Hydrogen Enhances Nickel Tolerance in the Purple Sulfur Bacterium Thiocapsa roseopersicina

A common microbial strategy for detoxifying metals involves redoxtransformation which often results in metal precipitation and/ or immobilization. In the present study, the influence of ionic nickel[Ni(II)] on growth of the purple sulfur bacterium Thiocapsa roseopersicina was investigated. The results suggest that Ni(II) in the bulk medium at micromolar concentrations results in growth inhibition, specifically an increase in the lag phase of growth, a decrease in the specific growth rate, and a decrease in total protein concentration when compared to growth controls containing no added Ni(II). The inhibitory effects of Ni(II) on the growth of T. roseopersicina could be partially overcome by the addition of hydrogen (H2) gas. However, the inhibitory effects of Ni/II) on the growth of T. roseopersicina were not alleviated by H2 in a strain containing deletions in all hydrogenase-encoding genes. Transmission electron micrographs of wild-type T. roseopersicina grown in the presence of Ni(II) and H2 revealed a significantly greater number of dense nanoparticulates associated with the cells when compared to wild-type cells grown in the absence of H2 and hydrogenase mutant strains grown in the presence of H2. X-ray diffraction and vibrating sample magnetometry of the dense nanoparticles indicated the presence of zerovalent Ni, suggesting Ni(II) reduction. Purified T. roseopersicina hyn-encoded hydrogenase catalyzed the formation of zerovalent Ni particles in vitro, suggesting a role for this hydrogenase in Ni(II) reduction in vivo. Collectively, these results suggest a link among H2 metabolism, Ni(II) tolerance, and Ni(II) reduction in T. roseopersicina.