Influence of Incubation Temperature on the Microbial Reductive Dechlorination of 2,3,4,6-Tetrachlorobiphenyl in Two Freshwater Sediments

We studied the impact of incubation temperatures on the dechlorination of 2,3,4,6-tetrachlorobiphenyl (2346-CB) in two sediments from different climates: polychlorinated biphenyl (PCB)-free sediment from Sandy Creek Nature Center Pond (SCNC) in Athens, Ga., and PCB-contaminated sediment from Woods Pond (WP) in Lenox, Mass. Sediment slurries were incubated anaerobically with 350 (mu)M 2346-CB for 1 year at temperatures ranging from 4 to 66(deg)C. Most of the 2346-CB was dechlorinated between 12 and 34(deg)C in both sediments and, unexpectedly, between 50 and 60(deg)C in WP sediment. This is the first report of PCB dechlorination at thermobiotic temperatures. The data reveal profound differences in dechlorination rate, extent, and products as a function of sediment and temperature. The highest observed rate of dechlorination of 2346-CB to trichlorobiphenyls occurred at 30(deg)C in both sediments, but the rate was higher for WP than for SCNC sediment (46 versus 16 (mu)mol liter(sup-1) day(sup-1)). For SCNC sediment the rate of dechlorination dropped sharply below 30(deg)C, but for WP sediments it was near optimal from 20 to 34(deg)C and then dropped sharply below 20(deg)C. In WP sediment most of the meta chlorines were removed between 8 and 34(deg)C and between 50 and 60(deg)C. para dechlorination was restricted from 18 to 34(deg)C and was optimal at 20(deg)C. ortho dechlorination occurred between 8 and 30(deg)C, with optima around 15 and 27(deg)C, but the extent was highly variable. In SCNC sediment complete meta dechlorination occurred from 12 to 34(deg)C and para dechlorination occurred from 18 to 30(deg)C; both were optimal at 30(deg)C. No ortho dechlorination was observed. Dechlorination products were 246-CB, 236-CB, and 26-CB (both sediments) and 24-CB, 2-CB, and 4-CB (WP sediment). The data suggest that in SCNC sediment similar factors controlled meta and para PCB dechlorination over a broad temperature range (18 to 30(deg)C) but that in WP sediment there were multiple temperature-dependent changes in the factors controlling ortho, meta, and para dechlorination. We attribute the differences observed in the two sediments to differences in their PCB-dechlorinating communities.