Abstract
The diversity within the genus Nostoc is still controversial and more studies are
needed to clarify its heterogeneity. Macroscopic species have been extensively
studied and discussed; however, the microscopic forms of the genus, especially
those from running waters, are poorly known and likely represented by many more
species than currently described. Nostoc isolates from biofilms of two Spanish
calcareous rivers were characterized comparing the morphology and life cycle in
two culture media with different levels of nutrients and also comparing the 16S
rRNA gene sequences. The results showed that trichome shape and cellular
dimensions varied considerably depending on the culture media used, whereas
the characteristics expressed in the course of the life cycle remained stable for each
strain independent of the culture conditions. Molecular phylogenetic analysis
confirmed the distinction between the studied strains established on morphological
grounds. A balanced approach to the evaluation of diversity of Nostoc in the
service of autecological studies requires both genotypic information and the
evaluation of stable traits. The results of this study show that 16S rRNA gene
sequence similarity serves as an important criterion for characterizing Nostoc
strains and is consistent with stable attributes, such as the life cycle.