In the summer of 2024, an ambitious deep-sea expedition embarked on a groundbreaking journey to explore the enigmatic ecosystems inhabiting the deepest oceanic trenches. Conducted aboard the research vessel Tan Suo Yi Hao, the TS42 cruise deployed the state-of-the-art human-occupied vehicle Fendouzhe, equipped with hydraulically powered manipulators mounted on dual swing arms. This sophisticated apparatus allowed precise and efficient collection of biological and geological samples from depths previously inaccessible to direct investigation. Guided by expert operators within the submersible, the team secured a diverse array of fauna and sedimentary materials, storing them meticulously in specialized compartments to preserve their integrity for subsequent analyses.

Upon resurfacing, the biological specimens were rapidly transferred under controlled conditions to the shipboard laboratory. There, initial sorting based on detailed visual inspections and stereomicroscopic examinations delineated samples into primary taxonomic categories, enabling focused identification and enumeration. Preservation methods were meticulously chosen according to taxonomic requirements, employing chilled, non-denatured 95% ethanol or buffered formaldehyde solutions for optimal tissue integrity. Select specimens underwent further long-term storage in 70% ethanol, ensuring sustained viability for future molecular and morphological research.

High-definition video footage, captured by dual cameras affixed to the Fendouzhe, provided an invaluable visual census of macro-epifaunal and mega-epifaunal communities on the seafloor. From these recordings, analysts extracted multiple representative frames illustrating the densest cold-seep habitats encountered during each dive. Using the submersible’s laser scale projection system, which emitted a pair of parallel laser points precisely 10 centimeters apart onto the benthic surface, researchers quantified the scanned areas. This spatial calibration enabled standardized density calculations by drawing virtual quadrats—typically measuring 50 by 50 centimeters—within the images. Through meticulous manual counting of organisms visible within these quadrats, the team derived mean individual densities per square meter, with statistical assessments computing the standard deviations to capture variance among samples.

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Molecular approaches complemented visual assessments by targeting the mitochondrial cytochrome c oxidase subunit I (coxI) gene from collected fauna tissue samples. DNA extractions, performed using the PowerSoil DNA Isolation Kit, yielded nucleic acids subsequently quantified via Qubit fluorometric assays. High-throughput metagenomic sequencing on the Illumina NovaSeq X Plus platform generated paired-end reads, which were quality-trimmed and assembled into contigs. Identification of coxI sequences within this assemblage permitted initial taxonomic classification by comparative analysis against the comprehensive GenBank repository, thus expanding insights into the genetic diversity present in these extreme habitats.

Concurrently, geochemical investigations focused intently on sediment pushcores retrieved during each descent, with 6 to 12 cores collected per dive by the submersible’s manipulators. These sediment blocks were promptly chilled in a 4°C cold room onboard to inhibit chemical alteration prior to analysis. Subsamples exhibiting significant methane concentrations underwent exhaustive isotopic scrutiny, underpinning studies of carbon and hydrogen cycling within the hadal environment. Advanced pore-water extraction techniques employed Rhizon samplers inserted at two-centimeter intervals, facilitating acquisition of uncontaminated fluids for subsequent hydrogen sulfide, sulfate, ammonium, and dissolved inorganic carbon measurements.

Gas composition and isotopic analyses leveraged a two-pronged approach. Sediment-derived gases were trapped in 2 ml aliquots of sodium hydroxide solution within sealed vials, creating headspaces through helium gas replacement for chromatographic and isotopic assays. Parallel direct gas sampling into evacuated vials allowed cross-validation of methane concentrations and stable isotope ratios. Key isotopes—including δ¹³C and δD of methane—were determined via coupled gas chromatography and isotope ratio mass spectrometry, delivering precision reproducibility of ±0.5‰ and ±2‰, respectively. These data illuminate methane sourcing and transformation pathways in these deep-sea seep systems.

