Photosynthetic adaptation mechanism of Microcystis (Cyanophyceae) related to changes of colony size in a eutrophic lake

Microcystis blooms occur frequently in eutrophic lakes, and they are composed of many different-sized colonies. Many studies have addressed the competitive advantage of colony size, e.g. colony buoyancy regulation mechanism, low nutrient stress tolerance, low irradiance stress; however, studies are lacking for growth strategies of different-sized Microcystis colonies responding to photosynthetic regulation. This study compared photosynthetic activities and photosynthetic pigment composition of different-sized Microcystis. The results showed that the photosynthetic activity caused a V-shaped curve as colony size increased, i.e. medium-sized colonies (125-400 mu m) had the lowest photosynthetic activity. The higher photosynthetic activity of small colonies (<125 mu m) was probably caused by the higher light-harvesting efficiency of phycobilisomes (PBS) and a relatively low nonphotochemical quenching (qN). The high photosynthesis of large colonies (>400 mu m) was attributed to the high efficiency of energy transition of the electron transport chains in photosystem II. Also, large colonies tolerated high irradiance stress because of their high qN, which was related to high cell quotas of PBS. All these results suggested that both small and large colonies had their own ecological adaptation strategies when they were suspended at different water layers, providing an explanation for how Microcystis survives adverse natural conditions, grows rapidly and forms serious surface blooms.