Abstract: Sulfate-reducing bacteria (SRB) play an important role in the process of organic matter mineralization and formation of
biogenic pyrite in lake and coastal sea sediments. Understanding how SRB respond to dissolved oxygen contents (D.O.) in fresh and
coastal sea waters is essential to predict future evolution of mineralization under increased eutrophication and seasonal hypoxia. So far
most pure cultures of sulfate reducing bacteria have been conducted under strict hypoxic conditions, in which SRB would gradually
lose oxygen tolerance in the original habitat. In order to address how SRB respond to D.O. contents with co-existing bacteria, we
collected water samples along the D.O. gradients and incubated them with D.O. concentrations strictly controlled the same as in situ. In
such enrichment cultures, genera Desulfovibrio and Desulfomicrobium were the main sulfate-reducing bacteria, and genera Shewanella
and Sulfurospirillum were the sulfur metabolism-related bacteria associated with SRB. The relative abundances of Desulfovibrio spp.
decreased when D.O. concentration increased. The upper D.O. limit in our enrichment cultures was 6.68 mg/L, significantly higher than
those previously reported in pure cultures or co-cultures, most likely resulted from the contribution of coexisting sulfur
metabolism-related bacteria which may provide ecological niches for Desulfovibrio spp. by consuming residual oxygen.

Key words: sulfate-reducing bacteria, enrichment culture, Desulfovibrio spp., dissolved oxygen, sulfur metabolism-related bacteria;
lakes and coastal seas