Transformation of dichlorprop began after a 6-month lag period and proceeded
20 times more slowly (half-life of about 14 days for dichlorprop compared with
about 0.7 days for methyl dichlorprop). The addition of inorganic fertilisers to forest
and pasture plots in Brazil had no measurable effect on enantioselectivity for methyl
dichlorprop (tested at 2 weeks and 5 months after fertilisation) or to plots in North
America, fertilised since May 1988. Laboratory enrichment experiments (addition of
beef extract and peptone) caused a shift in the preferential transformation characteristics: soil samples which showed a preferential transformation of the (+)-enantiomer
of methyl dichlorprop after addition of the organic nutrient shifted to a preferential
transformation of the inactive (À)-enantiomer in samples of North America and
Brazil (Table 8.20). Soils from Norway already in the original sample mainly
removed the (À)-enantiomer. Lewis et al. (1999) conjecture that micro-organisms
preferring the (+)-enantiomer may have increased in activity after organic nutrient
amendments (Table 8.20), but most samples (74%) still preferred the (À)-enantiomer. The authors pointed out that preferential removal of (À)-methyl dichlorprop
would render residues more phytotoxic than the same concentrations of the racemate, as long as demethylation is a prerequisite step and the (+)-enantiomer is not
preferentially removed after nutrient enrichment.
Ruelene, like dichlorprop, was transformed by micro-organisms only after a time
lag of about 6 months. The half-life, once transformation began, was about 40 days
in Brazilian soils and 20 days in Norwegian soils. Furthermore, Lewis et al. (1999)
concluded that the transformation rates of racemic ruelene were unaffected by
deforestation or soil warming. Enantioselectivity of the transformation process
was, however, influenced in both cases. In Brazil, forest soils, originally removing
preferentially the less toxic (À)-enantiomer [i.e. 22% (+), 67% (À), 11% (+/-); n ¼
17], after deforestation, shifted to transforming exclusively the (+)-ruelene. Soil
warming in Norway, on the other hand, caused soils to shift from all samples
preferentially removing the (+)-ruelene to some (22%) removing of the less toxic
(À)-enantiomer (n ¼ 13).
Delayed microbial transformation of chemical residues is commonly observed
and is often assumed to represent the amount of time required for low concentrations
of micro-organisms responsible for the transformation process to attain significant
numbers after using the chemicals as substrates or energy sources. In addition to this
general aspect, Lewis et al. (1999) gained deepened insight into the dependence of
the enantioselective transformation processes on the micro-organisms actually present in the respective soils. The diversity and biogeography of bacteria capable of
demethylating methyl dichlorprop were investigated by comparing small-subunit
(16S) ribosomal RNA (rRNA) genes of 50 soil bacterial isolates using amplified
ribosomal DNA restriction analysis (ARDRA). Furthermore, ARDRA has proved
useful in taxonomic assessments of bacterial culture collections and 16S rDNA clone
libraries at genus and species levels (for details of these methods, see refs in Lewis
et al. (1999)). In general, isolates from Norway and North America formed three
large clusters and appeared to be more closely related than to isolates from Brazil.
Conversely, Brazilian isolates formed smaller clusters that were more distantly
related to one another and to Norwegian and North American clusters. It appears
to be plausible that these relationships among bacterial isolates reflect ecological
194
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
20 times more slowly (half-life of about 14 days for dichlorprop compared with
about 0.7 days for methyl dichlorprop). The addition of inorganic fertilisers to forest
and pasture plots in Brazil had no measurable effect on enantioselectivity for methyl
dichlorprop (tested at 2 weeks and 5 months after fertilisation) or to plots in North
America, fertilised since May 1988. Laboratory enrichment experiments (addition of
beef extract and peptone) caused a shift in the preferential transformation characteristics: soil samples which showed a preferential transformation of the (+)-enantiomer
of methyl dichlorprop after addition of the organic nutrient shifted to a preferential
transformation of the inactive (À)-enantiomer in samples of North America and
Brazil (Table 8.20). Soils from Norway already in the original sample mainly
removed the (À)-enantiomer. Lewis et al. (1999) conjecture that micro-organisms
preferring the (+)-enantiomer may have increased in activity after organic nutrient
amendments (Table 8.20), but most samples (74%) still preferred the (À)-enantiomer. The authors pointed out that preferential removal of (À)-methyl dichlorprop
would render residues more phytotoxic than the same concentrations of the racemate, as long as demethylation is a prerequisite step and the (+)-enantiomer is not
preferentially removed after nutrient enrichment.
Ruelene, like dichlorprop, was transformed by micro-organisms only after a time
lag of about 6 months. The half-life, once transformation began, was about 40 days
in Brazilian soils and 20 days in Norwegian soils. Furthermore, Lewis et al. (1999)
concluded that the transformation rates of racemic ruelene were unaffected by
deforestation or soil warming. Enantioselectivity of the transformation process
was, however, influenced in both cases. In Brazil, forest soils, originally removing
preferentially the less toxic (À)-enantiomer [i.e. 22% (+), 67% (À), 11% (+/-); n ¼
17], after deforestation, shifted to transforming exclusively the (+)-ruelene. Soil
warming in Norway, on the other hand, caused soils to shift from all samples
preferentially removing the (+)-ruelene to some (22%) removing of the less toxic
(À)-enantiomer (n ¼ 13).
Delayed microbial transformation of chemical residues is commonly observed
and is often assumed to represent the amount of time required for low concentrations
of micro-organisms responsible for the transformation process to attain significant
numbers after using the chemicals as substrates or energy sources. In addition to this
general aspect, Lewis et al. (1999) gained deepened insight into the dependence of
the enantioselective transformation processes on the micro-organisms actually present in the respective soils. The diversity and biogeography of bacteria capable of
demethylating methyl dichlorprop were investigated by comparing small-subunit
(16S) ribosomal RNA (rRNA) genes of 50 soil bacterial isolates using amplified
ribosomal DNA restriction analysis (ARDRA). Furthermore, ARDRA has proved
useful in taxonomic assessments of bacterial culture collections and 16S rDNA clone
libraries at genus and species levels (for details of these methods, see refs in Lewis
et al. (1999)). In general, isolates from Norway and North America formed three
large clusters and appeared to be more closely related than to isolates from Brazil.
Conversely, Brazilian isolates formed smaller clusters that were more distantly
related to one another and to Norwegian and North American clusters. It appears
to be plausible that these relationships among bacterial isolates reflect ecological
194
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants