Hadal Sediments Research Articles

Spatial variations of microbial communities in abyssal and hadal sediments across the Challenger Deep.

Microbial communities in hadal sediments are least explored in hadal zone (>6,000 m), especially in the Challenger Deep with high pressure (∼110 M pa at the bottom). In this study, we investigated the microbial communities in the sediments of the slope and trench-axis bottom of the Challenger Deep in the Mariana Trench. Classification of the reads of the 16S rRNA gene amplicons showed vertical distribution of prokaryotic microbial inhabitants from the surface to up to 60 centimeter below surface floor (cmbsf). The most dominant phyla were Proteobacteria, Chloroflexi, Actinobacteria, Planctomycetes and candidate phyla Patescibacteria and Marinimicrobia. Distinct dominant groups in the microbial communities were observed in trench-axis sediment (water depth >8,600 m), compared to the slopes of the Challenger Deep. A sampling site at the northern slope was enriched with archaea from mesophilic Euryarchaeota Marine Group II (MGII) as a biomarker of specific geochemical setting. Among archaeal community, Thaumarchaeota represented by Nitrosopumilus were dominant in the upper layers and diminished drastically in the deeper layers. “Ca. Woesearchaeota”, however, became the dominant group in the deeper layers. Overall, our study provides a better understanding on the pattern of the microbial communities in the deepest hadal sediments on Earth, and highlights the extraordinary diversity still waiting to be discovered.

Organic matter diagenesis in hadal setting: Insights from the pore-water geochemistry of the Mariana Trench sediments

The diagenetic processes of organic matter (OM) in hadal sedimentary environments (>6000 m water depth) are much less studied than those in other accessible environments. In this study, we quantify OM oxidation processes using pore-water geochemical measurements, fluxes, and diagenetic rate calculations in sediments along a depth transect from abyssal to hadal sites (5500–10257 m) in the Mariana Trench. The total benthic O2 consumption, and depth-integrated rates of nitrification and denitrification are positively correlated with water depth, indicating that OM diagenesis is enhanced in the deep sites of the hadal zone. The negative linear correlation between water depth and oxygen/nitrate penetration depths further supports this conclusion. In the abyssal sites, aerobic respiration dominates OM degradation, and anaerobic processes are negligible. In contrast, denitrification plays an important role in anaerobic OM degradation, and accounts for approximately 5% of the depth-integrated total OM mineralization at the deepest hadal site. Moreover, our results suggest that direct coupling between nitrification and denitrification is significant, and may define the turnover of N in hadal sediments. Although they are not without uncertainties, our results shed new light on the importance of denitrification in sedimentary diagenesis in the deepest part of the Earth's ocean, and could have important implications for understanding the current state of biogeochemical cycles in the hadal zone.

Fungal diversity in the deep-sea hadal sediments of the Yap Trench by cultivation and high throughput sequencing methods based on ITS rRNA gene

The hadal zone is among the least studied habitats on the planet, and the composition, distribution and variations of the biological communities in the hadal zone are poorly understood. Recent explorations of the ocean trenches have revealed distinctive metabolic and functional microbial communities in the hadal biosphere. Compared to bacteria and archaea, fungal community in hadal trenches is poorly documented. In this study, we present, for the first time, results of a comparative survey of the sediment-associated fungal communities at the Yap Trench by using high-throughput sequencing and culture-based techniques. The fungal diversity obtained from these techniques were different, with 11.1% of the fungal genera shared between both techniques. Through ITS2 (internal transcribed spacer 2 of the rDNA) metabarcode sequencing from 42 sediment samples, 890 fungal operational taxonomic units (OTUs) were found based on clustering at a 97% sequence similarity cutoff level. Of these OTUs, 98 OTUs belonged to Ascomycota, 52 OTUs to Basidiomycota, 1 OTU to Chytridiomycota, 4 OTUs to Mortierellomycota, 19 OTUs to Mucoromycota and 2 OTUs to Rozellomycota, whereas 714 OTUs could not be assigned to any fungal phylum. The Ascomycota and the Basidiomycota comprised 49.74% and 5.84% of the total sequences, respectively t. Eurotiomycetes was identified as the most dominant fungal class. The culture-based technique also revealed Eurotiomycetes as the most abundant class, but some classes, such as Endogonomycetes and Umbelopsidomycetes, were not detected by the culture-based technique. Altogether, our findings provide valuable information for the understanding of fungal distribution and potential ecological roles of fungi in deep-sea hadal trenches.

