Efeitos de exposição ao cromo hexavalente (Cr(VI)) e resposta da defesa antioxidante em Exiguobacterium indicum HG8: estresse oxidativo e potencial de bioacumulação

Abstract

Chromium (Cr) contaminated areas pose a risk to the sustainability and biodiversity of ecosystems, demanding, therefore, the application of remediation processes to ensure its recovery and environmental safety. Hexavalent chromium (Cr(VI)) resistant bacteria have shown potential to be applied to bioremediation processes due to the development of various defense mechanisms, which can be used on Cr reduction. In this study, the rhizobacteria HG8 identified with a 99.88% similarity by 16S rRNA as Exiguobacterium indicum (KY439921.1) was isolated from the aquatic macrophyte Hymenachne grumosa, collected by the Santa Bárbara stream, an environment with heavy metal pollution, including Cr, by anthropogenic activities. For this purpose, the characteristics studied were the Cr(VI) removal capacity and the biomarkers (superoxide dismutase – SOD; catalase – CAT; glutathione peroxidase – GPx e produção de malondialdehyde – MDA production) response induced by the oxidative stress under culture medium supplemented with 50, 100, 200 and 300mg L -1 of Cr(VI), the minimal inhibitory concentration (MIC) and optimal pH and temperature conditions to maximize Cr(VI) removal. The isolated’s MIC was 400mg L -1 of Cr(VI) and results show that E. indicum HG8 was able to grow and remove Cr within a wide range of incubation temperatures (20-45ºC) and pH (5.0-9.0), indicating the capacity to adapt to extreme conditions. The ideal condition of growth and removal of Cr(VI) was verified at pH 6.0 and incubation temperature of 30ºC. E. indicum HG8 was able to effectively remove 99.6% of Cr(VI) and 89.4% of total Cr from the culture medium after 24 hours of incubation. The rise of MDA content in the extracellular extract shows that lipidic damage had occurred, in parallel to the rising in adaptative response of antioxidant enzymes CAT and SOD, which indicates that the oxidative stress has been reached. Data suggest that E. indicum HG8 probably has changed the permeability of the cell membrane, turning it into a sort of barrier, since there is no significant change in MDA concentration, nor antioxidant enzymes response in the intracellular extract. With such results, the isolated can be used as a potential agent for effective remediation of Cr(VI) contaminated areas, to reach environmental security and protect human and animal health.