SYNTROPHIC GROWTH OF SULFATE-REDUCING BACTERIA AND COLORLESS SULFUR BACTERIA DURING OXYGEN LIMITATION

Stable co-cultures of the sulfate-reducing bacterium Desulfovibrio des ulfuricans PA2805 and the colorless sulfur bacterium Thiobacillus thio parus T5 were obtained in continuous cultures supplied with limiting a mounts of lactate and oxygen while sulfate was present in excess. Neit her species could grow in purl culture under these conditions. Desulfo vibrio could grow only when the oxygen concentration was kept low by T hiobacillus. Zerovalent sulfur (S-0) produced by Thiobacillus was pref erred over sulfate as electron acceptor by Desulfovibrio, but the affi nity for S-0 seemed to be rather low. This substrate was more efficien tly used when sulfide was present suggesting that SO is preferably use d in the form of polysulfides. Through the use of SO as electron accep tor the sulfide production per lactate by Desulfovibrio was four times higher than with sulfate as acceptor. Thiobacillus produces less sulf ate and more S-0 when the amount of sulfide available per oxygen incre ases. The elevated sulfide production by Desulfovibrio thus resulted i n an increase of the S-0 production by Thiobacillus, again leading to a further increase of the sulfide production. This positive feedback m echanism stabilizes the syntrophic association. The yield on lactate o f Desulfovibrio was doubled in the mixed culture compared with growth on lactate and sulfate in pure culture. This yield increase was attrib uted to the use of zerovalent sulfur instead of sulfate as electron ac ceptor. Both organisms were thus shown to benefit from a syntrophic in teraction in which lactate was oxidized with oxygen, with a rapid cycl ing of sulfide and zerovalent sulfur serving the transfer of reducing equivalents between the species. These observations shed some light on the occurrence of colorless sulfur bacteria and sulfate-reducing bact eria at the same depth horizons in microbial mats.