Differential responses of different phenotypes of Microcystis (Cyanophyceae) to UV-B radiation

Microcystis can be single celled or colonial under certain conditions, thereby possessing phenotypic plasticity. Differential physiological and biochemical responses of colonial Microcystis aeruginosa FACHB 939 and its single-celled strain to ultraviolet type B radiation (UV-B) exposure were investigated. Results suggested that UV-B exposure exerted a lower inhibitory effect on the growth of colonial Microcystis, and that the single-celled strain had higher ability to recover after exposure to UV-B. Carotenoids/chlorophyll a (Chl a) ratios were increased, whereas phycobilisome/Chl a ratios were decreased by UV-B exposure. The photosynthetic activities of both phenotypes were inhibited, but colonial Microcystis showed higher tolerance and recovery ability to UV-B exposure. The sheath and shading effect, as well as changes of pigments of colonial Microcystis, were believed to play roles in resisting UV-B exposure. UV-B radiation can increase the sinking rate of the single-celled Microcystis and cause colonial Microcystis to shift from a floating to a sinking state. The observed reduction of colony size and increase in total carbohydrate content induced by UV-B radiation presumably caused the decrease in buoyancy in Microcystis. However, the increase in sinking rate is also an important way for Microcystis to avoid damage inflicted by UV-B radiation.