Due to the fact that Microcystis is a cyanobacteria it is autotrophic. An autotroph is an organism that creates its own food. Cyanobacteria do this with the aid of folds in their external membranes, this resembles the chloroplast of a plant. Like a chloroplast the color of cyanobacteria allow them to capture certain frequencies of light. The energy from said light is used to break down certain compounds to let the bacterium photosynthesize. Cyanobacteria are also carbon and nitrogen fixers. This fixations rely on aerobic processes, which is why cyanobacteria need to be at towards the surface of water to survive and flourish. Note that in the image below the cyanobacteria represented is bacillus.