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A Gram-stain-negative, tyrosine-metabolizing, non-motile, strictly aerobic, non-spore-forming, rod-shaped marine bacterium, designated strain CC-PW-75T, was isolated from the estuarine water off Pintung, Taiwan. Strain CC-PW-75T formed a distinct phyletic lineage associated with Gemmobacter species, sharing the highest 16S rRNA gene sequence similarity with G. megaterium CF17T and G. straminiformis CAM-8T (96.0% each) followed by G. aquatilis IFAM 1031T and G. nectariphilus AST4T (95.8% each). Analysis of the draft genome (3.76 Mbp) revealed the presence of genes encoding light-harvesting complexes, photosynthetic reaction centers and proteins involved in the metabolism of CO, CO2, HCO3‒ and H2S. However, bacteriochlorophyll a was not detected. Average nucleotide identity values between the genome sequence of CC-PW-75T and the related Gemmobacter species (n = 6) were estimated to be 72.8–76.3%. Polar lipid analysis revealed the presence of phosphatidylglycerol, phosphatidylethanolamine and an unidentified lipid in major amounts, and phosphatidylmonomethylethanolamine, phosphatidylcholine, an unidentified aminolipid and an unidentified lipid in minor amounts. C18:1ω7c and/or C18:1ω6c, C18:0 and C18:1ω7c 11-methyl were identified to be major fatty acids. The DNA G + C content was 66.2 mol% (draft genome sequence). Ubiquinone-10 (Q-10) was the sole respiratory quinone. Based on the polyphasic taxonomic evidence, CC-PW-75T is most likely a novel species of the genus Gemmobacter, affiliated to the family Rhodobacteraceae, for which the name Gemmobacter aestuarii sp. nov. is proposed. The type strain is CC-PW-75T (= JCM 19754T = BCRC 80759T). Also, we propose the reclassification of Cereibacter changlensis as Gemmobacter changlensis Chen et al. 2013 using the polyphasic data presented in this study.