Nostoc is a common and well-studied genus of cyanobacteria and, according to molecular phylogeny, is a polyphyletic group. Therefore, revisions of this genus are urged in an attempt to clarify its taxonomy. Novel strains isolated from underexplored environments and assigned morphologically to the genus Nostoc are not genetically related to the 'true Nostoc' group. In this study, four strains isolated from biofilms collected in Antarctica and five strains originated from Brazilian mangroves were evaluated. Despite their morphological similarities to other morphotypes of Nostoc, these nine strains differed from other morphotypes in ecological, physiological and genetic aspects. Based on the phylogeny of the 16S rRNA gene, the Antarctic sequences were grouped together with the sequences of the Brazilian mangrove isolates and Nostoc sp. Mollenhauer 1 : 1-067 in a well-supported cluster (74 % bootstrap value, maximum-likelihood). This novel cluster was separated phylogenetically from the 'true Nostoc' clade and from the clades of the morphologically similar genera Mojavia and Desmonostoc. The 16S rRNA gene sequences generated in this study exhibited 96 % similarity to sequences from the nostocacean genera mentioned above. Physiologically, these nine strains showed the capacity to grow in a salinity range of 1-10 % NaCl, indicating their tolerance of saline conditions. These results provide support for the description of a new genus, named Halotia gen. nov., which is related morphologically to the genera Nostoc, Mojavia and Desmonostoc. Within this new genus, three novel species were recognized and described based on morphology and internal transcribed spacer secondary structures: Halotia branconii sp. nov., Halotia longispora sp. nov. and Halotia wernerae sp. nov., under the provisions of the International Code of Nomenclature for Algae, Fungi and Plants.
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