Polyphasic evaluation of Aphanizomenon issatschenkoi and Raphidiopsis mediterranea in a Mediterranean lake

Aphanizomenon issatschenkoi and Raphidiopsis mediterranea were studied in the Mediterranean Lake Doirani during a cyanobacterial bloom (August-October 2004). Aphanizomenon issatschenkoi was clearly morphologically distinct from R. mediterranea on the basis of apical cells. The two species demonstrated an almost inverse temporal distribution, consistent preference for different water depths and negative correlation between their net growth rates. Raphidiopsis mediterranea exhibited a higher net growth rate (0.17 days(-1)) and dominated during warmer water periods, with high Z(max)/Z(sec) ratio and low but available inorganic nitrogen. Aphanizomenon issatschenkoi was more generalist, exhibiting slower net growth (0.10 days(-1)) and a broader distribution in relation to temperature and light, regularly forming heterocytes under low nitrogen conditions. Aphanizomenon issatschenkoi was also phylogenetically distinct from R. mediterranea on the basis of partial 16S rRNA gene sequence. The R. mediterranea typical morphotype corresponded to one phylotype in the well-defined 16S rRNA gene sequence cluster of Cylindrospermopsis raciborskii. The formation of two specialized reproductive cells in the same individual of R. mediterranea is described for the first time and may indicate unknown alternative reproductive modes and lifestyles in planktonic Cyanobacteria.