Flourishing chemosynthetic life at the greatest depths of hadal trenches

Abstract

Hadal trenches, some of the Earth's least explored and understood environments, have long been proposed to harbour chemosynthesis-based communities^1,2. Despite increasing attention, actual documentation of such communities has been exceptionally rare^3,4. Here we report the discovery of the deepest and the most extensive chemosynthesis-based communities known to exist on Earth during an expedition to the Kuril-Kamchatka Trench and the western Aleutian Trench using the manned submersible Fendouzhe. The communities dominated by siboglinid Polychaeta and Bivalvia span a distance of 2,500 km at depths from 5,800 m to 9,533 m. These communities are sustained by hydrogen sulfide-rich and methane-rich fluids that are transported along faults traversing deep sediment layers in trenches, where methane is produced microbially from deposited organic matter, as indicated by isotopic analysis. Given geological similarities with other hadal trenches, such chemosynthesis-based communities might be more widespread than previously anticipated. These findings challenge current models of life at extreme limits and carbon cycling in the deep ocean.

Fig. 1. Map showing the Kuril–Kamchatka Trench and western Aleutian Trench.

The study area, situated in the northwest Pacific, is demarcated by a white rectangle in the inset. Orange dots represent dive sites where chemosynthesis-based communities were observed and sampled and crosses indicate dive sites lacking such communities. Open orange circles delineate potential seep sites characterized by black sediments. White arrows illustrate the direction of subduction for the Pacific Plate beneath the Okhotsk Plate and the Bering Sea Plate. The dashed white lines indicate the transitional connection zones between the Kuril–Kamchatka Trench and the Aleutian Trench. Bathymetric data were acquired using the KM-EM122 multi-beam bathymetric system during the research expedition. Scale bar, 200 km. Credit: map created using Global Mapper 14 software, with background data sourced from GeoMapApp (http://www.geomapapp.org), under a CC BY 4.0 licence.

Fig. 2. Representative fauna of cold-seep sites in the Kuril–Kamchatka Trench and western Aleutian Trench.

a, Free-moving polychaetes Macellicephaloides grandicirra (white; reaching 6.5 cm in size) navigate among dense colonies of frenulate siboglinids, with tubes 20–30 cm in length and approximately 1 mm in diameter, at 9,532 m at The Deepest. b, Clusters of frenulate siboglinids extending red haemoglobin-filled tentacles, with small Gastropoda (white spots) on tops of the tubes near the tentacles, at 9,320 m at Wintersweet Valley. c, Tightly packed frenulate siboglinids are home to abundant free-moving polychaetes M. grandicirra (white) at 9,332 m at Cotton Field. d, Dense aggregation of vesicomyid bivalves A. phaseoliformis (reaching 23 cm in size) in the sediment, with approximately 6–8 cm of valves exposed and often hosting Actiniaria, at 5,743 m at Clam Bed. e, Tube-dwelling polychaetes Anobothrus sp. and Actiniaria are dominant at 6,870 m at Aleutian Deepest, with spots of white microbial mats. f, Dense aggregation of vesicomyid bivalves I. fossajaponicum (reaching 3 cm in size) associated with black sediments and accompanied by tube-dwelling polychaetes Anobothrus sp. at 6,928 m at Aleutian Deepest. g, Dark blue muds surrounded by clusters of frenulate siboglinids, mark methane seeps at 6,800 m at Blue Marsh. h, Large patches of white, snow-like microbial mats stretch tens of metres, accompanied by frenulate siboglinid tubeworms at 6,700 m at Icy River. The images were taken by the manned submersible equipped with a high-definition camera system. The name of each cold seep indicated in the lower left corner. The distance between laser beams is 10 cm. An expanded showcase of cold-seep fauna is given in Supplementary Video 1.

