Cell ultrastructure changes accompanying the annual life cycle of the cyanobacterium Microcystis aeruginosa
Cell ultrastructure of Microcystis aeruginosa in the Brno dam reservoir on the river Svratka (Moravia, Czech Republic) was followed during its annual life cycle. All the year round, the benthic vegetation lives in the sedimented swampy mud. Its cell metabolism, driven by heterotrophy, is rather slow. In comparison with the pelagic vegetation typical for the summer phase, the volume of most cells is slightly decreased and the division ratio falls beneath 20 %, though the cells permanently keep their division ability. Most thylakoids survive rather unchanged. Benthic cells are completely devoid of gas vesicles. Bended fibrils of the gas vesicle wall protein, keeping their tertiary structure tendency to form a helical curve, mark the previous position of aerotopes, completely collapsed now. Ribosomes survive. Metabolic inclusions are limited in number and volume; only polyphosphate granules are numerous. The cell wall gets thickened by increase of the mucopeptide, the homogeneous slime sheath is considerably wider. Usually during August, the benthic population gets strengthened in its cell number by sinking of up to then pelagic cells losing their aerotopes. Exceptionally, a few typical pelagic cells can be found in the overwintering phase of the benthic vegetation. More often, cells of a transitory ultrastructure can be found in periods of approaching movements of the population: up in spring and down in autumn. Usually, as soon as during the first half of March, a pelagic (planktic) vegetation starts to get established: cells are shifted up from the benthos into the water column towards its surface, starting practising autotrophy. Now, they construct their planktic ultrastructure. They grow slightly larger and the frequency of their division is enhanced up to its summer standard: some 65 % of cells dividing at any moment. Gas vesicles are re-built using the left gas vesicle wall protein filaments and the number of aerotopes increases up to its summer standard reaching some 65 % of the complete cell section area in May and nearly 95 % in July. The thylakoid appearance does not differ distinctly from its winter form. Also the typical picture of planktic ultrastructure is established during March to April. The cytoplasm is stuffed with ribosomes. Metabolic inclusions are represented by numerous granules of cyanophycin, glycogen, polyphosphates and lipid droplets. Carboxysomes reach their maximal volume. The cell wall is thin, just as its slime sheath. The autotrophic pelagic vegetation forms a metabolic outburst over the persistent benthic one, lasting from about March to about the end of August. Nevertheless, a typical benthic cell can exceptionally be found in plankton, too.