INFLUENCE OF OXYGEN ON SULFATE REDUCTION AND GROWTH OF SULFATE-REDUCING BACTERIA

The ambivalent relations of sulfate-reducing bacteria to molecular O2, have been studied with ten freshwater and marine strains. Generally, O2 was reduced prior to sulfur compounds and suppressed the reduction of sulfate, sulfite or thiosulfate to sulfide. Three strains slowly formed sulfide at O2 concentrations of below 15 muM (6% air saturation). In homogeneously aerated cultures, two out of seven strains tested, Desulfovibrio desulfuricans and Desulfobacterium autotrophicum, revealed weak growth with O2 as electron acceptor (up to one doubling of protein). However, O2 was concomitantly toxic. Depending on its concentration cell viability and motility decreased with time. In artificial oxygen-sulfide gradients with sulfide-containing agar medium and also in sulfide-free agar medium under an oxygen-containing gas phase, sulfate reducers grew in bands close to the oxic/anoxic interface. The speCifiC O2 tolerance and respiration capacity of different strains led to characteristically stratified gradients. The maximum O2 concentration at the surface of a bacterial band (determined by means of microelectrodes) was 9 muM. The specific rates of O2 uptake per cell were in the same order of magnitude as the sulfate reduction rates in pure cultures. The bacteria stabilized the gradients, which were rapidly oxidized in the absence of cells or after killing the cells by formaldehyde. The motile strain Desulfovibrio desulfuricans CSN slowly migrated in the gradients in response to changing O2 concentrations in the gas phase.