Comparative evaluation of lindane removal by free and immobilized Streptomyces sp. strains

Abstract

Lindane (γ-hexachlorocyclohexane) is a chlorinated insecticide which has a negative effect as a pollutant agent of soil, water and sediments. Nowadays it has been banned in almost all countries of the world, but its residues still remain in the environment. In this context, bioremediation, involving the use of microorganisms to degrade environmental contaminants, has received much attention as an effective biotechnological approach to clean up this kind of pollutants. Cell immobilization has been shown to have several advantages over conventional systems using free cells, such as the possibility of employing a high cell density, easier separation of cells from the system, repeated use of cells and better protection of cells from harsh environments. The goal of this study was to compare the cell growth and lindane removal by pure cultures of Streptomyces sp. using free cells and four different immobilization techniques. It was also evaluated the removal efficiency of a mixed immobilized culture and the reusability of the immobilized consortium. Four previously isolated actinobacteria (Streptomyces sp. A2, A5, A11 and M7) were individually grown in TSB medium and then cultured in minimal medium supplemented with lindane as carbon source. The strains were inoculated as free cells and immobilized in four supports: agar cubes, PVA-alginate beads, silicone tubes and silk sachets. After 96 h of incubation, cell growth was estimated by determining the colony forming units per gram of immobilization matrix, and residual lindane concentration was determined in the culture supernatants by gas chromatography. Streptomyces sp. A11 and M7 presented better growth when they were cultured immobilized rather than free, while Streptomyces sp. A2 showed a higher growth value in the free cell system and the silicone tube immobilization than in the other supports. Streptomyces sp. A5 presented the best growth value in the silicone tube immobilization. Lindane removal by microorganisms was more efficient for all the strains when they were immobilized in silk sachets (85-90% of removal), so this support was selected to evaluate the lindane removal ability of the mixed culture. When the four strains were entrapped together in silk sachets, there was no enhancement in lindane removal compared with the pure cultures; nevertheless the sachets could be reused for two more cycles of 96 h, with a removal efficiency of 59% in each. The use of immobilized indigenous actinobacteria is a promising alternative for bioremediation of polluted sites, even though further studies about the use of mixed cultures will be necessary.