Hadal biosphere: insight into the microbial ecosystem in the deepest ocean on Earth

Abstract

Hadal oceans at water depths below 6,000 m are the least-explored aquatic biosphere. The Challenger Deep, located in the western equatorial Pacific, with a water depth of ∼11 km, is the deepest ocean on Earth. Microbial communities associated with waters from the sea surface to the trench bottom (0∼10,257 m) in the Challenger Deep were analyzed, and unprecedented trench microbial communities were identified in the hadal waters (6,000∼10,257 m) that were distinct from the abyssal microbial communities. The potentially chemolithotrophic populations were less abundant in the hadal water than those in the upper abyssal waters. The emerging members of chemolithotrophic nitrifiers in the hadal water that likely adapt to the higher flux of electron donors were also different from those in the abyssal waters that adapt to the lower flux of electron donors. Species-level niche separation in most of the dominant taxa was also found between the hadal and abyssal microbial communities. Considering the geomorphology and the isolated hydrotopographical nature of the Mariana Trench, we hypothesized that the distinct hadal microbial ecosystem was driven by the endogenous recycling of organic matter in the hadal waters associated with the trench geomorphology.

Keywords: Challenger Deep; hadal; niche separation; nitrification; trench.

Fig. 1.

Temperature and salinity (A); oxygen, nitrate, nitrite, and phosphate concentrations and pH (B); the δ¹⁵N and δ¹⁸O of nitrate (C); and the abundance of prokaryotic cells and VLPs and VPR (D) along the water column in the Challenger Deep. The CTD profile of temperature and salinity was obtained in dive AB#11. Other profiles were obtained by a total of three dives at the same location.

Fig. 2.

Prokaryotic SSU rRNA gene community composition along the water column in the Challenger Deep. Numbers in parentheses indicate the number of tag sequences.

Fig. 3.

Similarity matrixes of Jaccard (A), Bray–Curtis (B), and UniFrac (C) of the SSU rRNA gene-tag sequences.

Fig. 4.

The abundance of whole prokaryotic, archaeal and nitrite oxidizer SSU rRNA and amoA genes (A), and the abundance of subgroups of amoA genes and SSU rRNA genes of nitrite-oxidizing bacteria (B) along the water column in the Challenger Deep obtained by quantitative PCR. Grouping of amoA are shown in Fig. S5.