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-B4 ^T , 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 °C (optimum, 28-30 °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-B4 ^T belongs to the genus Ruegeria and shares the highest sequence similarity with Ruegeria conchae TW15 ^T (99.64%), followed by Ruegeria profundi ZGT108 ^T (98.92%), Ruegeria lacuscaerulensis ITI-1157 ^T (98.46%), Ruegeria denitrificans CECT 5091 ^T (98.31%), Ruegeria arenilitoris G-M8 ^T (97.69%), Ruegeria atlantica NBRC 15792 ^T (97.55%), Ruegeria halocynthiae MA1-6 (97.11%), Ruegeria atlantica IAM 14463 ^T (96.98%), Ruegeria meonggei MA-E2-3 (96.83%), and Ruegeria pomeroyi DSS-3 ^T (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-B4 ^T and Ruegeria conchae TW15 ^T were 77.6% and 18.2-22.8%, respectively. These values are below the recommended thresholds for species delineation (95% and 70%). Strain MC10-B4 ^T shares the same predominant fatty acid, summed feature 8 (consisting of C _18:1 ω 7 c and/or C _18:1 ω 6 c ), with other Ruegeria strains; however, it also contains C _19:0 cyclo ω 8 c , which was not detected in Ruegeria conchae TW15 ^T and Ruegeria pomeroyi DSS-3 ^T . The type strain is MC10-B4 ^T (= GDMCC 1.6889 ^T = KCTC 18785 ^T ).