Humans have always been fascinated by the extremes of our world, from the highest mountain to the deepest ocean. Today we are beginning to understand these much more clearly. The latest scientific research at the deepest part of the ocean has revealed something extraordinary - biodiversity is extensive and flourishing despite the extreme conditions.

On March 6, the scientific journal, Cell, featured a cover story systematically revealing the ecological characteristics of the hadal zone at water depths exceeding 6,000 meters. These findings mark the latest results from the Mariana Trench Environment and Ecology Research (MEER) Project, a collaboration launched in 2021 by BGI Group, the Institute of Deep-sea Science and Engineering (IDSSE) of the Chinese Academy of Sciences, Shanghai Jiao Tong University, and other Chinese scientific institutions.

The hadal zone, while covering just 1–2% of the ocean floor, accounts for the deepest 45% of the ocean’s vertical depth. It is a realm of extreme conditions, where immense pressure, total darkness, limited food sources, and near-freezing temperatures create an environment that commonly considered inhabitable by only a few specialized organisms.

Microbes: Flourishing in the Extreme with Remarkable Adaptations

Thanks to the development of an integrated method using latest automation and sequencing technology for environmental samples, the largest and most comprehensive hadal microbial metagenome dataset to date was created.

Their findings uncovered an extraordinary diversity of hadal microorganisms, with over 7,564 newly identified species-level genomes, nearly 90% of which had never been documented in public databases.

To survive at the deepest sea levels on earth, hadal microorganisms have developed a unique set of evolutionary traits for energy intake and pressure resistance. By analyzing distinct ecological processes and microbial genomic/metabolic traits, scientists were able to determine two distinct adaptation strategies.

Amphipods: Tiny Titans of the Deep

Beyond microorganisms, the research team made fascinating discoveries about Hirondellea gigas (H. gigas), an amphipoda species that thrives at depths of 6,800 to 11,000 meters - where pressure is equivalent to balancing an SUV on a fingertip.

The research team successfully generated a chromosome level genome assembly for H. gigas, spanning a large genome size of 13.92 gigabytes (Gb). This represents the first-ever genome of the animal from the deepest ocean. Notably, more than 70% of the H. gigas genome consists of repetitive sequences, which may contribute to its ability to withstand deep-sea pressure.

Furthermore, they generated 245.97 terabytes (Tb) of whole-genome sequencing data, the largest dataset ever sequenced for a single marine species. Despite their extreme habitat, H. gigas populations showed no genetic differentiation across depths, suggesting they can migrate freely across a 4,000-meter vertical range. However, the genomes of H. gigas vary depending on their geographical location. Specifically, populations in the West Philippine Basin, located 1,500 km away, differ from those in the Mariana and Yap Trenches. This finding suggests that geographic isolation influences genetic evolution.

Deep-Sea Fish: Evolutionary Marvels of the Ocean

The researchers also examined 11 species of deep-sea fish, uncovering remarkable genetic adaptations that allow them to survive in extreme depths.

Genetic analysis suggests that some deep-sea fish lineages date back to the mid- or early Cretaceous period, surviving multiple mass extinction events. Most modern deep-sea fish colonized these depths after the Cretaceous–Paleogene extinction event, which wiped out the dinosaurs.

A key discovery found that many deep-sea fish species below 3,000 meters have undergone a unique genetic transformation in the rtf1 gene, which may be the key to maintaining stable gene expression under extreme pressure.

One of the most surprising findings challenges a long-standing scientific assumption about deep-sea adaptation. Previous research suggested that trimethylamine-N-oxide (TMAO), a compound that stabilizes proteins under high pressure, increases in fish as depth increases. However, this study found no significant rise in TMAO levels in fish living below 6,000 meters, prompting a reassessment of previous deep-sea research methods.

Disturbingly, the team discovered persistent organic pollutants - industrial waste compounds - in both fish and sediment samples from the Challenger Deep in the Mariana Trench and the Philippine Trench. This alarming finding underscores the far-reaching impact of human activities, even in the most remote and extreme environments on earth.

A New Era in Deep-Sea Exploration

These findings significantly advanced the world’s understanding of life in the deepest parts of the ocean, revealing unprecedented biodiversity and evolutionary adaptations in these remote ecosystems. All genomic data generated by this research, including information on deep-sea microbes, amphipods, and fish, has been made freely accessible to the global scientific community through online platforms.

"Our study not only redefines our understanding of the limits of deep-sea life but also unveils an 'extreme survival manual' written through hundreds of millions of years of evolution,” explained Dr. Xun Xu, Director of BGI-Research, the scientific research arm of BGI Group. “Deciphering these life forms is both a journey of scientific exploration and a renewed responsibility for humanity. In the future, with technological advancements and global scientific collaboration, the deep sea may reveal more secrets about the origins, adaptation, and symbiosis of life - offering critical insights for the sustainable stewardship of earth's ecosystems."

Photo Caption: A fish observed swimming in the deep sea during a deep dive exploration. (Source: BGI Group)