Polyphasic approach revealed complex bacterial community structure and function in deep sea sediment of ultra-slow spreading Southwest Indian Ridge

The Southwest Indian Ridge (SWIR) is one of the least explored mid-ocean ridges and contains a high frequency of hydrothermal vents. We examined the bacterial community structure and metabolic potential of surface sediments collected from a sea mount at 2681 m depth and a ridge-flank at 3776 m in SWIR. High throughput 16S rRNA amplicon sequencing combined with Community Level Physiological Profiling (CLPP) revealed that hydrothermal sediments hosted a diverse microbial community with significant difference in structure and function between seamount and ridge-flank. The ridge-flank community was primarily comprised of facultative chemoautotrophic members of Burkholderiales (43.0%) while copiotrophic members of Alteromonadales (56.9%) and Oceanospirillales (14.6%) were dominant in seamount sample. These results coincided with CLPP of microbial community with higher utilization of carbohydrates such as D-cellobiose, β-methyl-D-glucoside, glucose 1-phosophate and pyruvic acid methyl ester observed in seamount. Culture-dependent approach using array of nutrient media enabled the isolation of heterotrophic genera that exhibited ability to degrade refractory organic matter such as phenanthrene, anthracene, pyrene, fluorene and fluoranthene. These results indicate that bacterial community present in the deep-sea hydrothermal sediment can act as bioreactor for mineralization of refractory organic matter. Thus our study emphasize on the significance of polyphasic approach in comprehensive and rapid assessment of physiological and taxonomic characteristics of the deep seabed bacterial community and further suggested that the bacterial community can act as ecological indicators of the metabolic sources prevalent in the ridge environment.