Abstract
A taxogenomic study was conducted to describe two new Thalassobius species and to analyze the internal consistency of the genus Thalassobius along with Shimia and Thalassococcus. Strains CECT 5113T, CECT 5114, CECT 5118T, 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 5113T and CECT 5114 were C18 : 1 ω7c/ω6c and C10 : 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, Thalassobius maritimus. Based on these data, strains CECT 5113T and CECT 5114 are recognized as a new species, for which the name Thalassobius activus is proposed, with strain CECT 5113T (=LMG 29900T) as type strain. Strains CECT 5118T and CECT 5120 were found to constitute another new species, with major cellular fatty acids C18 : 1 ω7c/ω6c and C18 : 1 ω7c 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 Thalassobius mediterraneus, with values of 99.6% 16S rRNA gene sequence similarity, 79.1% ANIb and 23.2% digital DDH compared to the type strain, CECT 5383T. The name Thalassobius autumnalis is proposed for this second new species, with strain CECT 5118T (=LMG 29904T) as type strain. To better determine the phylogenetic relationship of the two new species, we submitted 12 genomes representing species of Thalassobius, Shimia, and Thalassoccocus, 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 Thalassobius was divided in three clades, Thalassobius sensu stricto (T. mediterraneus, T. autumnalis sp. nov., and T. gelatinovorus), Thalassobius aestuarii plus the three Shimia spp (S. marina, S. haliotis, and Shimia sp. SK013) and finally, Thalasobius maritimus plus T. activus sp. nov. Thalassococcus halodurans remained apart from the two genera. Phenotypic inferences from explored genomes are presented.
Highlights
The genus Thalassobius was established to accommodate the species Thalassobius mediterraneus and the reclassified Thalassobius gelatinovorus by Arahal et al (2005)
Strains CECT 5113T, CECT 5114, CECT 5118T, and CECT 5120 were affiliated to the genus Thalassobius based on partial 16S rRNA gene sequence comparison performed among routine identification and authentication procedures at CECT
The similarities of their respective 16S rRNA gene sequences to T. mediterraneus CECT 5383T were low for strains CECT 5113T and CECT 5114, which showed a closer position to T. maritimus (98.3%)
Summary
The genus Thalassobius was established to accommodate the species Thalassobius mediterraneus and the reclassified Thalassobius gelatinovorus (formerly Ruegeria gelatinovorans) by Arahal et al (2005). Since the description of the first pair of species, five more have been added: Thalassobius aestuarii (Yi and Chun, 2006), Thalassobius maritimus (Park et al, 2012), Thalassobius aquaeponti (Park et al, 2014), Thalassobius abyssi (Nogi et al, 2016), and Thalassobius litorarius (Park et al, 2016). All species so far characterized are aerobic chemoorganotrophic marine bacteria able to accumulate polyhydroxybutyrate (PHB). They have been isolated from marine environments, surface coastal seawater and tidal flat samples, but one species (T. abyssi) was isolated from deep seawater (around 1,000 m depth). None of the species produces BChl a or synthetizes carotenoid pigments
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