We compared the community structures of cyanobacteria in four biological desert crusts from Utah's Colorado Plateau developing on different substrata. We analyzed natural samples, cultures, and cyanobacterial filaments or colonies retrieved by micromanipulation from field samples using microscopy, denaturing gradient gel electrophoresis, and sequencing of 16S rRNA genes. While microscopic analyses apparently underestimated the biodiversity of thin filamentous cyanobacteria, molecular analyses failed to retrieve signals for otherwise conspicuous heterocystous cyanobacteria with thick sheaths. The diversity found in desert crusts was underrepresented in currently available nucleotide sequence databases, and several novel phylogenetic clusters could be identified. Morphotypes fitting the description of Microcoleus vaginatus Gomont, dominant in most samples, corresponded to a tight phylogenetic cluster of probable cosmopolitan distribution, which was well differentiated from other cyanobacteria traditionally classified within the same genus. A new, diverse phylogenetic cluster, named “Xeronema,” grouped a series of thin filamentous Phormidium -like cyanobacteria. These were also ubiquitous in our samples and probably correspond to various botanical Phormidium and Schizothrix spp., but they are phylogenetically distant from thin filamentous cyanobacteria from other environments. Significant differences in community structure were found among soil types, indicating that soil characteristics may select for specific cyanobacteria. Gypsum crusts were most deviant from the rest, while sandy, silt, and shale crusts were relatively more similar among themselves.

Photosynthetic microbial mats are complex, stratified ecosystems in which high rates of primary production create a demand for nitrogen, met partially by N₂ fixation. Dinitrogenase reductase (nifH) genes and transcripts from Cyanobacteria and heterotrophic bacteria (for example, Deltaproteobacteria) were detected in these mats, yet their contribution to N₂ fixation is poorly understood. We used a combined approach of manipulation experiments with inhibitors, nifH sequencing and single-cell isotope analysis to investigate the active diazotrophic community in intertidal microbial mats at Laguna Ojo de Liebre near Guerrero Negro, Mexico. Acetylene reduction assays with specific metabolic inhibitors suggested that both sulfate reducers and members of the Cyanobacteria contributed to N₂ fixation, whereas ¹⁵N₂ tracer experiments at the bulk level only supported a contribution of Cyanobacteria. Cyanobacterial and nifH Cluster III (including deltaproteobacterial sulfate reducers) sequences dominated the nifH gene pool, whereas the nifH transcript pool was dominated by sequences related to Lyngbya spp. Single-cell isotope analysis of ¹⁵N₂-incubated mat samples via high-resolution secondary ion mass spectrometry (NanoSIMS) revealed that Cyanobacteria were enriched in ¹⁵N, with the highest enrichment being detected in Lyngbya spp. filaments (on average 4.4 at% ¹⁵N), whereas the Deltaproteobacteria (identified by CARD-FISH) were not significantly enriched. We investigated the potential dilution effect from CARD-FISH on the isotopic composition and concluded that the dilution bias was not substantial enough to influence our conclusions. Our combined data provide evidence that members of the Cyanobacteria, especially Lyngbya spp., actively contributed to N₂ fixation in the intertidal mats, whereas support for significant N₂ fixation activity of the targeted deltaproteobacterial sulfate reducers could not be found.

Desulfovibrio vulgarissubsp.oxamicus(type strain, DSM 1925^T) was found to use nitrate as a terminal electron acceptor, the latter being reduced to ammonium. Phylogenetic studies indicated that strain DSM 1925^Twas distantly related to the type strain ofDesulfovibrio vulgaris(95.4 % similarity of the small-subunit rRNA gene) and had as its closest phylogenetic relatives two other nitrate- and sulfate-reducing bacteria, namelyDesulfovibrio termitidis(99.4 % similarity) andDesulfovibrio longreachensis(98.4 % similarity). Additional experiments were conducted to characterize better strain DSM 1925^T. This strain incompletely oxidized lactate and ethanol to acetate. It also oxidized butanol, pyruvate and citrate, but not glucose, fructose, acetate, propionate, butyrate, methanol, glycerol or peptone. The optimum temperature for growth was 37 °C (range 16–50 °C) and the optimum NaCl concentration for growth was 0.1 % (range 0–5 %). Because of significant genotypic and phenotypic differences fromDesulfovibrio termitidisandDesulfovibrio longreachensis, reclassification ofDesulfovibrio vulgarissubsp.oxamicusasDesulfovibrio oxamicussp. nov., comb. nov., is proposed. The type strain is strain Monticello 2^T(=DSM 1925^T=NCIMB 9442^T=ATCC 33405^T).