Cadmium Tolerance in the Thermo-Acidophilic Red Alga C. merolae, Possible Mechanisms and Implications for Bioremediation

Cyanidioschyzon merolae, a unicellular extremophilic red algae, is found in hot, acidic groundwater with high concentrations of heavy metals. The association makes it an ideal species to investigate mechanisms of heavy metal tolerance, which may lead to its use in phyco- remediation wherein photosynthetic algae use biological processes to bind and remove toxic substances. Two strains of C. merolae, MS1 and 10D, are genetically very similar, despite the latter lacking a cell wall. To investigate heavy metal toxicity and the role of the cell wall, the two strains of C. merolae were exposed to various concentrations of cadmium and cultures were evaluated spectrophotometrically to assess the impact on growth over a 7-day period. The IC50 values of MS1 and 10D were estimated to be 15 and 0.5 ppm CdCl2 respectively, indicating that the cell wall provides protection under the presence of heavy metals. Cadmium uptake was also measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to investigate metal ion exclusion and acidocalcisome-Cd2+ chelation as potential tolerance mechanisms. ICP-OES data indicated that 10D inoculum pretreated with phosphate depletion and re-supplementation, to induce Cd chelation in acidocalcisomes, then cultured in MA2 had the highest biomass Cd content of all strains and treatments (0.321 ppm; 31.55%). The cell wall clearly promotes survival and resistance to higher concentrations of environmental heavy metals, however, neither MS1 nor 10D seemed to be strains primed for phyco-remediation of heavy metal contamination through cellular uptake and sequestration.