Diversity of culturable aquatic bacteria at different depths of the Mariana Trench

Abstract:

[Objective] The Mariana Trench is the deepest site on earth with diverse extreme conditions (e.g., ultra-high hydrostatic pressure, low temperature and lack of light). It contains abundant microbial resources. The aim of this study is to explore aquatic microbial morphological characteristics and to mine culturable bacterial resources from different depths of the Mariana Trench. [Methods] Seawaters of seven depths (2-8727 m) from the Mariana Trench were collected. Aquatic microbial morphological characteristics were observed by atomic force microscopy and scanning electron microscopy. Combined with enrichment culture by using tangential flow system and high pressure device, two kinds of conventional media (1/5×2216E and 1/30×2216E) and six kinds of selective media (made up of combinations of organic carbon and nitrogen) were used for isolation and cultivation of aquatic bacteria. [Results] Various aquatic bacteria with different sizes (130 nm-1.5 μm) were found at different depths, dominated by spherical or rod-shaped bacteria. Particle-associated bacteria were common in surface seawaters and free-living bacteria were common in deep seawaters. Belonging to 3 phyla, 31 genera and 56 species, a total of 365 strains were identified. Gammaproteobacteria dominated (accounting for 62.7% of the total isolates) and its relative abundance in deep waters was higher than that in shallow waters. Alteromonas (21.8%) and Sulfitobacter (19.1%) were the most abundant genera, dominated in shallow water samples. The diluted 2216E and amino acid medium were found to have a better selectivity to Sulfitobacter. The selectivity of glucose-mannose medium (GM) or taurine-glycolic acid medium (TG) to rare bacteria was better. Seven strains (five species) were potential novel bacteria. In addition, 70 0.22-μm-passable strains (22 genera) and 33 piezotolerant strains (8 genera) were isolated, respectively. [Conclusion] There were rich diversities of different trophic, 0.22-μm-passable and piezotolerant bacteria and their morphologies at different water depths of the Mariana Trench. This study provided unique resources of aquatic microorganisms to investigate their trophic types, high-pressure adaptation mechanisms and biogeochemical roles in the Mariana Trench.