Four heterotrophic bacterial strains belonging to the globally distributed marine RCA (Roseobacter clade-affiliated) cluster (family Rhodobacteraceae, class Alphaproteobacteria) were obtained from coastal seawater samples. Strain RCA23(T) was isolated from a 10(-7) dilution culture inoculated with seawater from the German Wadden Sea (southern North Sea), reflecting the high abundance of RCA bacteria in this habitat. Strains IMCC1909, IMCC1923 and IMCC1933 were isolated from diluted seawater (10(-3)) of the Yellow Sea, South Korea. Based on 16S rRNA gene sequence comparison, Octadecabacter antarcticus 307(T) is the closest described relative of the RCA strains, with 95.4-95.5 % sequence similarity. Cells of RCA23(T), IMCC1909, IMCC1923 and IMCC1933 are small motile rods requiring sodium ions. Optimal growth of RCA23(T) occurs at 25 °C and within a very narrow pH range (pH 7-8, optimum pH 7.5). The DNA G+C base content of RCA23(T) is 53.67 mol%. The major respiratory lipoquinone is ubiquinone-10 (Q-10) and the dominant fatty acids (>1 %) are 12 : 1 3-OH, 16 : 1ω7c, 16 : 0, 18 : 1ω7c, 18 : 0 and 11-methyl 18 : 1ω7c. The polar lipid pattern indicated the presence of phosphatidylglycerol, two unidentified aminolipids and two unidentified phospholipids. On marine agar, RCA23(T) forms non-pigmented, transparent to light beige, small (<1 mm), circular, convex colonies. Strain RCA23(T) harbours all genes for the production of bacteriochlorophyll a (BChl a). Genes encoding the light-harvesting reaction centre of BChl a (pufM) were identified in all RCA strains. No visible pigmentation was observed for any of the strains under laboratory conditions, but spectrophotometric analysis revealed weak production of BChl a by RCA23(T). Morphological, physiological and genotypic features of strain RCA23(T) suggest that it represents a novel species of a new genus within the Rhodobacteraceae, for which we propose the name Planktomarina temperata gen. nov., sp. nov., described previously by Giebel et al. [ISME J 5 (2011), 8-19] as 'Candidatus Planktomarina temperata'. The type strain of Planktomarina temperata is RCA23(T) ( = DSM 22400(T) = JCM 18269(T)).