Further chemical profiling elucidated pore-water geochemistry with a suite of sophisticated analytical tools. Hydrogen sulfide, a hallmark of reducing sedimentary environments, was measured colorimetrically using the sensitive methylene blue method. Anion concentrations, particularly sulfate, were quantified via ion chromatography with high precision, while ammonium levels were determined through fluorescence spectrometry. Dissolved inorganic carbon (DIC) concentrations and isotopic compositions were analyzed by Gas Bench II isotope ratio mass spectrometry at the Institute of Deep-Sea Science and Engineering, offering detailed insights into carbon cycling dynamics. Additional metrics such as dissolved organic carbon concentrations and salinity provided contextual parameters essential for interpreting biogeochemical processes shaping microbial and macrofaunal habitats.

To contextualize methane behaviors in these sediments, researchers applied rigorous thermodynamic models to delineate methane phase equilibria and hydrate stability boundaries. The Van der Waals–Platteeuw model, enhanced with angle-dependent ab initio potential functions, underpinned calculations of chemical potentials within hydrate phases, while the Gibbs–Thomson equation accounted for the influence of pore-scale capillary effects on phase equilibria. Such modeling, incorporating activity coefficients calculated via the Pitzer framework and refined with Poynting corrections, enabled accurate prediction of methane hydrate solubility in seawater under varying pressure-temperature regimes. This integrative approach sheds light on physical constraints governing methane storage and release in the trench environment.

This comprehensive suite of investigations revealed a surprisingly vibrant ecosystem thriving under extreme pressure and darkness at the hadal trench’s abyssal bottom. The detection of flourishing chemosynthetic life forms demonstrates adaptive strategies harnessing methane and sulfide seepage to fuel complex food webs independent of sunlight. Coupling ecological observations with molecular genetics and environmental geochemistry provides an unprecedented multidimensional portrait of life at Earth’s deepest marine frontiers. These findings challenge prevailing assumptions about life’s limits, offering profound implications for biogeochemical cycling and deep-ocean biodiversity.

The expedition successfully marries cutting-edge deep-sea technologies with multidisciplinary scientific inquiry, pushing the boundaries of oceanographic exploration. The deployment of the human-occupied vehicle Fendouzhe exemplifies the synergy of engineering innovation and biological discovery, enabling direct observations and precise manipulations in environments otherwise accessible only via remote instruments. This fusion of approaches amplifies our capacity to document, characterize, and understand the extreme biosphere, shedding light on ecosystems hidden beneath kilometers of water.

Moreover, the integration of metagenomic and isotopic techniques with traditional taxonomy and videography exemplifies a holistic study design, crucial for untangling complex ecological and geochemical interactions within these isolated habitats. Through robust sampling protocols and meticulous analytical procedures, the study lays a foundation for longitudinal monitoring and comparative analyses across geographic regions and depth gradients. The insights generated extend beyond pure science, informing predictions about the impacts of climate change and anthropogenic disturbances on fragile deep-sea environments.

Ultimately, this research not only enriches our understanding of hadal biodiversity and methane dynamics but also inspires broader curiosity about Earth’s least explored ecosystems. The scale and depth of the endeavor invite contemplation of the resilience and versatility of life, inviting reevaluation of ecological paradigms in the context of planet-wide environmental heterogeneity. As the scientific community continues to probe these depths, such pioneering studies will undoubtedly redefine our conception of the deep ocean as a vibrant and vital realm.

Subject of Research: Flourishing chemosynthetic life and biogeochemical processes at hadal trench depths.

Article Title: Flourishing chemosynthetic life at the greatest depths of hadal trenches.

Article References:
Peng, X., Du, M., Gebruk, A. et al. Flourishing chemosynthetic life at the greatest depths of hadal trenches. Nature (2025). https://doi.org/10.1038/s41586-025-09317-z

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Tags: chemosynthetic life formsdeep-sea biological samplingdeep-sea ecosystemsdeep-sea expedition research methodshadal trenches explorationmarine biodiversity assessmentoceanic trench ecosystems analysispreservation techniques for biological samplessedimentary sample collection methodssubmersible technology in ocean researchtaxonomic categorization of marine speciesunderwater video documentation