Studies On The Growth And Photosynthetic Responses Of The Bloom-forming Cyanobacterium Microcystis Aeruginosa (Cyanophyceae) To Elevated Levels Of Cadmium

Cadmium (Cd) is a bivalent, non-essential trace element and considered as one of the most phytotoxic heavy-metal contaminants. Its concentration in air, soil and waters of the earth is continuously increasing due to industrial activities and agricultural practices. The toxic effects of Cd to phytoplanktons including cyanobacteria have received considerable attention. Microcystis aeruginosa Kiitz. has been widely recognized as one of the most common bloom-forming cyanobacteria found in both fresh and marine waters. In this research, the author reviewed the mechanisms of heavy metal detoxification and tolerance in algae, and the growth and photosynthetic responses of the bloom-forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) to elevated levels of cadmium were studied in the present paper, in the hope to elucidate the toxicity mechanisms of Cd on M. aeruginosa.Effects of cadmium on the growth and photosynthesis of the bloom-forming cyanobacterium Microcystis aeruginosa were investigated. Growth, pigments content, chlorophyll fluorescence, oxygen evolution, dark respiration, electron transport activities, and Cd content of cells were measured on Cd-exposed material. The growth was markedly inhibited when it was treated with 4 μM Cd. However, the biomass production was almost not influenced after a prolonged exposure of low Cd concentrations. Chlorophyll content was more sensitive to Cd toxicity than phycobiliproteins under low Cd conditions. However, the decrease of phycobiliproteins was larger than chlorophyll at the highest Cd concentration. A significant increase of Fv/Fm value was observed at Cd levels ≤ 2 μM. It was significantly increased at the initial phase but was decreased at 48 h at 4 μM Cd. During the initial period, the true photosynthesis was decreased with the increase of Cd concentration. However, the recovery was occurred during subsequent treatment. At the highest Cd level, photosynthetic oxygen evolution was markedly inhibited but dark respiration was increased by 67% at 48 h. Cellular Cd contents were increased with the increase of Cd-exposed concentration. PSII and PSI werenot the inhibitory sites of Cd, it could situate to the FNR at terminal of whole electron transport chain. The increase of PSI activity indicated the enhancement of cyclic electron transport around PSI. The increase of cyclic electron transport and dark respiration activities, and the decrease of phycobiliproteins might be adaptive mechanisms of M. aeruginosa under high Cd conditions.