Microbiomes in the Challenger Deep slope and bottom-axis sediments.

Hadal trenches are the deepest and most remote regions of the ocean. The 11-kilometer deep Challenger Deep is the least explored due to the technical challenges of sampling hadal depths. It receives organic matter and heavy metals from the overlying water column that accumulate differently across its V-shaped topography. Here, we collected sediments across the slope and bottom-axis of the Challenger Deep that enable insights into its in situ microbial communities. Analyses of 586 metagenome-assembled genomes retrieved from 37 metagenomes show distinct diversity and metabolic capacities between bottom-axis and slope sites. 26% of prokaryotic 16S rDNA reads in metagenomes were novel, with novelty increasing with water and sediment depths. These predominantly heterotrophic microbes can recycle macromolecules and utilize simple and complex hydrocarbons as carbon sources. Metagenome and metatranscriptome data support reduction and biotransformation of arsenate for energy gain in sediments that present a two-fold greater accumulation of arsenic compared to non-hadal sites. Complete pathways for anaerobic ammonia oxidation are predominantly identified in genomes recovered from bottom-axis sediments compared to slope sites. Our results expand knowledge of microbially-mediated elemental cycling in hadal sediments, and reveal differences in distribution of processes involved in nitrogen loss across the trench.

In situ meta-omic insights into the community compositions and ecological roles of hadal microbes in the Mariana Trench.

The low temperature and elevated hydrostatic pressure in hadal trenches at water depths below 6000 m render sample collection difficult. Here, in situ hadal water microbial samples were collected from the Mariana Trench and analysed. The hadal microbial communities at different depths were revealed to be consistent and were dominated by heterotrophic Marinimicrobia. Thirty high-quality metagenome-assembled genomes (MAGs) were retrieved to represent the major hadal microbes affiliated with 12 prokaryotic phyla. Most of the MAGs were newly reported and probably derived from novel hadal inhabitants as exemplified by a potentially new candidate archaeal phylum in the DPANN superphylum. Metabolic reconstruction indicated that a great number of the MAGs participated in nitrogen and sulfur cycling, in which the nitrification process was driven sequentially by Thaumarchaeota and Nitrospirae and sulfur oxidization by Rhodospirillales in the Alphaproteobacteria class. Moreover, several groups of hadal microbes were revealed to be potential carbon monoxide oxidizers. Metatranscriptomic result highlighted the contribution of Chloroflexi in degrading recalcitrant dissolved organic matter and Marinimicrobia in extracellular protein decomposition. The present work provides an in-depth view on the hadal microbial communities regarding their endemism and element cycles.

Periodic and Spatial Spreading of Alkanes and Alcanivorax Bacteria in Deep Waters of the Mariana Trench.

In subduction zones, serpentinization and biological processes may release alkanes to the deep waters, which would probably result in the rapid spread of Alcanivorax However, the timing and area of the alkane distribution and associated enrichment of alkane-degrading microbes in the dark world of the deep ocean have not been explored. In this study, we report the richness (up to 17.8%) of alkane-degrading bacteria, represented by Alcanivorax jadensis, in deep water samples obtained at 3,000 to 6,000 m in the Mariana Trench in two cruises. The relative abundance of A. jadensis correlated with copy numbers of functional almA and alkB genes, which are involved in alkane degradation. In these water samples, we detected a high flux of alkanes, which probably resulted in the prevalence of A. jadensis in the deep waters. Contigs of A. jadensis were binned from the metagenomes for examination of alkane degradation pathways and deep sea-specific pathways, which revealed a lack of nitrate and nitrite dissimilatory reduction in our A. jadensis strains. Comparing the results for the two cruises conducted close to each other, we suggest periodic release of alkanes that may spread widely but periodically in the trench. Distribution of alkane-degrading bacteria in the world's oceans suggests the periodic and remarkable contributions of Alcanivorax to the deep sea organic carbon and nitrogen sources.IMPORTANCE In the oligotrophic environment of the Mariana Trench, alkanes as carbohydrates are important for the ecosystem, but their spatial and periodic spreading in deep waters has never been reported. Alkane-degrading bacteria such as Alcanivorax spp. are biological signals of the alkane distribution. In the present study, Alcanivorax was abundant in some waters, at depths of up to 6,000 m, in the Mariana Trench. Genomic, transcriptomic, and chemical analyses provide evidence for the presence and activities of Alcanivorax jadensis in deep sea zones. The periodic spreading of alkanes, probably from the subductive plates, might have fundamentally modified the local microbial communities, as well as perhaps the deep sea microenvironment.

Genomics insights into ecotype formation of ammonia-oxidizing archaea in the deep ocean.

Various lineages of ammonia-oxidizing archaea (AOA) are present in deep waters, but the mechanisms that determine ecotype formation are obscure. We studied 18 high-quality genomes of the marine group I AOA lineages (alpha, gamma and delta) from the Mariana and Ogasawara trenches. The genomes of alpha AOA resembled each other, while those of gamma and delta lineages were more divergent and had even undergone insertion of some phage genes. The instability of the gamma and delta AOA genomes could be partially due to the loss of DNA polymerase B (polB) and methyladenine DNA glycosylase (tag) genes responsible for the repair of point mutations. The alpha AOA genomes harbour genes encoding a thrombospondin-like outer membrane structure that probably serves as a barrier to gene flow. Moreover, the gamma and alpha AOA lineages rely on vitamin B12 -independent MetE and B12 -dependent MetH, respectively, for methionine synthesis. The delta AOA genome contains genes involved in uptake of sugar and peptide perhaps for heterotrophic lifestyle. Our study provides insights into co-occurrence of cladogenesis and anagenesis in the formation of AOA ecotypes that perform differently in nitrogen and carbon cycling in dark oceans.