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Shanghai Jiao Tong University along with multiple collaborating institutions including the University of Copenhagen and Lawrence Berkeley National Laboratory, have conducted an extensive investigation into microbial ecosystems in the deep ocean hadal zone.

Findings reveal an unprecedented level of taxonomic novelty, with 89.4% of identified microbial species previously unreported. The study demonstrated that selection pressures, favoring either streamlined or versatile adaptation strategies, dominate over neutral drift in shaping these extreme microbial ecosystems.

Hadal environments, located at depths exceeding 6,000 meters below sea level, remain among the least explored ecosystems on Earth. Manned submersibles capable of reaching full-ocean depth have been rare, with less than a dozen individuals visiting the deepest point of the Mariana Trench before 2019.

Immense hydrostatic pressure, near-freezing temperatures, and extreme nutrient limitations of the hadal zone create seemingly impossible conditions for life to flourish. Previous research suggested microbial diversity in these depths might be limited, yet a lack of direct sampling has limited the certainty of these suggestions.

Advances in deep-sea technology, particularly the Fendouzhe submersible, have revolutionized hadal research, enabling more precise and frequent sampling. With a 220 kg sampling capacity and an extended six-hour benthic operation time, Fendouzhe has outperformed traditional methods such as gravity and box cores.

The Mariana Trench Environment and Ecology Research (MEER) project provides the first systematic exploration of microbial and macrofaunal life in these depths, revealing an unprecedented level of biological diversity and adaptation strategies.

In the study, "Microbial Ecosystems and Ecological Driving Forces in the Deepest Ocean Sediments," in Cell, researchers conducted large-scale metagenomic and 16S rRNA gene amplicon sequencing to investigate microbial diversity and adaptation strategies in the hadal zone.

During the TS21 expedition (August–November 2021), researchers conducted 33 dives, collecting 1,648 sediment samples, 12 in situ seawater filtrates, and multiple macroorganism specimens across 6,000 to 10,900 meters in the Mariana Trench, Yap Trench, and Philippine Basin.

Sampling covered diverse geological features, including concave and convex faults, trench slopes, and abyssal plains, laying the groundwork for the most comprehensive hadal ecological dataset to date.

Metagenomic sequencing of 1,194 samples generated the 92-terabasepair (Tbp) Mariana Trench Environment and Ecology Research (MEER) dataset, identifying 7,564 microbial species, 89.4% of which had not been previously reported. Comparative genomic analysis confirmed that the MEER dataset had little overlap with global deep-sea datasets, reinforcing its novelty.

Analyses integrated geophysical, geochemical, and biological methodologies, employing a semiautomated DNA extraction-to-sequencing process. Data revealed exceptionally high levels of microbial novelty, diversity, and heterogeneity, particularly among prokaryotes and viruses, influenced by both extreme environmental pressures and intricate topographical variations.

Beyond microorganisms, the research highlights two representative hadal macrofauna species, Hadal amphipods (Hirondellea gigas)—with horizontal population exchanges across trenches and the Hadal snailfish (Pseudoliparis swirei)—exhibiting a vertical invasion of vertebrates from the deep sea into the hadal zone.

Shared adaptation mechanisms across microbes and macrofauna suggest convergent evolutionary strategies in hadal environments. Enhanced antioxidation mechanisms and intracellular accumulation of compatible solutes were observed across biological domains, reinforcing the concept of universal physiological responses to ultra-high-pressure conditions.

Conclusions drawn from the data confirm that environmental selection, rather than random drift, is the key determinant of microbial community assembly in the deep ocean. Findings further suggest that hadal microbial ecosystems operate under distinct evolutionary pressures that differ from those in more shallow marine environments.

The newly generated dataset provides a springboard for future studies on deep-sea microbial adaptation with potential applications in biotechnology and environmental science.

MEER datasets are now publicly accessible, offering researchers a comprehensive repository of new hadal microbial and macrofaunal data.

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