Nitrogen fixation by chemolithoautotrophic bacteria is an important biogeochemical process in tailing environments

The tailings generated during mining productions represent harsh environments to both microorganisms and plants. In addition to metal toxicity, biological remediation efforts are often impeded by reduced availability of nutrients in the tailings. Nitrogen deficiency is one of the limiting factors for microbial growth in tailings. To overcome this deficiency, nitrogen fixation was suggested to be one of the major ecosystem services required by the indigenous bacterial community. However, the community of nitrogen-fixers in tailings, especially their energy production and carbon fixation pathways, remains unclear. In the current study, a large scale field investigation, involving six different metal contaminated regions across southern China , was conducted using a combination of geochemical characterization, sequence analysis, molecular techniques, and activity measurements to elucidate the potential metabolism of the nitrogen-fixers in tailings. Significant differences between tailings and their adjacent soils in both prokaryotic and nitrogen-fixing communities were observed. Moreover, the abundance of genes related to chemolithoautotrophic functions, including cbbL (carbon fixation), soxB (sulfur oxidation), and aioA (arsenic oxidation), showed strong and significant correlations with the nitrogenase gene (nifH) in the tailings, but not in the adjacent soils. In corroboration to this observation, the functional pathways related to carbon fixation and energy metabolism were enriched in the metagenomes obtained from tailings compared to the soils. Metagenome binning identified complete nitrogenase gene clusters in six metagenome-assembled genomes (MAGs), suggesting they are potential nitrogen fixers. The carbon fixation related genes were identified in all six MAGs, while the sulfur oxidation pathway (SOX) were found in four MAGs. Three out of the four SOX containing MAGs were significantly more abundant in tailings. Collectively, these observations suggest that chemolithoautotrophic nitrogen-fixing bacteria may be prevalent in tailing environments but not in the adjacent soils.