Growth and photosynthetic responses of the bloom-forming cyanobacterium Microcystis aeruginosa to elevated levels of cadmium

Effects of cadmium (Cd) on the growth and photosynthesis of the bloom-forming cyanobacterium Microcystis aeruginosa Kütz 854 were investigated. 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 at Cd concentrations less-than-or-equals, slant2 μM. Chlorophyll content was more sensitive to Cd toxicity than phycobiliproteins at 0.5 μM Cd. 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 concentrations less-than-or-equals, slant2 μM. On the other hand, when cells were treated with 4 μM Cd, Fv/Fm was significantly increased after 12 h of treatment but decreased after 48 h. The true photosynthesis was decreased with the increase of Cd concentration at 2 h. However, we noticed a recovery when the treatment was prolonged. After 48 h of exposure at the highest Cd concentration, photosynthetic oxygen evolution was markedly inhibited but dark respiration increased by 67%. Cellular Cd contents were augmented with the increase of Cd concentration. To our knowledge, we have demonstrated for the first time that the inhibitory site of Cd in M. aeruginosa is not located at the PSII or PSI level, but is probably situated on the ferredoxin/NADP+-oxidoreductase enzyme at the terminal of whole electron transport chain. We noticed also an increase of PSI activity, which is probably linked to the enhancement of cyclic electron transport around PSI. We can conclude that the increase of cyclic electron transport and dark respiration activities, and the decrease of phycobiliproteins might be adaptive mechanisms of M. aeruginosa 854 under high Cd conditions.