Genomic insights into metabolism and phylogeography of the mesophilic Thermotogae genus Mesotoga

The genus Mesotoga, the only described mesophilic Thermotogae lineage, is commonly detected in low−temperature anaerobic hydrocarbon−rich environments. Besides mesophily, Mesotoga displays lineage−specific phenotypes, such as no or little H2 production, dependence on sulfur−compound reduction and ability to oxidize acetate, which may influence its ecological role. We used comparative genomics of 18 Mesotoga strains and a transcriptome of M. prima to investigate how life at moderate temperatures affects phylogeography, and to interrogate the genomic features of its lineage−specific metabolism. Phylogenetic analysis revealed three distinct Mesotoga lineages having different geographic distributions patterns and high levels of intra−lineage recombination but little gene−flow between lineages. All 16S rRNA genes from the genomes had ≥ 99% identity whereas average nucleotide identity among genomes was 90.6−98.6% within groups and 80−86% between groups. Including data from metagenomes, phylogeographic patterns reveled that geographical separation is more important for Mesotoga than their thermophilic relatives, and we suggest its distribution is constrained by their strictly anaerobic lifestyle. We propose that H2 oxidation and thiosulfate reduction are accomplished by a sulfide dehydrogenase and a hydrogenase-complex and that a pyruvate:ferredoxin oxidoreductase acquired from Clostridia is responsible for oxidizing acetate.