Accumulation, metabolic effects, and mechanisms of toxicity of nickel in the microalga Chlorella sorokiniana

Nickel (Ni) contamination is a serious threat to aquatic ecosystems. The application of freshwater green microalgae that show capacity to sequester this metal, such as Chlorella sorokiniana , in bioremediation, requires filling certain knowledge gaps. Herein, the response of C. sorokiniana to a high nominal concentration of Ni 2+ was examined at late exponential / early stationary growth phase. The fraction of Ni in the biomass reached a maximum of ~6% within the first 24 h of exposure. Ni in the biomass remained in the 2+ redox form that was coordinated by (poly)phosphates and water molecules in distorted octahedral geometry within cluster structures in vacuoles, and by sulphated polysaccharides in the cell wall. In response to Ni 2+ exposure, microalgae displayed altered cell wall structure with increased rigidity and reduced thickness and partial detachment of the outer layer. Metabolic responses to Ni 2+ included decreased lipid reserves with reduced fatty acid saturation and substantial decrease in protein-glutathione adducts. Ni 2+ induced increased intracellular generation of reactive oxygen species. However, this did not cause significant oxidative damage or antioxidant activation. We hypothesize that the inhibition of photosynthesis may be the main reason behind the toxic effects of Ni in this microalga. Photosynthetic activity could be a useful biomarker of Ni toxicity. Despite some toxicity, high intracellular Ni storage ability supports the bioremediation potential of C. sorokiniana biomass.