| Bacterial enrichment cultures developed with Baltimore Harbor (BH) sediments were found to reductively dechlorinate 2,3,5,6-tetrachlorobiphenyl (2,3,5,6-CB) when incubated in a minimal estuarine medium containing short-chain fatty acids under anaerobic conditions. Primary enrichment cultures formed both meta and ortho dechlorination products from 2,3,5,6-CB. Only ortho dechlorination of the 2,3,5,6-CB was observed following sequential transfers of the cultures. Sequential transfer into medium without added sediment eventually resulted in the development of a strict ortho-dechlorinating culture producing only 2,3,5-CB and 3,5-CB from 2,3,5,6-CB. This was the first report of sustained anaerobic PCB dechlorination in the complete absence of soil or sediment. In order to test whether acetate or hydrogen could serve as an electron donor for microbial growth and ortho PCB dechlorination, sediment-free cultures were amended with 2,3,5,6-CB and either acetate or H2:CO2 (80:20). The addition of 20 mM sodium acetate was optimal for dechlorination. Dechlorination ceased in the absence of acetate. Dechlorinating activity was inhibited when cultures were incubated under H2:CO2 in the presence or absence of acetate. Although acetate-amended cultures were methanogenic, the addition of 3 mM bromoethanesulfonate (BES) inhibited methanogenesis but not dechlorination. These data indicate that the dechlorinating bacteria within this community will utilize acetate, but not hydrogen, as an electron donor for ortho PCB dechlorination. Incubation of the culture with other PCBs revealed that the dechlorinating community dechlorinated doubly-flanked meta or single-flanked ortho chlorines, but was unable to cleave para-substituted or non-flanked ortho chlorines or dechlorinate congeners with a 2,6-2,6-substitution. Denaturing gradient gel electrophoresis (DGGE) analysis of community 16S rDNA showed that the non-methanogenic 2,3,5,6-CB ortho-dechlorinating culture was dominated by organisms with highest sequence similarity to green non-sulfur, low G+C Gram positive and δ eubacterial groups. The bacterium related to the green non-sulfur group, previously reported as unidentified eubacterium RFLP 17, and an organism related to Desulfovibrio aminophilus were not detected in non-dechlorinating cultures. The growth of the bacterium related to the green non-sulfur group was shown to be dependent on the presence and dechlorination of 2,3,5,6-CB. This is the first identification of a microorganism that catalyzes the reductive dechlorination of a PCB for growth. |
