Life cycle of Nostoc sphaericum (Nostocales, Cyanoprokaryota) in tropical wetlands

We studied the life cycle of Nostoc sphaericum Vaucher ex Bornet & Flahault in a wetland in southeast Mexico, collecting in the only two sites where this species is growing: a tropical forest dominated by Haematoxylon campechianum L. and a subdeciduous tropical forest, inundated for part of the year. Cultures grown under controlled conditions revealed the sequence of life cycle stages observed in isolated collections, from the dry and from the flooded period. Nostoc sphaericum was observed both in culture and in situ and the expression of morphological life stages were studied in relation to the hydrological changes. In the studied wetland, N. sphaericum does not form either aseriate phases or buds; these appeared only in Culture but aseriate phases remain inside the thallus forming the new trichomes; buds remain attached to the main thallus. Thus, none of these structures seems to have a central role in the thallus dispersion. On the contrary, the most frequent dispersion mechanisms could be the production and liberation of hormogonia and akinetes. Moreover, the length of the dry period (7-8 months) seems to regulate the duration of two typical life cycle stages in wild conditions: the production of akinetes and mature thalli represented by compact colonies surrounded by thick and intensely pigmented mucilaginous envelopes. In addition, since the dry period is long and flooding is intermittent during each hydrological cycle, limited growth is also a characteristic of N. sphaericum in the tintal (H. campechianum forest).