Evolutionary differentiation of the sister cyanobacterial genera Cyanospira (Florenzano, Sili, Pelosi et Vincenzini) and Anabaenopsis (Woloszynska) Miller in response to extreme life conditions

To clarify the distinctiveness of the genus Cyanospira from the related genus Anabaenopsis, a morphological study of several undescribed strains of Cyanospira and an extensive phylogenetic analysis of Anabaenopsis and Cyanospira were performed. Heterocytous cyanobacterial strains were isolated from natron salt samples from the Republic of Chad containing large numbers of dormant dry akinetes. Morphology, and the entire life cycle, from akinete germination to the development of mature vegetative forms, were documented. Cultures of Cyanospira, obtained from the germination of desiccated akinetes, were characterized by variable filament dimensions and coiling degrees. Akinete development in vegetative filaments was typically apoheterocytic; akinetes developed in chains continuously expanding along the filament; in old cultures, the whole filaments could be transformed into chains of akinetes. The development pattern of akinetes in Cyanospira was definitely different from that of Anabaenopsis. Phylogenetic analysis was performed using the ARB and SILVA phylogenetic tools and databases. All publicly available sequences of strains belonging to the genera Anabaenopsis and Cyanospira, and sequences of our new strains, were compared with selected entries of Nostocaceae. The two genera Anabaenopsis and Cyanospira formed two phylogenetically coherent and well defined sister units, mostly related to the genus Nodularia. Inside the Anabaenopsis branch, a cluster exclusively composed by strains of A. elenkinii and another corresponding to A. nadsonii could be identified. Sequences of Cyanospira formed two distinct phylogenetic clusters, respectively corresponding to the species C. rippkae and C. capsulata. Sequences of the alkaliphilic species A. abijatae were found to be members of the C. rippkae subcluster. As already known for halophilic cyanobacteria, also alkaliphilic cyanobacteria of the genus Cyanospira were phylogenetically separated from their mesophilic counterparts, classified under the genus Anabaenopsis. We support that the differentiation of sister genera in response to selective conditions of extreme environments is a well represented evolutionary mechanism in cyanobacteria.