Temperature and irradiance influences on cadmium and zinc uptake and toxicity in a freshwater cyanobacterium, Microcystis aeruginosa.

Temperature and light irradiance are important factors affecting the occurrence of cyanobacterial blooms. In this study, we examined the influences of different temperatures (15, 24, and 30°C) and irradiances (18, 32, and 55 μmol photons m(-2)s(-1)) on the uptake and toxicity of cadmium (Cd) and zinc (Zn) in a freshwater cyanobacterium Microcystis aeruginosa. The subcellular distribution of Cd and Zn was analyzed. Enhanced growth rates were observed for the cyanobacterial cells incubated at higher temperature or irradiance conditions with lower metal concentrations. With increasing ambient Cd or Zn concentrations, both cellular growth rate and photosynthesis were significantly inhibited at elevated irradiance conditions. The observed increase in Cd and Zn toxicity might be attributed to the enhanced metal uptake and accumulation in Microcystis. Based on the intracellular Cd concentration, the 50% inhibition concentration (IC(50)) values were higher at the higher temperature or irradiance treatment. The subcellular distribution demonstrated that Cd in the metal rich granule (MRG) faction increased with elevated [Cd(2+)] concentration, suggesting that MRG may partially detoxify the Cd toxicity in the cyanobacterial cells. This study implied that temperature and irradiance may influence the biogeochemical cycling of metals during cyanobacterial blooming in eutrophic freshwater ecosystems.