Rhizoremediation of lindane by root‐colonizing Sphingomonas

We used a two‐step enrichment approach to isolate root‐colonizing hexachlorocyclohexane (HCH)‐degrading microorganisms. The first step consists of the use of classical liquid enrichment to isolate γ‐HCH degraders. The γ‐HCH‐degrading microbes were attached in mass to corn seeds sown in soil with γ‐HCH, and after plant development we rescued bacteria growing on root tips. Bacteria were then subjected to a second enrichment round in which growth on liquid medium with γ‐HCH and inoculation of corn seeds were repeated. We then isolated bacteria on M9 minimal medium with γ‐HCH from root tips. We were able to isolate four Sphingomonas strains, all of which degraded α‐, β‐, γ‐ and δ‐HCH. Two of the strains were particularly good colonizers of corn roots, reaching high cell density in vegetated soil and partly removing γ‐HCH. In contrast, these bacteria performed poorly in unplanted soils. This study supports the hypothesis that the removal of persistent toxic chemicals can be accelerated by combinations of plants and bacteria, a process generally known as rhizoremediation.