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

Abstract

Abstract: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 и [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 и [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.

Aerobic bioreduction of nickel(II) to elemental nickel with concomitant biomineralization

“…A similar result has been found in the reduction of 0.09 μM Hg(II) to Hg(0) by Shewanella with 10 mM lactate under aerobic growth (Wiatrowski et al 2006). The use of oxygen as an electron acceptor by microbes for their growth coupled to metal reduction is related to co-metabolic reduction (Middleton et al 2003).…”

“…The coatings inhibited not only nitrate but also Ni(II) from interacting with the cells. Neal et al (2003) -N. In a previous study, Middleton et al (2003) also pointed out that nitrate and nitrite reduction was immediately inhibited upon the addition of 45 μM Cr(VI). On the contrary, Han et al (2008) have indicated that NO 3 − inhibited the removal of Cr(VI) using the microalgal isolate Chlorella miniata.…”

“…Hence, the microbial reduction of most metals has commonly been studied in anoxic environments instead of aerobic conditions (Straub et al 2001). Only recent research has reported that metal-reducing bacteria such as Pseudomonas and Shewanella have the ability to reduce Cr (VI) to Cr(III) in the presence of oxygen (McLean and Beveridge 2001;Middleton et al 2003;Wani et al 2007). To date, a number of metal-reducing bacteria have been isolated from a variety of habitats, and much work has been focused on the metal-reducing bacteria Geobacter spp.…”

Evaluation of the effects of various culture conditions on Cr(VI) reduction by Shewanella oneidensis MR‐1 in a novel high‐throughput mini‐bioreactor

“…Once the lactate was depleted, the cells used the acetate as a carbon source for 16 growth. With respect to oxygen's effect on Cr(VI) reduction, it has been suggested that oxygen 17 may not directly inhibit reduction of Cr(VI) (Middleton et al 2003). Rather the stress of the high 18 dissolved oxygen inhibits cell growth, and thus there is less biomass for chromate reduction.…”

Hexavalent Chromium Reduction and Accumulation by Acinetobacter AB1 Isolated from Fez Tanneries in Morocco

“…TEM photograph of morphological changes in AB1 strain due to the exposure of chromate revealed circular electron dense (dark black point) inclusions within the cell cytoplasm indicating that Cr(III) was adsorbed on microbial surface cells, the presence of electron dense particles in the cytoplasmic region of the bacteria suggested deposition of chromium in these cells. These intracellular and extracellular electron-dense precipitates were confirmed by Middleton et al as chromium through examination using electron energy loss spectroscopy (EELS) where Cr precipitated as Cr(III) and not Cr(VI) [26].…”