Comparative Genomics of Thalassobius Including the Description of Thalassobius activus sp. nov., and Thalassobius autumnalis sp. nov.

Abstract

A taxogenomic study was conducted to describe two new <i>Thalassobius</i> species and to analyze the internal consistency of the genus <i>Thalassobius</i> along with <i>Shimia</i> and <i>Thalassococcus</i>. Strains CECT 5113^T, CECT 5114, CECT 5118^T, and CECT 5120 were isolated from coastal Mediterranean seawater, Spain. Cells were Gram-negative, non- motile coccobacilli, aerobic chemoorganotrophs, with an optimum temperature of 26°C and salinity of 3.5-5%. Major cellular fatty acids of strains CECT 5113^T and CECT 5114 were C_{18 : 1} ω7c/ω6c and C_{10 : 0} 3OH, G+C content was 54.4-54.5 mol% and were able to utilize propionate, L-threonine, L- arginine, and L-aspartate as carbon sources. They exhibited 98.3% 16S rRNA gene sequence similarity, 75.0-75.1 ANIb and 19.5-20.9 digital DDH to type strain of their closest species, <i>Thalassobius maritimus</i>. Based on these data, strains CECT 5113^T and CECT 5114 are recognized as a new species, for which the name <i>Thalassobius activus</i> is proposed, with strain CECT 5113^T (=LMG 29900^T) as type strain. Strains CECT 5118^T and CECT 5120 were found to constitute another new species, with major cellular fatty acids C_{18 : 1} ω<i>7c</i>/ω<i>6c</i> and C_{18 : 1} ω<i>7c</i> 11-methyl and a G+C content of 59.8 mol%; they were not able to utilize propionate, L-threonine, L- arginine or L-aspartate. Their closest species was <i>Thalassobius mediterraneus</i>, with values of 99.6% 16S rRNA gene sequence similarity, 79.1% ANIb and 23.2% digital DDH compared to the type strain, CECT 5383^T. The name <i>Thalassobius autumnalis</i> is proposed for this second new species, with strain CECT 5118^T (=LMG 29904^T) as type strain. To better determine the phylogenetic relationship of the two new species, we submitted 12 genomes representing species of <i>Thalassobius, Shimia</i>, and <i>Thalassoccocus</i>, to a phylogenomic analysis based on 54 single protein-encoding genes (BCG54). The resulting phylogenomic tree did not agree with the current genera classification, as <i>Thalassobius</i> was divided in three clades, <i>Thalassobius</i> sensu stricto (<i>T. mediterraneus, T. autumnalis</i> sp. nov., and <i>T. gelatinovorus</i>), <i>Thalassobius aestuarii</i> plus the three <i>Shimia</i> spp (<i>S. marina, S. haliotis</i>, and <i>Shimia</i> sp. SK013) and finally, <i>Thalasobius maritimus</i> plus <i>T. activus</i> sp. nov. <i>Thalassococcus halodurans</i> remained apart from the two genera. Phenotypic inferences from explored genomes are presented.