Fig. 3. Origins and phases of methane in hadal cold seeps.

a, δ¹³C_VPDB and δD_VSMOW diagram classifying the source of methane. The base diagram is adapted from previous studies^,,, which used field measurements of stable carbon and hydrogen isotopes of methane (CH₄) from diverse sedimentary environments to distinguish between different types of microbial and thermogenic methanogenesis. Yellow dots in the CO₂ reduction (CR) category represent data from the methane samples analysed in this study. b, Schematic phase diagram of methane hydrate–sea water system. Solid black lines represent the phase boundaries between H+I – H+L and H+L – L+V ; the shaded (grey blue) H+L phase zone and grey lines therein indicate the methane hydrate solubilities in sea water (mCH₄, in ppm). Red diamonds represent the temperature and pressure conditions at stations where seeps are present. H + I, methane hydrate + ice; H + L, methane hydrate + liquid; L + V, liquid + vapour; M, methyl-based methanogenesis.

Fig. 4. Formation of hadal trench cold seeps.

a, Schematic geological model presenting the cross-section view of the subducting plate and the overriding plate along the trench, as indicated by seismic survey data from these areas. The light green arrows depict the migration of organic matter into the trench, encompassing both downward and lateral movements. The white arrow denotes the direction of subduction, and the dashed line signifies the trench axis, which is nearly parallel to the striped zone of cold seeps. Black triangles point to the trench’s axis. Note the prevalence of normal faults developed in the subducting plate. b, Detailed view of the area outlined by the black rectangle in a showing the formation of gas hydrates in deep sediment layers. Methane-rich fluids migrate horizontally towards the accretionary wedge as a result of the compression forces associated with subduction. Normal faults at the leading edge of the accretionary wedge create a pathway that facilitates the upward migration of seep fluids. Figure created using previously published location data of subducting bending faults.

Extended Data Fig. 1. Some samples of Siboglinidae collected during cruise.

a, FDZ 271, short tubeworms cf. Spirobrachia (from the Deepest seep field). b, FDZ 294, mix of ultra slender & long wide tubeworms (from Blue Marsh, Icy River). c, FDZ 295, beard of ultra slender tubeworms (from Blue Marsh, Icy River). Scale bars=20 mm.

Extended Data Fig. 2. Representative fauna at three hadal cold seeps.

a, Dead Valley. b, Clam Valley. c, Turtle Egg. Scale bars=20 mm.

Extended Data Fig. 3. Representative samples of Bivalvia recovered from seeps.

a, FDZ 286, Tartarothyasira cf. hadalis. b, FDZ 296, Isorropodon fossajaponicum. c, FDZ 298, Axinus sp. d, FDZ 297, Abyssogena phaseoliformis. Scale bars=20 mm.

Extended Data Fig. 4. Vertical profiles of porewater geochemistry in two sediment pushcores (FDZ292-S05 and FDZ294-S08).

a, CH₄. b, ${\mathrm{SO}}_4^{2-}$. c, H₂S. d, ${\mathrm{NH}}_4^{+}$. e, DOC. f, DIC. g, δ¹³C-DIC. h, Salinity.

Extended Data Fig. 5. Representative ikaite samples recovered from cold seeps.

a, Ikaite specimens. b, Photomicrograph of ikaite. Scale bar=1 cm in a and 2 mm in b.

Extended Data Fig. 6. A phase diagram illustrating hydrate stability conditions in the sediments of the investigated hadal zone.

The horizontal black dashed line denotes the 9,533-meter water depth, corresponding to 98.0 MPa of hydrostatic pressure (assuming an average water density of 1.045 g/cm³), above which lies the unconsolidated sub-seafloor sediments. The gray circle denotes the seafloor temperature-pressure conditions at 9,533 meters, with a temperature of 2.25 °C and a pressure of 98.0 MPa. The purple and red lines represent the minimum and maximum geothermal gradients (ranging from 25 to 60 °C/km) within the unconsolidated sediments, respectively. The point of intersection between these geothermal gradients and HLV delineates the vertical limits of the hydrate stability zone (HSZ) within the sediment.

Extended Data Fig. 7. A coexistence example of frenulate siboglinids with Elpidia hanseni.

FDZ 271, at 9532 m at The Deepest. Scale bar=3 cm.