Photosynthetic oxidation of MnS and FeS by Chlorobium spp

The ability of two species of green phototrophic sulfur bacteria (Chlorobium limicola and Chlorobium phaeobacteroides) to photosynthetically oxidize several metal sulfides (MnS, FeS, NiS, CuS, ZnS, CdS and PbS) by has been tested in laboratory batch cultures. Both species only oxidized MnS and FeS, which are the ones having higher solubilities (pKs = 13.5 and 18.1, respectively). The specific oxidation rates were directly related to the solubility of the metal sulfide involved. C. limicola oxidized MnS and FeS at specific rates of 11.8 and 0.9 mumol S2-h-1 mg protein-1, respectively. Specific oxidation rates of C. phaeobacteroides for MnS and FeS were 7.1 and 1.8 mumol S2-h-1 mg protein-1, respectively. The oxidation of both metal sulfides resulted in the release of the free-soluble metal ions in the culture media, but no toxic effect of these cations on the photosynthetic activity of the cells was observed. The anaerobic photosynthetical oxidation of MnS and FeS by Chlorobium reveals an adaptation of this bacterial species to sulfide-poor environments, and introduces a new process in the Mn, Fe, and S biogeochemical cycles to be considered.