Variance in bacterial communities, potential bacterial carbon sequestration and nitrogen fixation between light and dark conditions under elevated CO2 in mine tailings.

This study is the first to show the response of bacterial communities with primary carbon and nitrogen fixers to elevated CO2 (eCO2) in light and dark conditions based on 6 months of culture growth. Carbon sequestration and nitrogen fixation were analyzed by 13C and 15N isotope labeling using 13C-labeled CO2 and 15N-labeled N2, followed by pyrosequencing and DNA-based stable isotope probing (SIP) to identify carbon fixers and nitrogen fixers. The results indicated that eCO2 decreased the Chao 1 richness, and the eCO2-light treatment exhibited the highest Shannon diversity. In addition, eCO2 (in either light or dark conditions) greatly increased the relative abundances of bacteria belonging to the classes Betaproteobacteria and Alphaproteobacteria. The 13C atom % in the mine tailings increased from 1.108 to 1.84 ± 0.11 under light conditions and 1.52 ± 0.17 under dark conditions after 6 months of culture growth. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) form I-coding gene (cbbL) copy numbers were 164.30-fold and 40.36-fold higher than RubisCO form II-coding gene (cbbM) copy numbers in the heavy fractions with a buoyant density of 1.7388 g·mL-1 relative to the buoyant density gradients of DNA fractions obtained under eCO2-light and eCO2-dark treatment, respectively. The Proteobacteria-like cbbL genes were dominant in the carbon fixers. In addition, the 15N atom % in the mine tailings increased from 0.366 to 0.454 ± 0.021 in light conditions and 0.437 ± 0.018 in dark conditions. Furthermore, uncultured nitrogen-fixing bacteria were the dominant nitrogen fixers in light conditions, and bacteria harboring the Bradyrhizobium-like nifH and Leptospirillum-like nifH genes were the dominant nitrogen fixers in dark conditions. These first data for a mine tailing ecosystem are inconsistent with those obtained for a range of other ecosystems, in which the effects of CO2 were limited to several nonphotoautotrophic communities and different nitrogen fixers.