Responses of bacterial communities along vertical soil profile to the chromium-contamination stress

Driven by rainfall and agricultural irrigation, chromium slag causes serious pollution to the subsurface envi-ronment and damages the indigenous soil bacterial community. The study of the ecological responses of bacterial community to chromium pollution can provide support for bioremediation of chromium pollution. In this study, soil samples at different depths (0, 2, 5, 7 m) in a long-term chromium contaminated site were selected to analyze the vertical changes and responses of bacterial community structure using high-throughput 16S rRNA gene sequencing. Total Cr and soluble Cr(VI) concentrations were highest in topsoil and decreased with depth. Compared with the topsoil of uncontaminated land, the alpha-diversity in the contaminated topsoil was significantly reduced, and the bacterial community composition was altered. With increasing soil depth, the alpha-diversity decreased, and the species distribution became more uneven. Topsoil and subsurface soil showed different bacterial community structures, associated with the Cr concentration and physicochemical properties. Proteobacteria in all samples were dominant, and their relative abundances increased with depth. Bacterial community alpha-diversity and dominant taxa can be used as indicators of response to chromium pollution. The predicted metagenomic functional annotation (using Tax4Fun) showed that the relative abundance of chromate transporters (chrA, K07240) in the topsoil is 20-30 times of that in deeper soil, indicating potential bioreme-diation capability by topsoil bacterial community. The bioremediation feasibility of reducing Cr(VI) using indigenous bacterial community showed that Cr-polluted topsoil bacterial community has ability to reduce Cr (VI).