Solitary terminal cells of Aphanizomenon gracile (Cyanobacteria,Nostocales) can divide and renew trichomes

An attempt was made to find evidence that morphologically distinct terminal cells of filamentous cyanobacterium Aphanizomenon gracile strain CCALA 8 are capable of dividing and forming trichomes. Based on our current knowledge, the division of morphologically diversified terminal cells is possible in nostocalean cyanobacteria. However, this process has been observed only in a few species. Terminal cells of A. gracile differ morphologically from other vegetative cells of a trichome, as they are not hyaline and can sometimes be found as solitary cells in cultures. Hence, it was reasonable for us to suspect that these cells are capable of dividing and forming trichomes. We observed terminal cells under a light and transmission electron microscope. Microscopic observations revealed that the septum formed in both solitary terminal cells and in terminal cells attached to trichomes. Our study is the first to demonstrate division and renewal of trichomes in terminal cells of A. gracile. Previously, such mode of reproduction was described only for another nostocalean cyanobacterium Raphidiopsis mediterranea. Moreover, our findings further emphasize the variability among members that belong to the genus Aphanizomenon , in which a type species (A. flos‐aquae) has hyaline cells incapable of dividing and renewing trichomes, while A. gracile can additionally propagate by solitary terminal cells division. This additional feature distinguishing A. gracile from typical species of Aphanizomenon, such as A. flos‐aquae, might be valuable for resolving taxonomic position of the species considering ambiguous genetic relationship between A. gracile and A. flos‐aquae.