and iron-reducing media. Amplicon sequencing of these enrichments confirmed growth of Firmicutes members and proved their sulfate- and iron-reduction abilities. This study provided a better insight on ecological perspective of Firmicutes members within the AMD impacted sites, particularly their involvement in sulfate- and iron-reduction processes, in situ pH management and bioremediation.
Highlights
Acid mine drainage (AMD) is considered to be a global environmental problem faced by mining industries due to the biological oxidation of sulfidic minerals (Johnson and Hallberg, 2005; Neculita and Zagury, 2008; Qian et al, 2017)
These acidophilic, autotrophic and Fe/S oxidizing microorganisms mainly contribute toward AMD generation and were studied extensively for their physiology, molecular mechanisms and ecological relevance (Denef et al, 2010; Kuang et al, 2013; Méndez-García et al, 2014; Chen et al, 2015; Goltsman et al, 2015; Chen et al, 2016), whereas the small heterotrophic populations thriving in the same niches could be of great significance in reducing AMD generation process and attenuating the overall hazard of these systems remain less explored
The increased abundance of gene encoding dissimilatory sulfite reductase and Firmicutes specific 16S rRNA gene detected in qPCR, reduction of -sulfate/-iron and rise in pH were all in strong agreement
Summary
Acid mine drainage (AMD) is considered to be a global environmental problem faced by mining industries due to the biological oxidation of sulfidic minerals (Johnson and Hallberg, 2005; Neculita and Zagury, 2008; Qian et al, 2017). Microbial sulfur- and iron-metabolisms through redox transformations coupled with or without energy generation constitute the major biochemical reactions within AMD (Baker and Banfield, 2003; Druschel et al, 2004). These transformation reactions facilitate generation of acidity and contribute toward raising the soluble -sulfate or -iron concentrations, while on the other hand could lead to reversal of such processes and aid to restoration of such environments. Enhancing the activities of indigenous microorganisms capable of sulfate- and/or iron-reduction and generation of alkalinity have gained interest for developing in situ bioremediation strategies (Neculita et al, 2007; Hiibel et al, 2008, 2011; Becerra et al, 2009; Bijmans et al, 2009; Burns et al, 2012; Xingyu et al, 2013; Lefticariu et al, 2015)
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