Dechlorinating microorganisms

When other sediments known to support PCB dechlorination were inoculated with PCB-dechlorinating microorganisms and amended with incremental increases of pure petroleum hydrocarbons (0-4 wt%) or 6.2 % petroleum hydrocarbons extracted from Silver Lake sediments, a reduction in both the rate and extent of PCB dechlorination occurred. 

The following section discusses the priming of PCB-dechlorinating microorganisms with various compounds, such as (1) chlorobiphenyls, (2) bromobi-phenyls, and (3) benzoate ions. 

DeWeerd and Bedard [253] investigated novel approaches for enhancing microbial PCB dechlorination in aquatic sediments of the Housatonic River. They reported that PCB dechlorinating microorganisms were activated (i.e., primed) to dechlorinate the PCBs that have persisted for years in these sediments. Several PCB congeners, especially 2,3,4,5,6-pentachlorobiphenyl (23456-CB), 2,3,4,6-tetrachlorobiphenyl (2346-CB), and 2,3,6-trichloro-biphenyl (236-CB), were shown to prime and sustain mefa-dechlorination of Aroclor 1260 in the river sediment, whereas the PCB congener 245-CB primed para-dechlorination of PCBs in the same sediments. 

Three mechanisms have been identified in the metabolic dechlorination of chlorinated aliphatic hydrocarbon. Metabolic processes are coupled to energy conservation and, as a result, are interpreted to be beneficial to the dechlorinating microorganisms. These include... 

Reductive dechlorination of chlorinated aUphatic hydrocarbons, eg, lindane (11) (eq. 17) is extremely facile and occurs almost exclusively via chemical mechanisms, although microorganisms are typically the source of electron donors (30). 

There have been only a few studies have evaluated membrane microfiltration of secondary wastewater effluent. Microfiltration membranes might be used to achieve very low turbidy effluents with very little variance in treated water quality. Because bacteria and many other microorganisms are also removed, such membrane disinfection might avoid the need for chlorine and subsequent dechlorination. Metal... 

The refractory nature of some pollutants, notably, persistent polyhalogenated compounds, has raised problems of bioremediation of contaminated sites (e.g., sediments and dumping sites). There has been interest in the identification, or the production by genetic manipulation, of strains of microorganisms that can metabolically degrade recalcitrant molecules. For example, there are bacterial strains that can reductively dechlorinate PCBs under anaerobic conditions. 

Under anaerobic conditions, p,p -DDT is converted to p,p -DDD by reductive dechlorination, a biotransfonnation that occurs postmortem in vertebrate tissues such as liver and muscle and in certain anaerobic microorganisms (Walker and Jefferies 1978). Reductive dechlorination is carried out by reduced iron porphyrins. It is carried out by cytochrome P450 of vertebrate liver microsomes when supplied with NADPH in the absence of oxygen (Walker 1969 Walker and Jefferies 1978). Reductive dechlorination by hepatic microsomal cytochrome P450 can account for the relatively rapid conversion of p,p -DDT to p,p -DDD in avian liver immediately after death, and mirrors the reductive dechlorination of other organochlorine substrates (e.g., CCI4 and halothane) under anaerobic conditions. It is uncertain to what extent, if at all, the reductive dechlorination of DDT occurs in vivo in vertebrates (Walker 1974). 

Bryant FO, DD Hale, JE Rogers (1991) Regiospecific dechlorination of pentachlorphenol by dichlorophenol-adapted microorganisms in freshwater anaerobic sediment slurries. Appl Environ Microbiol 57 2293-2301. 

Wu Q, DL Bedard, J Wiegel (1999) 2,6-dibromobiphenyl primes extensive dechlorination of Arochlor 1260 in contaminated sediment at 8-30°C by stimulating growth of PCB-dehalogenating microorganisms. 

Mikesell MD, S A Boyd (1986) Complete reductive dechlorination and mineralization of pentachlorophenol by anaerobic microorganisms. Appl Environ Microbiol 52 861-865. 

Cometabolism refers to the degradation of the chlorinated solvent as a by-product of the degradation of other substrates by microorganisms, and does not benefit the microorganism. As the degree of dechlorination decreases, the cometabolism rates increase. Thus, less oxidized or chlorinated solvents such as chlorinated ethenes (excluding PCE) biodegrade more favorably under aerobic conditions. 

TCDD may be in a state of flux, resulting from dechlorination of octa-and hepta-CDD and being further dechlorinated to 2-mono-CDD [54]. Beside dechlorination reactions in sediments [4],dioxin dechlorination reactions have been demonstrated in the presence of microorganisms ([5, 12, 13, 431-433], dihydroxylated monoaromatic compounds [433], vitamin B12, and zero valent metals [3]. 

Based on information by Fu et al. [219], other reactions include trans-chlorinations (migration of chlorine from PCDD to organic matter) and polymerizations, which have not been quantified. In spiked sediments from the Hudson and Passaic Rivers and sediment microorganisms, lesser-chlorinated products accounted for 10-15% of the decrease in octa-CDD. Hepta-, tetra-, tri-, and 2-mono-CDD congeners tend to dominate the dechlorination pattern [432, 433]. The microbial dechlorination sequence of octa-CDD is provided in Fig. 29 [432], which distinguishes a pathway via... 

Reductive dechlorination of PCBs is important because it reduces their potential toxicity and persistence. In situ dechlorination of PCBs attributed to microorganisms in the anaerobic sediments has been documented in the Hudson River, Silver Lake (MA), the St. Lawrence River (NY), and New Bedford Harbor (MA) [76,371,451,454-456]. 

Wu et al. [157] applied the most probable number (MPN) method to test the hypothesis that 2,6-dibromobiphenyl (26-BB) primes PCB dechlorination by stimulating the growth of microorganisms which dehalogenate 26-BB and PCBs. The experiments were conducted in anaerobic microcosms of Aroclor 1260-contaminated sediment from Woods Pond (Lenox, MA). They reported... 

Dingyi, Y., Quensen, J. F., Ill, Tiedje, J.S. Boyd, S. A. (1992). Anaerobic dechlorination of polychlorobiphenyls (Aroclor 1242) by pasteurized and ethanol-treated microorganisms from sediments. Applied and Environmental Microbiology, 58, 1110-14. 

---