Growth and photosynthesis acclimated response of the cyanobacterium Fischerella sp. FS 18 exposed to extreme conditions: alkaline pH, limited irradiance, and carbon dioxide concentration.

The true-branching heterocystous cyanobacterium Fischerella sp. FS 18 is widely distributed in paddy fields (North) and petroleum polluted soils (South) in Iran. This investigation tested the hypothesis that the cyanobacterium can acclimatize under the combined effect of extreme environmental conditions. Here, we analysed the physiological response of the cyanobacterium under extremely limited irradiance (2 μmol photon m s); limited carbon dioxide concentration (no aeration) at alkaline pHs (9 and 11) for up to 96 h. When the cyanobacterium was exposed to these extreme conditions at pH 11, we observed a decline in growth, oxygen liberation, photosystems ratio, chlorophyll a, and phycobilisomes activity compared to pH 9 after 24 h. Besides, we registered a significant decrease in maximum photochemical efficiency and activity of photosystem II at pH 11. The comparative single-cell study revealed that pH 9 caused higher efficiency of photosystem II and I, while increasing alkalinity pH 11 led to disturbed phycobilisomes activity after 24 h. This strain was able to recover its structures after 96 h. In addition, spectroscopy analyses revealed the presence of the Mycosporine-like amino acid at pH 9.