Abstract
Thermal and cave habitats on nearly all continents have been a substantial source of new cyanobacterial genotypes and morphotypes that expanded with the dawn of the era of molecular phylogenetics. In this study, we investigated the cyanobacterial flora of an extreme habitat of recently discovered caves with sulfur-rich thermal springs, using the polyphasic approach. The methods included cultivation, light and transmission electron microscopy, and molecular methods, including those that can be employed on samples that are not unialgal. Here, we present data on morphological and ultrastructural characteristics, 16S rRNA gene and 16S-23S internal transcribed spacer (ITS) rRNA region sequences, and folding structures. We identified one new trichal genus Xomosiella with the type species X. audyi forming a distinctly isolated clade and three new species in Loriellopsis, Mastigocladus, and Pegethrix. Apart from genetic distance, Xomosiella is distinguished from Limnothrix by its high trichome motility and benthic habitat, with granules likely composed of cyanophycin rather than aerotopes. The coccal cyanobacterium initially identified as "Cyanosarcina" sp. has been proposed as a new species, Loriellopsis vromonerensis, although its classification is complicated by morphological plasticity and phylogenetic uncertainties. The erection of Mastigocladus boudae was supported by a significant genetic divergence and distinct morphological characteristics. A description of a newly revealed cryptic species, Pegethrix sulphurea, has been provided. These results advance our knowledge of the diversity of cyanobacteria in extreme and understudied environments, which could enrich our understanding of microbial adaptability.