Diversity of culturable heterotrophic bacteria from the Mariana Trench and their ability to degrade macromolecules

The Mariana Trench is the deepest location on earth and harbors unique microbial communities as evidenced by 16S rRNA gene amplicon and metagenomic sequencing. Obtaining culturable microorganisms from the Mariana Trench will contribute to a further understanding of hadal biogeochemical processes and act as a unique microbial reservoir with potential applications. Here, 825 bacterial strains, identifed by 16S rRNA gene sequencing, were isolated from 12 water depths (0–10, 400 m) of the Mariana Trench with 2216E and R2A media at 4 ℃ or 28 ℃ on four cruises during 2015–2017. These bacteria belong to four phyla, nine classes, 27 orders, 45 families and 108 genera. Alphaproteobacteria, Gammaproteobacteria, Actinobacteria_c, Bacilli and Flavobacteriia were the most abundant classes, accounting for 37.9%, 33.0%, 11.8%, 8.6% and 8.0% of the total bacterial isolates, respectively. 2216E and R2A media were found to have a better selectivity to Bacilli and Flavobacteriia, respectively. Fifty strains were potential novel bacterial species with a 16S rRNA gene similarity & 98.65%, and a higher percentage of novel strains were obtained from R2A than 2216E medium. Additionally, 301 (150 species) out of 354 strains (178 species) selected from each depth could degrade at least one of the ten kinds of macromolecules tested. These results indicate that there is a high diversity of culturable bacteria in the Mariana Trench and they can produce a variety of extracellular enzymes. Our study provides a valuable resource of microorganisms for investigating their biogeochemical roles in the Mariana Trench and for industrial applications.