Biotransformation of polychlorinated biphenyls in St. Lawrence River sediments: reductive dechlorination and dechlorinating microbial populations

Polychlorinated biphenyl dechlorinating microbial populations in St. Lawrence River sediments were fractionated and estimated based on the dechlorination pattern using a combination of serial dilution and most probable number techniques. Two distinctive dechlorination patterns were found in most probable number sediments spiked with Aroclor 1248. A high-dilution inoculum decreased the average number of chlorines per biphenyl from 4.0 to 3.4 but was unable to dechlorinate meta-substituted congeners consisting mainly of 2,5,2',5'-, 2,4,2',5'-, and 2,5,2'-chlorobiphenyl (pattern B). On the other hand, a low-dilution inoculum did dechlorinate the meta-rich congeners and reduced the average number of chlorines to 2.9 (pattern A). These results indicate that there are at least two populations. While pattern B was produced by pattern B producing dechlorinators, pattern A was produced by a combination of pattern B producers plus another population that dechlorinates the meta-substituted congeners. When the population size was calculated based on the frequency of respective dechlorination patterns, the populations yielding pattern B were approximately 2.4 x 10(6) cells . g dry weight sediment(-1) whereas the dechlorinators of the meta-rich congeners were two orders of magnitude less at 3.5 x 10(4) cells . g dry weight sediment(-1). Despite lower numbers, these meta-dechlorinators in pattern A increased the overall dechlorination by almost twofold.