Early Divergence and Gene Exchange Highways in the Evolutionary History of Mesoaciditogales.

The placement of a nonhyperthermophilic order Mesoaciditogales as the earliest branching clade within the Thermotogota phylum challenges the prevailing hypothesis that the last common ancestor of Thermotogota was a hyperthermophile. Yet, given the long branch leading to the only two Mesoaciditogales described to date, the phylogenetic position of the order may be due to the long branch attraction artifact. By testing various models and applying data recoding in phylogenetic reconstructions, we observed that early branching of Mesoaciditogales within Thermotogota is strongly supported by the conserved marker genes assumed to be vertically inherited. However, based on the taxonomic content of 1,181 gene families and a phylogenetic analysis of 721 gene family trees, we also found that a substantial number of Mesoaciditogales genes are more closely related to species from the order Petrotogales. These genes contribute to coenzyme transport and metabolism, fatty acid biosynthesis, genes known to respond to heat and cold stressors, and include many genes of unknown functions. The Petrotogales comprise moderately thermophilic and mesophilic species with similar temperature tolerances to that of Mesoaciditogales. Our findings hint at extensive horizontal gene transfer (HGT) between, or parallel independent gene gains by, the two ecologically similar lineages and suggest that the exchanged genes may be important for adaptation to comparable temperature niches.

Marinitoga lauensis sp. nov., a novel deep-sea hydrothermal vent thermophilic anaerobic heterotroph with a prophage.

A novel moderately thermophilic, heterotrophic anaerobe, designated strain LG1T, was isolated from the Mariner deep-sea hydrothermal vent field along the Eastern Lau Spreading Center and Valu Fa Ridge. Cells of strain LG1T were motile rods, occurring singly or in pairs, 0.6µm in width and 1.2µm in length. The strain LG1T grew between 40 and 70°C (optimum 50-55°C), at a pH between 5 and 8 (optimum pH 6.5) and with 7.5-50gL-1 NaCl (optimum 30gL-1). Sulfur, cystine and thiosulfate were reduced to sulfide, and cell yield was improved in the presence of cystine. Strain LG1T was an organotroph able to use a variety of organic compounds. Phylogenetic analysis based on 16S rRNA gene sequence comparisons indicated that strain LG1T was affiliated to the genus Marinitoga within the order Petrotogales. It shared 95.34-96.31% 16S rRNA gene sequence similarity with strains of other Marinitoga species, and is most closely related to Marinitoga okinawensis. Genome analysis revealed the presence of a prophage sharing high sequence homology with the viruses MPV1, MCV1 and MCV2 hosted by Marinitoga strains. Based on the data from the phylogenetic analyses and the physiological properties of the novel isolate, we propose that strain LG1T is a representative of a novel species, for which the name Marinitoga lauensis sp. nov. is proposed; the type strain is LG1T (=DSM 106824=JCM 32613).