| Chromium (Cr) is a contaminant found at ∼600 Superfund sites (hazardous wastes sites specified for intense clean-up initiatives) (US EPA, 1999). It's two most abundant valences, Cr(VI) and Cr(III), have contrasting properties. Cr(VI) is readily bioavailable, carcinogenic, and toxic, whereas Cr(III) serves as an essential trace element but is relatively insoluble and non-toxic unless complexed with organic compounds. The primary objective for the detoxification of Cr is to reduce Cr(VI) to Cr(III). In marine environments, the general assumption is that sulfate-reducing and iron-reducing bacteria (SRB and FeRB) should indirectly reduce Cr(VI) via their metabolic end-products, HS- and Fe(II).; The thesis objective was to identify and characterize the pathways for microbial Cr(VI) reduction in marine sediments. Initial research demonstrated that bacterial consortia from marine environments can reduce Cr(VI). An important result was that SRB were inhibited by Cr(VI) and that FeRB were more resistant to Cr(VI), and thus likely key players in reduction and detoxification. To study the effect of Cr(VI) on sediment bacterial communities, two mesocosm experiments were performed in which sediments from San Diego Bay were exposed to different concentrations of Cr(VI) for 2 or 6 months. Cr(VI) was reduced and immobilized in the surficial sediments. Molecular community analysis demonstrated that the bacterial community was most affected. These results implicated bacteria to be responding to and contributing to Cr(VI) immobilization. Sequence analysis of the 16S rRNA gene indicated that most bacteria cloned from these sediments were not related to known bacteria; hence, their physiological capacities could not be assessed. Bacterial enrichments of SRB and FeRB indicated that SRB were inhibited under Cr(VI) exposure. FeRB were present and enriched in the presence of 0.05 mM Cr(VI), but were inhibited by 0.25 mM Cr(VI). In sum, these investigations have shown that SRB are sensitive to Cr(VI) and probably do not play key roles in Cr(VI) reduction and detoxification. To assess whether Cr(VI) reduction in these sediments was abiotic or biotic, Cr(VI) reduction was measured at different incubation temperatures. The results indicate that bacteria contribute substantially to Cr(VI) reduction in clay and in sandy sediments. |
