Cometabolism of Cr(VI) by Shewanella oneidensis MR-1 produces cell-associated reduced chromium and inhibits growth

Microbial reduction is a promising strategy for chromium remediation, but the effects of competing electron acceptors are still poorly understood. We investigated chromate (Cr(VI)) reduction in batch cultures of Shewanella oneidensis MR-1 under aerobic and denitrifying conditions and in the absence of an additional electron acceptor. Growth and Cr(VI) removal patterns suggested a cometabolic reduction; in the absence of nitrate or oxygen, MR-1 reduced Cr(VI), but without any increase in viable cell counts and rates gradually decreased when cells were respiked. Only a small fraction (1.6%) of the electrons from lactate were transferred to Cr(VI), The 48-h transformation capacity (Tc) was 0.78 mg (15 μmoles) Cr(VI) reduced · [mg protein]^-1 for high levels of Cr(VI) added as a single spike. For low levels of Cr(VI) added sequentially, Tc increased to 3.33 mg (64 μmoles) Cr(VI) reduced · [mg protein]^-1, indicating that it is limited by toxicity at higher concentrations. During denitrification and aerobic growth, MR-1 reduced Cr(VI), with much faster rates under denitrifying conditions. Cr(VI) had no effect on nitrate reduction at 6 μM, was strongly inhibitory at 45 μM, and stopped nitrate reduction above 200 μM. Cr(VI) had no effect on aerobic growth at 60 μM, but severely inhibited growth above 150 μM. A factor that likely plays a role in Cr(VI) toxicity is intracellular reduced chromium. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of denitrifying cells exposed to Cr(VI) showed reduced chromium precipitates both extracellularly on the cell surface and, for the first time, as electron-dense round globules inside cells.