Ruegeria persistens sp. nov., a persistent symbiotic bacterium isolated from coral

A Gram-stain-negative, aerobic, moderately halophilic, non-motile, rod-shaped bacterium, designated MC10-B4T, was isolated from the coral Oulastrea crispata collected from Hong Kong waters. The strain showed the ability to persist in a coral holobiont during an eight-month monitoring period in a field trial, indicating a long-term association with the coral host. Growth occurred at 10-37 {degrees}C (optimum, 28-30 {degrees}C), at NaCl concentrations up to 4.0% (w/v; optimum, 0.5-2.0%), and at pH 7.0-10.0 (optimum, pH 7.0-8.0) under standard laboratory conditions. Colonies on Marine Agar 2216 were round, smooth, convex, white, opaque, and had entire margins. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain MC10-B4T belongs to the genus Ruegeria and shares the highest sequence similarity with Ruegeria conchae TW15T (99.64%), followed by Ruegeria profundi ZGT108T (98.92%), Ruegeria lacuscaerulensis ITI-1157T (98.46%), Ruegeria denitrificans CECT 5091T (98.31%), Ruegeria arenilitoris G-M8T (97.69%), Ruegeria atlantica NBRC 15792T (97.55%), Ruegeria halocynthiae MA1-6 (97.11%), Ruegeria atlantica IAM 14463T (96.98%), Ruegeria meonggei MA-E2-3 (96.83%), and Ruegeria pomeroyi DSS-3T (96.65%). Genome sequencing revealed a genome size of 4.55 Mbp and a DNA G+C content of 57.5 mol%. The average nucleotide identity and digital DNA-DNA hybridization values between strain MC10-B4T and Ruegeria conchae TW15T were 77.6% and 18.2-22.8%, respectively. These values are below the recommended thresholds for species delineation (95% and 70%). Strain MC10-B4T shares the same predominant fatty acid, summed feature 8 (consisting of C18:1 {omega}7c and/or C18:1 {omega}6c), with other Ruegeria strains; however, it also contains C19:0 cyclo {omega}8c, which was not detected in Ruegeria conchae TW15T and Ruegeria pomeroyi DSS-3T. The type strain is MC10-B4T (= GDMCC 1.6889T = KCTC 18785T).