Natural and acquired mechanisms of tolerance to chromium in a Scenedesmus dimorphus strain

Abstract

The development of strategies for the bioremoval of heavy metals from the environment will greatly benefit from a deep understanding of how resistance arises. For the purpose of advancing in the knowledge of resistance to chromium in algae, we generated a resistant strain of the green alga Scenedesmus dimorphus by chronically exposing cells to this metal for a 95-day period. This procedure generated a strain that showed normal growth in the presence of the metal. This strain, termed chromium resistant strain (CRS), was able to overcome oxidative stress generated by chromium by employing antioxidant mechanisms based on the augmentation of accessory pigments and the release to the medium of substances that reduced metal toxicity. In addition, both the WT (wild type) and CRS strains appeared to exclude most of the metal from entering the cells, which suggests that this is a natural mechanism of metal tolerance in algae. Although CRS resistance significantly improved strain performance in the presence of chromium, it had no impact on the metal removal capacity of the strain. We observed that chromium bioremediation only occurred at low pH with the biomass working as a biosorbent, but the WT and CRS strains were equally efficient in metal removal.