Microcystis aeruginosa (Cyanobacteria): ultrastructure in a pelagic and in a benthic ecosystem

Populations of chroococcal cyanobacterium Microstys aeruginosa are known in two ecosystems during their annual life cycle, carrying out different ways of metabolism: in summer in the plankton (pelagic biotope) and in winter in the sediments (benthic biotope). In fact, they live in sediments all the year long. The objective of this work was to study the ultrastructure of the cells from both biotopes. Aerotopes occupy some 75% of the inner space of the pelagic cells, while there are none in the benthic ones. The cell envelope is thin, formed by a thin cell wall covered with an also thin layer of the superficial slime sheath in the pelagos, while being much wider in its amplified glycopeptide and slime compartments in the benthos, with the sheath surface adsorbing mineral materials. There is no substantial difference between both cell types in the appearance of thylakoids or of ribosomes. Metabolic inclusions in the pelagos are represented by ample and relatively large carboxysomes and cyanophycin granules, just as by smaller polyphosphate and glycogen granules. In the benthos, there are hardly any carboxysomes, no glycogen granules, while the global amount of cyanophycin granules is kept about the same and polyphosphate bodies are larger and more frequent. Some 55% of cells undergo binary fission at any moment in a pelagic population, while not more than 20% divide in the benthic one.