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A slow-growing, strictly aerobic, Gram-negative, coccus bacterial strain, designated KAUST100406-0324T, was isolated from sea-floor sediment collected from the Red Sea, Saudi Arabia. The catalase- and oxidase-positive strain was non-sporulating and only slightly halophilic. Optimum growth occurred at 20–25 °C and at pH values ranging from 7.0 to 8.0. The major cellular fatty acids of the strain were unsaturated C18 : 1ω6c and/or C18 : 1ω7c, C18 : 1ω7c 11-methyl and C16 : 1ω7c and/or C16 : 1ω6c. The major polar lipids were phosphatidylglycerol, phosphatidylethanolamine and two unidentified phospholipids. Ubiquinone 10 was the predominant lipoquinone. The DNA G+C content of strain KAUST100406-0324T was 64.0 mol%. Phylogenetic analysis of 16S rRNA gene sequences revealed that the novel strain belonged to the family Rhodobacteraceae of the class Alphaproteobacteria but formed a distinct evolutionary lineage from other bacterial species with validly published names. The 16S rRNA gene sequence of the novel strain was distantly related, but formed a monophyletic cluster with, those of bacteria from two moderately halophilic genera, Hwanghaeicola and Maribius . The similarity of the sequence between the novel strain KAUST100406-0324T and the type strains Hwanghaeicola aestuarii Y26T (accession number FJ230842), Maribius pelagius B5-6T (DQ514326) and Maribius salinus CL-SP27T (AY906863) were 94.5 %, 95.2 % and 95.3 %, respectively. Based on the physiological, phylogenetic and chemotaxonomic characteristics presented in this study, we propose that this strain represents a novel species of a new genus in the family Rhodobacteraceae , for which the name of Profundibacterium mesophilum gen. nov., sp. nov. was proposed, with KAUST100406-0324T ( = JCM 17872T = NRRL B-59665T) as the type strain.