Blue-green bacteria - Microcystis
Microcystis forms big colonies mainly consisting of mucilage and with very small cells (3-7 my). The cells have gas vesicles, which are too small to be seen by light microscope. The cells with gas vesicles will have a lesser weight or density than the water they are in and so float upwards rather than sink downwards. Microcystis is common in plankton of waters rich in nutrients and can produce massive water bloom. In the images the small cells outside the colonies are cells of the gold algae Dinobryon.