For many years, chlordecone (sold under the brand name Kepone^®) and lindane (also called γ-hexachlorocyclohexane) were used as pesticides in the French West Indies, causing soil and water pollution that persists there to this day. In two earlier studies, researchers from Genoscope's Genomics Metabolics mixed research unit had identified bacteria (Citrobacter sp.86 and Desulfovibrio sp.86) able to anaerobically transform chlordecone into a number of degradation products.

In those bacteria, no genes coding for reductive dehalogenases^a or other gene candidates were identified in genome analyses. However, the team did identify an anaerobic cobalamin biosynthesis pathway^b, common also to other corrinoids^c. The fact that the majority of the effective chemical protocols for chlordecone degradation use cobalamin as a catalyst gives a whole new meaning to that simple observation.

In their recent study published in Frontiers in Microbiology, the researchers hypothesized that the cobalamin (or one of its analogous corrinoids) produced by these bacteria may explain the microbiological transformation of chlordecone.

To validate that hypothesis, the team constructed four Citrobacter sp.86 strains in which several genes associated with cobalamin biosynthesis were deleted. Those strains were placed in long-term liquid culture in the presence of chlordecone and analytically monitored by gas chromatography–mass spectrometry.

The team detected transformation products in the culture containing the control Citrobacter sp.86 strain (able to produce cobalamin and/or other corrinoids) and in those of mutant strains capable of producing only an incomplete cobalamin (missing the lower element that contains the nucleotide loop to which the dimethylbenzimidazole ligand or an aromatic analogue attaches). In contrast, the mutant strains unable to insert the cobalt atom in the precorrin-2 (an intermediate in the synthesis of cobalamin and other corrinoids^c) could not degrade chlordecone.

The study also showed that lindane, which can be transformed in anaerobiosis by Citrobacter freundii (although the mechanisms remained unknown), was also degraded by the control Citrobacter sp.86 strain and the mutants able to insert the cobalt atom, but not those unable to insert it.

This latest study thus showed a role for cobalt-containing corrinoids (cobalamin and other analogues) in the bacterial degradation of chemically different chlorine compounds. This suggests that their increased production in contaminated environments could accelerate decontamination processes.

The results show the role of corrinoids produced by Citrobacter sp.86 not only in the biodegradation of chlordecone, but also in that of at least one other organochlorine compound. Indeed, the observation of degradation activity against lindane, with its chemical structure quite different from that of chlordecone, suggest that the mechanisms underlying this action may extend to a range of organochlorine pesticides, which, in turn, suggests the possibility of novel bioremediation applications.

^a: Reductive dehalogenases are enzymes involved in the substitution of a chlorine atom for a hydrogen atom. The energy thus produced enables bacterial respiration in the absence of O₂.

^b: Cobalamin is another name for the water-soluble vitamin B12. This latter is important in brain and nervous system function and DNA synthesis. It is also a cofactor in enzymatic functions involved in the reductive dechlorination of chlorine pesticides.

^c: Corrinoids are compounds comprising a corrin, i.e., a tetrapyrrole related to porphyrins. The best-known corrinoid is vitamin B12 (cobalamin).