enantiomers as separate entities in a biochemical and environmental toxicological
context.
8.1 Microbial Transformation of Chiral Environmental
Pollutants
Microbial transformation processes are today considered a major transformation
pathway for many chiral pollutants entering the environment via aqueous, terrestrial
or aerial routes (Hühnerfuss and Shah 2009). Thus, the thorough examination of
microbial transformation processes is an important part of an environmental risk
assessment. A first evaluation of the microbial transformation of HCHs is dating
back to 1975 (Tu 1975). However, the first studies on the enantiomer-selective
microbial transformation were not attempted before the early 1990s.
8.1.1 Laboratory Experiments
In 1992, Ludwig et al. (1992a) applied for the first time enantioselective cGC to a
microbial transformation experiment that included the environmental pollutants γand α-hexachlorocyclohexane (γ-HCH and α-HCH, respectively, Figs. 8.1, top, and
8.2, bottom), as well as their transformation products γ-pentachlorocyclohexene and
β-pentachlorocyclohexene (γ-PCCH and β-PCCH, respectively). α-HCH is the only
chiral isomer of the eight conceivable HCH isomers (Fig. 8.2), while γ-HCH is
achiral, but it is the only HCH isomer that exhibits an insecticidal activity. The
transformation products of the two HCH parent compounds, γ-PCCH and β-PCCH,
are chiral; therefore, γ-HCH is considered as prochiral (Fig. 8.1).
In order to allow the comparability between the laboratory experiments and
natural conditions in the open sea, the experiments were performed with a microbial
community obtained from aerobic sediment and surface water collected in the
vicinity of the island Heligoland (54
o 11
0 N, 7
o , 53
0 E, German Bight, North Sea)
and in the presence of additional carbon sources. Thus, the formation of specific
microbial cultures had been avoided, and it was tentatively assumed that the original
microbial community could be perpetuated at least for the experimental period.
Furthermore, the cultures were kept under conditions that are known to promote
the growth of marine bacteria much better than the growth of terrestrial bacteria, that
is, at room temperature and at natural saltwater conditions. In addition, microscopic
examinations confirmed that both the size and the mobility of the micro-organisms
indicated the nearly exclusive presence of marine bacteria in the culture obtained.
For the transformation experiment, the sterile aged seawater medium was inoculated
with 0.2 mL of the enrichment culture, 2 mg of γ-HCH and 0.4 mg of α-HCH and
shaken under the same conditions as the sterile control. For further experimental
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8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
context.
8.1 Microbial Transformation of Chiral Environmental
Pollutants
Microbial transformation processes are today considered a major transformation
pathway for many chiral pollutants entering the environment via aqueous, terrestrial
or aerial routes (Hühnerfuss and Shah 2009). Thus, the thorough examination of
microbial transformation processes is an important part of an environmental risk
assessment. A first evaluation of the microbial transformation of HCHs is dating
back to 1975 (Tu 1975). However, the first studies on the enantiomer-selective
microbial transformation were not attempted before the early 1990s.
8.1.1 Laboratory Experiments
In 1992, Ludwig et al. (1992a) applied for the first time enantioselective cGC to a
microbial transformation experiment that included the environmental pollutants γand α-hexachlorocyclohexane (γ-HCH and α-HCH, respectively, Figs. 8.1, top, and
8.2, bottom), as well as their transformation products γ-pentachlorocyclohexene and
β-pentachlorocyclohexene (γ-PCCH and β-PCCH, respectively). α-HCH is the only
chiral isomer of the eight conceivable HCH isomers (Fig. 8.2), while γ-HCH is
achiral, but it is the only HCH isomer that exhibits an insecticidal activity. The
transformation products of the two HCH parent compounds, γ-PCCH and β-PCCH,
are chiral; therefore, γ-HCH is considered as prochiral (Fig. 8.1).
In order to allow the comparability between the laboratory experiments and
natural conditions in the open sea, the experiments were performed with a microbial
community obtained from aerobic sediment and surface water collected in the
vicinity of the island Heligoland (54
o 11
0 N, 7
o , 53
0 E, German Bight, North Sea)
and in the presence of additional carbon sources. Thus, the formation of specific
microbial cultures had been avoided, and it was tentatively assumed that the original
microbial community could be perpetuated at least for the experimental period.
Furthermore, the cultures were kept under conditions that are known to promote
the growth of marine bacteria much better than the growth of terrestrial bacteria, that
is, at room temperature and at natural saltwater conditions. In addition, microscopic
examinations confirmed that both the size and the mobility of the micro-organisms
indicated the nearly exclusive presence of marine bacteria in the culture obtained.
For the transformation experiment, the sterile aged seawater medium was inoculated
with 0.2 mL of the enrichment culture, 2 mg of γ-HCH and 0.4 mg of α-HCH and
shaken under the same conditions as the sterile control. For further experimental
108
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants