Microbial Degradation of Pesticides in Tropical Soils
95
although in small quantities of less than 1 % of added 14C, did implicate microbial
participation (MacRae et al., 1967). Moreover, several bacteria isolated from
flooded soil treated with HCH attacked 'V-HCH with ease, but only under anaerobic conditions (Anonymous, 1966). The most active bacterium (MacRae et
al., 1969), identified as Clostridium sphenoides (Heritage and MacRae, 1977),
metabolized a- and 'V-HCH, but not ~- and 8-HCH, under anaerobic conditions
(MacRae et al., 1969; Sethunathan et al., 1969; Heritage and MacRae, 1977,
1979). During bacterial degradation, 'V-HCH was converted to 'V-3,4,5,6,-tetrachloro-l-cyclohexene (TCCH) and a-HCH to 8-TCCH (Heritage and MacRae,
1977, 1979); 'V- and 8-TCCH were further metabolized, leading to a nearly
complete dechlorination of 'V-HCH with the formation of chloride ions in stoichiometric amounts (MacRae et al., 1969; Jagnow et al., 1977). Temperature
and pH optima for the metabolism of'V-HCH by washed cell suspensions of C.
sphenoides were 40°C and 8.0, respectively, (Heritage and MacRae, 1979). The
'V-HCH-adapted cells also metabolized the ~-, 8-, and 'V-isomers of TCCH,
2,3,4,5,6-pentachloro-2-cyclohexen-l-01 and'V-1,2,3,4,5-pentachlorocyclohexl-ene, and 1,2,3,4,5-pentachlorocyclohexane, but not a selection of chlorinated
benzenes, dichlorocyclohexane and chlorocyclohexane. Carbon dioxide and
methane, the common end products of carbon metabolism, were not detected.
Anaerobic degradation of'V-HCH in flooded Japanese soils, activated sludge,
and rumen fluid has also led to the formation of 'V-TCCH (Tsukano and Kobayashi, 1972; Beland et al., 1976; Haider et al., 1976), but 'V-TCCH formation in
tropical rice soils is yet to be reported.
Other microorganisms actively implicated in the anaerobic degradation of
'V-HCH included Clostridium butyricum, Clostridium pasteurianum, Citrobacter
freundii (Jagnow et al., 1977), and Clostridium rectum (Ohisa and Yamaguchi,
1978a). Interestingly, 'V-HCH was decomposed by both growing and resting
cells of C. sphenoides (Heritage and MacRae, 1977) and only by growing cells
of C. rectum (Ohisa and Yamaguchi, 1978a). Addition of peptone, but not 'VHCH, to the flooded soil significantly increased the population of'V-HCH-decomposing microorganisms, Clostridium sp. in particular (Ohisa and Yamaguchi,
1978b). Besides, no proliferation of C. rectum occurred during 'V-HCH degradation, suggesting cometabolism (Ohisa and Yamaguchi, 1978a). Although all
four isomers of HCH are almost equally biodegradable in predominantly anaerobic flooded soils, degradation of ~- and 8-isomers in isolated cultures of microorganisms is yet to be demonstrated. More in-depth studies on the stepwise
degradation of the ~-isomer in pure and/or mixed cultures of microorganisms
are warranted in view of the concern over its entry into the food chain.
4.3.1.2. Other Chlorinated Hydrocarbons
As with HCH isomers, several related chlorinated hydrocarbon insecticides,
known to persist for several years in aerobic environments, showed a relatively
95
although in small quantities of less than 1 % of added 14C, did implicate microbial
participation (MacRae et al., 1967). Moreover, several bacteria isolated from
flooded soil treated with HCH attacked 'V-HCH with ease, but only under anaerobic conditions (Anonymous, 1966). The most active bacterium (MacRae et
al., 1969), identified as Clostridium sphenoides (Heritage and MacRae, 1977),
metabolized a- and 'V-HCH, but not ~- and 8-HCH, under anaerobic conditions
(MacRae et al., 1969; Sethunathan et al., 1969; Heritage and MacRae, 1977,
1979). During bacterial degradation, 'V-HCH was converted to 'V-3,4,5,6,-tetrachloro-l-cyclohexene (TCCH) and a-HCH to 8-TCCH (Heritage and MacRae,
1977, 1979); 'V- and 8-TCCH were further metabolized, leading to a nearly
complete dechlorination of 'V-HCH with the formation of chloride ions in stoichiometric amounts (MacRae et al., 1969; Jagnow et al., 1977). Temperature
and pH optima for the metabolism of'V-HCH by washed cell suspensions of C.
sphenoides were 40°C and 8.0, respectively, (Heritage and MacRae, 1979). The
'V-HCH-adapted cells also metabolized the ~-, 8-, and 'V-isomers of TCCH,
2,3,4,5,6-pentachloro-2-cyclohexen-l-01 and'V-1,2,3,4,5-pentachlorocyclohexl-ene, and 1,2,3,4,5-pentachlorocyclohexane, but not a selection of chlorinated
benzenes, dichlorocyclohexane and chlorocyclohexane. Carbon dioxide and
methane, the common end products of carbon metabolism, were not detected.
Anaerobic degradation of'V-HCH in flooded Japanese soils, activated sludge,
and rumen fluid has also led to the formation of 'V-TCCH (Tsukano and Kobayashi, 1972; Beland et al., 1976; Haider et al., 1976), but 'V-TCCH formation in
tropical rice soils is yet to be reported.
Other microorganisms actively implicated in the anaerobic degradation of
'V-HCH included Clostridium butyricum, Clostridium pasteurianum, Citrobacter
freundii (Jagnow et al., 1977), and Clostridium rectum (Ohisa and Yamaguchi,
1978a). Interestingly, 'V-HCH was decomposed by both growing and resting
cells of C. sphenoides (Heritage and MacRae, 1977) and only by growing cells
of C. rectum (Ohisa and Yamaguchi, 1978a). Addition of peptone, but not 'VHCH, to the flooded soil significantly increased the population of'V-HCH-decomposing microorganisms, Clostridium sp. in particular (Ohisa and Yamaguchi,
1978b). Besides, no proliferation of C. rectum occurred during 'V-HCH degradation, suggesting cometabolism (Ohisa and Yamaguchi, 1978a). Although all
four isomers of HCH are almost equally biodegradable in predominantly anaerobic flooded soils, degradation of ~- and 8-isomers in isolated cultures of microorganisms is yet to be demonstrated. More in-depth studies on the stepwise
degradation of the ~-isomer in pure and/or mixed cultures of microorganisms
are warranted in view of the concern over its entry into the food chain.
4.3.1.2. Other Chlorinated Hydrocarbons
As with HCH isomers, several related chlorinated hydrocarbon insecticides,
known to persist for several years in aerobic environments, showed a relatively