Natural attenuation of Cr(VI) contamination in laboratory mesocosms

The processes leading to the natural attenuation of hexavalent chromium (Cr(VI)) in marine systems are not well understood. To determine the rate at which Cr(VI) could be reduced and the effect of Cr(VI) on bacterial communities in marine sediments, we performed mesocosm experiments with 37.85 Laquaria containing San Diego Bay sandy sediments and seawater. Constant levels of 0, 0.25 (low), and 1.5 mM (high) Cr(VI) were maintained in the water column for 2 months. Chemical analyses of sediment cores taken from the mesocosms indicated that Cr accumulated in the upper 5 mm of the sandy sediments. In general, the distribution of total Cr did not correlate with Fe, Mn, or total organic carbon. Enrichment cultures of metal (iron and chromium), and sulfate-reducing bacteria from the upper horizon (0-5 mm) of sediments were performed to look for the potential contributors in the detoxification/removal process. PCR of 16S rRNA genes and denaturing gradient gel electrophoresis (DGGE) was used to examine the microbial community structure in sediment depth profiles. When Cr(VI) was present, the number of DGGE bands decreased only in the upper 5 mm of sediments indicating an inhibition of certain bacterial populations and/or a selection for Cr-resistant bacteria in this region. Analysis of the DGGE bands was not especially helpful as most sequences were related to unknown, unidentified, or uncharacterized bacterial cloned sequences.