In this study, sediments from eight sites were collected from hydrothermal areas (e.g., the Tiancheng, Tianzuo, and Longqi hydrothermal areas) and non-hydrothermal area on the Southwest Mid-Indian Ocean Ridge. Using crude oil as the only carbon and energy source, 162 strains of culturable oil-degrading bacteria were isolated and obtained. The rate of oil degradation of the consortia was 39.48-46.00% in hydrothermal and non-hydrothermal areas. High-throughput sequencing found that the alpha diversity indices (e.g., Shannon and Simpson) of the communities in hydrothermal areas were higher than those in non-hydrothermal area. The species diversities of the oil-degrading bacteria were different among different hydrothermal areas. The composition of the oil-degrading bacterial species in the Tianzuo hydrothermal area tended to be more similar to that in the non-hydrothermal area. This similarity is attributed to the changes in the bacterial community that followed the cessation of hydrothermal vent eruptions at this site. The Alphaproteobacteria abundance of the oil-degrading bacteria was significantly different in oil-degrading bacteria between the hydrothermal and non-hydrothermal areas.

A taxonomic study was carried out on strain GC03-9T, which was isolated from deep-sea sediment of the Indian Ocean. The bacterium was Gram-stain-negative, catalase-positive, oxidase-negative, rod-shaped and gliding motile. Growth was observed at salinities of 0–9 % and at temperatures of 10–42 °C. The isolate could degrade gelatin and aesculin. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain GC03-9T belonged to the genus Gramella , with the highest sequence similarity to Gramella bathymodioli JCM 33424T (97.9 %), followed by Gramella jeungdoensis KCTC 23123T (97.2 %) and other species of the genus Gramella (93.4–96.3 %). The average nucleotide identity and the digital DNA–DNA hybridization estimate values between strain GC03-9T and G. bathymodioli JCM 33424T and G. jeungdoensis KCTC 23123T were 25.1 and 18.7 % and 82.47 and 75.69 %, respectively. The principal fatty acids were iso-C15 : 0 (28.0 %), iso-C17 : 0 3OH (13.4 %), summed feature 9 (iso-C17 : 1  ω9c and/or 10-methyl C16 : 0; 13.3 %) and summed feature 3 (C16 : 1  ω7c and/or C16 : 1  ω6c; 11.0 %). The G+C content of the chromosomal DNA was 41.17 mol%. The respiratory quinone was determined to be menaquinone-6 (100 %). Phosphatidylethanolamine, one unknown phospholipid, three unknown aminolipids and two unknown polar lipids were present. The combined genotypic and phenotypic data showed that strain GC03-9T represents a novel species within the genus Gramella , for which the name Gramella oceanisediminis sp. nov. is proposed, with the type strain GC03-9T (=MCCC M25440T=KCTC 92235T).

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