Effects of hexavalent chromium (Cr(VI)) exposure and antioxidant defense response in <i>Exiguobacterium indicum</i> HG8

Chromium (Cr) contamination represents a risk to the biodiversity of ecosystems, requiring the application of remediation processes for its recovery. One form of bioremediation can be done by bacteria resistant to hexavalent chromium (Cr(VI)). In the present study, the rhizobacterium <i>Exiguobacterium indicum</i> was isolated from the aquatic macrophyte <i>Hymenachne grumosa</i>, collected in the Santa Bárbara channel, located in southern Brazil. The Cr(VI) removal capacity and the response of oxidative stress biomarkers were analysed, in addition to the optimal pH and temperature conditions for maximum removal. The minimum inhibitory concentration of growth (MIC) of the isolate was 400 mg L^-1 of Cr(VI) and the results showed that <i>E. indicum</i> HG8 was able to grow and remove Cr in a wide range of incubation temperatures (20-45°C) and pH (5.0-9.0), evidencing its ability to adapt to different factors. The ideal conditions for cultivation and removal of Cr(VI) were verified at pH 6.0 and at 30°C. <i>E. indicum</i> HG8 was able to efficiently remove 99.6% of Cr(VI) and 89.4% of total Cr in 24 h of incubation. The increase in malondialdehyde levels in the extracellular extract demonstrates that there was lipid damage, in parallel with the increase in the adaptive response of antioxidant enzymes, indicating that oxidative stress was established. The data suggest that <i>E. indicum</i> HG8 possibly altered the permeability of the cell membrane, forming a kind of barrier.