In drylands worldwide, biocrusts, topsoil microbial communities, are prevalent, contributing to the biostabilization of soils and allowing the subsequent establishment and growth of vascular plants. In early successional biocrusts, cyanobacteria are the first dominant colonizers of bare ground, largely determining their functioning. However, there are large gaps in our knowledge of the cyanobacterial diversity in biocrusts, particularly in understudied geographic regions, such as the tropical latitudes. We analyzed the diversity of the cyanobacteria inhabiting the biocrusts of semideserts from Central Mexico in two localities belonging to the same desert system (Chihuahuan Desert) that are separated by a cordillera that crosses the center of Mexico. Morphological identification of the cyanobacteria was carried out after cultivation in parallel with the direct observation of the environmental samples and was supported by genetic characterization through analysis of the 16S rRNA gene of the isolated strains and by next-generation sequencing of the soil samples. Taxonomic assignment revealed a clear dominance of heterocystous cyanobacteria at one of the studied locations (Actopan, Hidalgo state). Although heterocystous forms were abundant at the other location (Atexcac, Puebla state), almost a third of the cyanobacterial phylotypes were represented by unicellular/colonial cyanobacteria, mostly Chroococcidiopsis spp. Only 28.4% of the phylotypes were found to be common to both soils. Most of the other taxa, however, were biocrust-type specific, and approximately 35% of the phylotypes were found to be unique to the soil they were collected in. In addition, differences in the abundances of the shared cyanobacteria between the locations were also found. These differences in the cyanobacterial distribution were supported by the distinct responses of the isolated strains representative of the sites to extreme heat and desiccation in bioassays. Some cyanobacteria with high abundance or only present at the hottest Actopan site, such as Scytonema hyalinum , Scytonema crispum, Nostoc commune , Nostoc sp., and Calothrix parietina , survived extreme heat and desiccation. However, Tolypothrix distorta and Chroococcidiopsis spp. were clearly sensitive to these extreme conditions in relation to their lower abundances at Actopan as opposed to Atexcac. Since novel biocrust-associated phylotypes were also found, the emergence of endemic cyanobacterial taxa is discussed.

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Twelve strains of Leptolyngbya-like filaments were isolated from biological soil crust samples from two localities in the Central Region of Mexico. The strains were morphologically distinguished from most Synechococcalean species by the obligate presence of a blackish sheath. Phylogenetic analysis based on 16S rRNA gene sequence placed all strains into a strongly supported single clade sister to Scytolyngbya. The genetic identity between our strains and all other Synechococcales, including Scytolyngbya, was less than 95%, and the strains were further distinguished by morphology and terrestrial ecology. The conserved domains of the 16S-23S ITS region had secondary structures distinct from all other closely related genera, which included Scytolyngbya, Stenomitos, Chamaethrix, and Pantanalinema. Based on the combination of morphological, molecular, and ecological evidence, we here describe two species: Chroakolemma opaca gen. et sp. nov. and C. pellucida gen. et sp. nov. Based on the ability to form blackish sheaths, these two species are morphologically similar to Leptolyngbya edaphica, Chamaethrix vaginata and Trichocoleus badius. The latter two species have been sequenced and are phylogenetically distant from Chroakolemma. Leptolyngbya edaphica is a soil species described from Russia and shares other morphological similarities with Chroakolemma, including wide sheaths, coiled filaments, pale blue-green trichomes, and constricted cross-walls. We consider these characteristics diagnostic of Chroakolemma, and accordingly propose Chroakolemma edaphica comb. nov.

Two isolates of Oculatella from biological soil crusts in the arid and semi-arid tropical upland region of México were characterized using a polyphasic approach which included morphology, ecology, 16S rRNA and 16S-23S ITS phylogenetic analysis, percent identity of 16S rRNA gene sequence, percent dissimilarity of 16S-23S ITS sequence, and secondary structure of conserved ITS domains. The two species shared the generic synapomorphy of a reddish granule at the tip of mature apical cells, and possessed a single ribosomal operon with both tRNA genes (tRNA Ile and tRNA Ala) based upon our analysis and the analysis of the other species in the genus. After characterization, the strains were compared with 10 previously described and similarly characterized species in the genus, and based upon the evidence obtained, the strains were described as two new species, O. dilatativagina and O. leona. O. dilatativagina has an extensively widened sheath, and thus is morphologically separated from all other taxa described thus far. O. leona is morphologically highly similar to soil species found in North and South America, and should be considered a cryptic species among these taxa. The recently developed criterion for species recognition using percent dissimilarity among ITS gene sequences in orthologous ribosomal operons was found to be effective in separating cryptic species of Oculatella, and has consistently been useful and sufficient for separation of closely related species in other cyanobacterial genera.

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Only two complete genomes of the cyanobacterial genus Gloeobacter from two very different regions of the world currently exist. Here we present the complete genome sequence of a third member of the genus isolated from a waterfall cave in Mexico. Analysis of the average nucleotide identities (ANI) between published Gloeobacter genomes revealed that the complete genome of this new member is only 92.7% similar to Gloeobacter violaceus and therefore we determined it to be a new species. We propose to name this new species Gloeobacter morelensis after the location in Mexico where it was isolated. The complete genome consists of one circular chromosome (4,921,229 bp), one linear plasmid (172,328 bp), and one circular plasmid (8,839 bp). Its genome is the largest of all completely sequenced genomes of Gloeobacter species. Pangenomic comparisons revealed that G. morelensis encodes 759 genes not shared with other Gloeobacter species. Despite being more closely related to G. violaceus, it features an extremely divergent psbA gene encoding an atypical D1 core subunit of Photosystem II previously only found within the genome of G. kilaueensis. In addition, we detected evidence of concerted evolution of psbA genes encoding identical D1 in all three Gloeobacter genomes, a characteristic that seems widespread in cyanobacteria and may therefore be traced back to their last common ancestor.

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