Cyanobacteria as agents for the control of pollution by pesticides and chlorinated organic compounds

Cyanobacteria are phototrophic aquatic micro-organisms that are found in a variety of environments, including polluted ones. Fifteen strains of cyanobacteria that belong to three taxonomic groups are able to degrade lindane (λ-hexachlorocyclohexane, a recalcitrant pesticide). The initial degradation pathway has been studied in two filamentous nitrogen-fixing strains of Anabaena sp. PCC 7120 and Nostoc ellipsosporum. These cyanobacteria dechlorinated loindane first to pentachlorocyclohexene and then to a mixture of trichlorobenzenes, and possibly further. Lindane dechlorination by these organisms occurred only in the presence of nitrate in the medium. Both ammonium and darkness inhibited the process. This combination of observations led us to the hypothesis that the nitrate-reduction system of cyanobacteria may be involved in dechlorination. The hypothesis was proven by the analysis of Anabaena sp. transpositional mutants in four genes of the nir operon. The mutants were unable to dechlorinate lindane. However, there was no correlation between lindane dechlorination and activities of individual proteins encoded by this operon. Genetic engineering of Anabaena sp. and N. ellipsosporum that introduced linA lindane dechlorination operon from Psuedomonas paucimobilis allowed us to uncouple dechlorination from nitrate requirement. Introduction, by genetic engineering, of fcABC (the 4-chlorobenzoate dechlorination system from Arthrobacter globiformis) to Anabaena sp and N. ellipsosporum rendered these strains newly capable of 4-chlorobenzoate dechlorination both constitutively and inducible by an environmental factor.