A depocenter of organic matter at 7800 m depth in the SE Pacific Ocean

The Atacama trench, the deepest ecosystem of the southern Pacific Ocean (ca. 8000 m depth) was investigated during the Atacama Trench International Expedition. Sediments, collected at three bathyal stations (1040–1355 m depth) and at a hadal site (7800 m) were analyzed for organic matter quantity and biochemical composition (in terms of phytopigments, proteins, carbohydrates and lipids), bacterial abundance, biomass and carbon production and extracellular enzymatic activities. Functional chlorophyll- a (18.0±0.10 mg m −2), phytodetritus (322.2 mg m −2) and labile organic carbon (16.9±4.3 g C m −2) deposited on surface sediments at hadal depth (7800 m) reached concentrations similar to those encountered in highly productive shallow coastal areas. High values of bacterial C production and aminopeptidase activity were also measured (at in situ temperature and 1 atm). The chemical analyses of the Atacama hadal sediments indicate that this trench behaves as a deep oceanic trap for organic material. We hypothesize that, despite the extreme physical conditions, benthic microbial processes might be accelerated as a result of the organic enrichment.

Biodiversity of nematode assemblages from deep-sea sediments of the Atacama Slope and Trench (South Pacific Ocean)

Nematode assemblages were investigated (in terms of size spectra, sex ratio, Shannon diversity, trophic structure and diversity, rarefaction statistics, maturity index, taxonomic diversity and taxonomic distinctness) at bathyal and hadal depths (from 1050 to 7800 m) in the deepest trench of the South Pacific Ocean: the Trench of Atacama. This area, characterised by very high concentrations of nutritionally-rich organic matter also at 7800-m depth, displayed characteristics typical of eutrophic systems and revealed high nematode densities (>6000 ind. 10 cm −2). Nematode assemblages from the Atacama Trench displayed a different composition than at bathyal depths. At bathyal depths 95 genera and 119 species were found (Comesomatidae, Cyatholaimidae, Microlaimidae, Desmodoridae and Xyalidae being dominant), whereas in the Atacama Trench only 29 genera and 37 species were encountered (dominated by Monhysteridae, Chromadoridae, Microlaimidae, Oxystominidae and Xyalidae). The genus Monhystera (24.4%) strongly dominated at hadal depths and Neochromadora, and Trileptium were observed only in the Atacama Trench, but not at bathyal depths. A reduction of the mean nematode size (by ca. 67%) was observed between bathyal and hadal depths. Since food availability was not a limiting factor in the Atacama Trench sediments, other causes are likely to be responsible for the reduction of nematode species richness and body size. The presence of a restricted number of families and genera in the Atacama Trench might indicate that hadal sediments limited nematode colonisation. Most of the genera reaching very high densities in Trench sediments (e.g., Monhystera) are opportunistic and were responsible for the significant decrease of the maturity index. The dominance of opportunists, which are known to be characterised by small sizes, might have contributed to the reduced nematode size at hadal depths. Shannon diversity and species richness decreased in hadal water depth and this pattern was more evident at genus than at species level. Epistrate feeders dominated and increased their relevance, determining a reduction of the index of trophic diversity at hadal depths. According to trophic diversity, taxonomic diversity and distinctness also decreased with depth. All diversity indices from the Atacama Slope and Trench were lower than in other equally deep areas world wide (e.g. Puerto Rico Trench). We suggest that such reduction was related to the high nutrient loading observed in this system (up to two orders of magnitude higher than in typical oligotrophic deep-sea sediments).

Meiofauna hotspot in the Atacama Trench, eastern South Pacific Ocean

Meiofaunal assemblages were investigated (in terms of abundance, biomass, individual size and community structure) at bathyal and hadal depths (from 1050 to 7800 m) in the Atacama Trench in the upwelling sector of the eastern South Pacific Ocean, in relation to the distribution and availability of potential food sources (phytopigments, biochemical compounds and bacterial biomass) in this highly productive region. Meiofaunal density and biomass in the Atacama Trench were one to two orders of magnitude higher than values reported in other “oligotrophic” hadal systems. The Atacama Trench presented very high concentrations of nutritionally rich organic matter at 7800-m depth and displayed characteristics typical of eutrophic systems. Surprisingly, despite a decrease in chlorophyll- a and organic matter concentrations of about 50% from bathyal to hadal depths, meiofaunal abundance in hadal sediments was 10-fold higher than at bathyal depths. As indicated by the higher protein to carbohydrate ratio observed in trench sediments, the extraordinarily high meiofaunal density reported in the Atacama Trench was more dependent upon organic matter quality than on its quantity. The trophic richness of the system was reflected by a shift of the size structure of the benthic organisms. In contrast with typical trends of deep-sea systems, the ratio of bacterial to meiofaunal biomass decreased with increasing depth and, in the Atacama Trench, meiofaunal biomass largely dominated total benthic biomass. Nematodes at 7800-m depth accounted for more than 80% of total density and about 50% of total meiofaunal biomass. In hadal sediments a clear meiofaunal dwarfism was observed: the individual body size of nematodes and other taxa was reduced by 30–40% compared to individuals collected at bathyal depths. The peculiarity of this trophic-rich system allows rejection of previous hypotheses, which explained deep-sea dwarfism by the extremely oligotrophic conditions typical of deep-sea regions.