Substantially Enhanced Degradation of Hexachlorocyclohexane Isomers by a Microbial Consortium on Acclimation.

Widespread contamination of the environment, globally, has been caused by extensive and
indiscriminate use of hexachlorocyclohexane (HCH) as an insecticide since the 1940s, threatening
the biota including humans, and there is an urgent need to eliminate it, preferably through
bioremediation technologies. A γ-HCH-degrading microbial consortium was isolated by enrichment
of a soil sample from a sugar cane field having a long history of technical grade HCH application. On
acclimation the degrading ability improved substantially. The consortium, which took 10 days to
degrade 25 μg mL-1 of γ-HCH, initially could mineralize even 300 μg mL-1 of the substrate within
108 h on acclimation. With 300 μg mL-1 substrate, the rate of degradation, as calculated for the
early exponential phase, was 216 μg mL-1 day-1, the highest reported so far. An amount of 400 μg
mL-1 of γ-HCH, however, was mineralized partially with only 78% Cl- release. No apparent
accumulation of intermediary metabolites was observed up to 300 μg mL-1 substrate, indicating
a fast rate of mineralization. Aeration, mesophilic temperatures (20-35 �C), and near neutral pH
(6.0-8.0) were favorable conditions for degradation. The presence of glucose at 1000 μg mL-1
retarded the degradation, whereas cellulose and sawdust at 1600 μg mL-1 and glucose at 100 μg
mL-1 did not show any marked effect. The consortium also mineralized R-, β-, and δ-HCH
efficiently. The consortium consisted of nine bacterial strains and a fungal strain, and individually
they were able to degrade 10 μg mL-1 of γ-HCH. This mixed culture holds high potential for
deployment in bioremediation of HCH-contaminated soils, waste dumpsites, and water bodies.