Effective Se reduction by lactate-stimulated indigenous microbial communities in excavated waste rocks

Waste rocks generated from tunnel excavation contain the metalloid selenium (Se) and its concentration sometimes exceeds the environmental standards. The possibility and effectiveness of dissolved Se removal by the indigenous microorganisms are unknown. Chemical analyses and high-throughput 16S rRNA gene sequencing were implemented to investigate the functional and structural responses of the rock microbial communities to the Se and lactate amendment. During anaerobic incubation of the amended rock slurries from two distinct sites, dissolved Se concentrations decreased significantly, which coincided with lactate degradation to acetate and/or propionate. Sequencing indicated that relative abundances of Desulfosporosinus burensis increased drastically from 0.025 % and 0.022% to 67.584% and 63.716 %, respectively, in the sites. In addition, various Desulfosporosinus spp., Symbiobacterium-related species and Brevibacillus ginsengisoli, as well as the Se(VI)-reducing Desulfitobacterium hafniense, proliferated remarkably. They are capable of incomplete lactate oxidation to acetate as only organic metabolite, strongly suggesting their involvement in dissimilatory Se reduction. Furthermore, predominance of Pelosinus fermentans that ferments lactate to propionate and acetate implied that Se served as the electron sink for its fermentative lactate degradation. These results demonstrated that the indigenous microorganisms played vital roles in the lactate-stimulated Se reduction, leading to the biological Se immobilization treatment of waste rocks.