A Gram-negative, aerobic, non-motile bacterium, designated strain KC90B(T), was isolated from the surface of a cell of the marine diatom Thalassiosira delicatula. The bacterial cells were pleomorphic and formed very small, beige colonies on marine agar. Optimal growth was obtained at 25 degrees C, at pH 6.5-7.5 and in the presence of 1.5-2.0% (w/v) NaCl. Phylogenetic analyses based on its 16S rRNA gene sequence revealed that strain KC90B(T) belonged to the Roseobacter clade and formed a monophyletic cluster with the sequences of Boseongicola aestuarii, Profundibacterium mesophilum, Hwanghaeicola aestuarii, Maribius pelagius and M. salinus, showing 91.4-95.7% sequence similarities. Ubiquinone Q-10 was the predominant lipoquinone but a significant amount of ubiquinone Q-9 was also detected. The major cellular fatty acids were C-18:1 omega 7c, 11-methyl C-18:(1)omega 7c and C-18:0. Strain KC90B(T) also contained specific fatty acids (C-17:0, anteiso-C-15:0 and anteiso-C-17:0) that were not detected in its closest described relatives. The major polar lipids of strain KC90B(T) comprised phosphatidylglycerol, phosphatidylcholine, diphosphatidylglycerol and an unidentified aminolipid. The DNA G+C content of strain KC90B(T) was 65.2 mol%. The phylogenetic analysis of strain KC90B(T), together with the differential phenotypic and chemotaxonomic properties demonstrate that strain KC90B(T) is distinct from type strains of B. aestuarii, P. mesophilum, H. aestuarii, M. pelagius and M. salinus. Based on the data presented in this study, strain KC90B(T) represents a novel genus and species within the family Rhodobacteraceae, for which the name Silicimonas algicola gen. nov., sp. nov. is proposed. The type strain is KC90B(T) (=DSM 103371(T)=RCC 4681(T)).