Degradation of organic contamination by anaerobic bacteria in Lake Ontario sediments

The nature of this research study was to investigate the extent of anaerobic biodegradation of halogenated aromatics in sediments collected at pre-selected sites from Lake Ontario along the Toronto Waterfront. Anaerobic bacteria from sediments collected at different sites in Lake Ontario were cultivated in the presence of halogenated benzoates. Biodegradation was assessed by net gas production (CH4, CO2), measured by pressure transducer and gas chromatography (GC), and by substrate disappearance using high pressure liquid chromatography (HPLC). The ability of anaerobic bacteria in the sediments to adapt to halogenated benzoates and to subsequently cross-adapt to more complex halogenated aromatics was assessed. This was done by suspending the sediments from each site in Revised Anaerobic Mineral Medium (RAMM) in the presence of halogenated benzoates. Total and viable bacterial counts were performed in order to ensure that there was no significant loss during prolonged storage of sediments as necessitated by the experimental plan. To ascertain the types of bacteria present, anaerobic organisms were cultured in roll tubes according to V.P.I. procedures and examined by phase-contrast microscopy. A characteristic metabolic pattern was observed with preferred biodegdradation of 3-iodobenzoate (3-IBZ) and 3-bromobenzoate (3-BrBZ) over 3-chlorobenzoate (3-ClBZ). Upon addition of further substrates, degradation occurred immediately without a significant lag period. Following adaptation, the sediments were tested for the biodegradation of more complex halogenated aromatics. 3,5,-Dichlorobenzoate (3,5-diClBZ) was depleted in 3-5 weeks, whereas the amino derivative, 4-amino-3,5-diClBZ, was incompletely degraded even after 16 weeks, following initial adaptation to 3-ClBZ. However, when initially adapted to 3-BrBZ, the 4-amino-3,5-diClBZ was depleted within 6 weeks. The cross-adaptation results thus showed that 3-BrBZ is preferred for initial microbial adaptation. HPLC analyses detected a transient accumulation of benzoic acid in the cultures during biodegradation, indicating a reductive dehalogenation pathway with benzoic acid as the main intermediate. Bacterial numbers (total counts, viable counts) did not decrease significantly during prolonged storage and were found to be higher than those in nonpolluted sediments of most lakes. Bacteria grown on agar roll tubes and characterized by phase contrast microscopy were highly diverse. The consortium consisted of gram negative and positive cocci, gram negative rods, and spiral forms. The findings demonstrate a potential for break-down of halogenated contaminants by anaerobic cultures found in Lake Ontario sediment. This points to the feasibility of developing applications for -anaerobic degradation of xenobiotics under controlled conditions prior to their release into the environment.