Sexual reproduction underlies the bloom dynamics of toxicogenic diatoms of the genus Pseudo-nitzschia H. Peragalo

Over six years (from March 2018 to the present), representatives of the genus Pseudo-nitzschia have been monitored in the waters of the Karadag Biological Station (44. 91 N, 35. 20 E). The data obtained indicate a periodic increase in the abundance of representatives of the genus in spring and autumn. During the early spring periods of 2021 and 2024, the number of cells in the population increased sharply. The potentially harmful Pseudo-nitzschia calliantha Lundholm, Moestrup et Hasle was continuously present and was dominant during bloom periods. Before this period, the relative number of small, reproductively potent cells increases in the population that enhances the probability of meeting sexual partners and facilitates fusion of gametes during sexual reproduction. Mass development up to phenomenon known as "blooms" is thus caused by the need for sexual reproduction in populations of P. calliantha. During the blooms, a redistribution of cell sizes was observed with the gradual disappearance of smaller cells and the appearance of larger ones, resulting from sexual reproduction, the size of which approached the maximum species-specific size. An increase in the sexually effective proportion of the population may predict bloom occurrence. We hypothesize that a sharp increase in cell numbers associated with sexual reproduction is a trait shared by all representatives of Pseudo-nitzschia.