Temporal bacterial diversity associated with metal-contaminated river sediments

The temporal activity, abundance and diversity of microbial communities were evaluated across a metal-contamination gradient around a Superfund site in Montana. In order to analyze short-term variability, samples were collected from six sites on four occasions over 12 months. Measurements of community activity, diversity and richness, quantified by dehydrogenase activity and through denaturant gradient gel electrophoresis (DGGE), respectively, were higher at contaminated sites adjacent to the smelter, relative to reference sites. 16S rRNA gene copy numbers, measured by quantitative PCR, showed seasonal variability, yet were generally higher within polluted sediments. Jaccard similarity coefficients of DGGE profiles, found sites to cluster based primarily on geographical proximity rather than geochemical similarities. Intra-site clustering of the most polluted sites also suggests a stable metal-tolerant community. Sequences from DGGE-extracted bands were predominantly Beta and Gammaproteobacteria, although the communities at all sites generally maintained a diverse phylogeny changing in composition throughout the sampling period. Spearman's rank correlations analysis found statistically significant relationships between community composition and organic carbon (r-value = 0.786) and metals (r-values As = 0.65; Cu = 0.63; Zn = 0.62). A diverse and abundant community at the most polluted site indicates that historical contamination selects for a metal-resistant microbial community, a finding that must be accounted for when using the microbial community within ecosystem monitoring studies. This study highlights the importance of using multiple time-points to draw conclusions on the affect of metal contamination.