Hadal trenches are the least explored habitats on the planet. Situated below six thousand meters, they experience hydrostatic pressures that would instantly destroy most surface-dwelling organisms, and they receive only a sparse rain of organic matter drifting down from the sunlit waters far above. Despite these constraints, trenches host distinct microbial communities whose composition differs markedly from those of shallower seafloor sediments and the open ocean. Understanding how such communities arise, and what genetic innovations allow them to colonize and thrive in these extreme conditions, has become a central question in deep-sea microbiology, and the Kermadec Trench, one of the deepest trenches in the world, offers an ideal natural laboratory for investigating it.

Horizontal gene transfer is a well-known force in microbial evolution. Unlike vertical inheritance, in which genes pass from parent cells to daughter cells during division, horizontal transfer allows microorganisms to acquire genes from unrelated lineages, sometimes across large evolutionary distances. This can happen through several mechanisms: transformation, in which cells take up free DNA from their surroundings; transduction, in which viruses shuttle DNA between hosts; and conjugation, in which cells exchange genetic material through direct contact. In many environments, these processes contribute to the rapid spread of useful traits, such as antibiotic resistance or the ability to metabolize new compounds. The new research suggests that in the hadal zone, this gene-swapping activity is not merely occasional but pervasive, leaving a deep imprint on the genomes of trench-dwelling microbes.

By analyzing the genomes of microorganisms recovered from Kermadec Trench sediments, the researchers identified an unusually high prevalence of genes that appear to have been acquired from other lineages rather than inherited vertically. These transferred genes are not randomly distributed across microbial genomes. Instead, they cluster in functional categories that are directly relevant to life in the trench, including genes involved in coping with high hydrostatic pressure, breaking down complex organic compounds, and managing the energetic demands of a nutrient-poor environment. This functional bias indicates that horizontal gene transfer in the hadal zone is not simply background noise but a targeted engine of adaptation, preferentially enriching microbial populations with traits that improve survival under trench-specific stresses.

One of the most striking implications of the study concerns the role of the trench environment itself as a conduit for gene exchange. Hadal trenches are geographically isolated basins, separated from one another by vast stretches of open ocean and from shallower habitats by steep pressure and chemical gradients. Yet the genetic evidence suggests that DNA moves readily within the trench microbial community, crossing the boundaries between species and even between higher-level taxonomic groups. Viruses, plasmids, and other mobile genetic elements appear to act as vehicles for this exchange, stitching the trench microbiome into a connected genetic network. In such a network, a beneficial adaptation that arises in one lineage can, in principle, spread to many others, accelerating the pace at which the community as a whole adjusts to its extreme surroundings.

The study also sheds light on how diversification unfolds in the deep ocean. Microbial species boundaries are notoriously difficult to define, and horizontal gene transfer complicates the picture further by allowing lineages to share genes while otherwise diverging. In the Kermadec Trench, the researchers found evidence that extensive gene exchange has contributed to the genetic distinctiveness of trench microbial populations relative to their relatives in shallower waters. Rather than erasing differences between lineages, the pattern of transfer appears to have promoted diversification, as different microbial groups assembled distinct combinations of acquired genes, each tailored to particular ecological niches within the trench, from the sediment surface to deeper layers where organic matter is even scarcer and chemical conditions differ.

These results carry broader significance for how scientists think about microbial evolution in extreme environments. For decades, studies of adaptation focused primarily on mutation and selection, the slow accumulation of small genetic changes that gradually equip organisms for new conditions. Horizontal gene transfer offers a shortcut, allowing microbes to acquire fully formed, functional genes from organisms that have already solved similar problems. In an environment like the hadal zone, where energy is scarce and conditions are unforgiving, this shortcut may be essential. The Kermadec Trench findings suggest that gene flow between lineages is a first-order driver of adaptation in the deep ocean, comparable in importance to mutation and vertical inheritance, and that models of microbial evolution that neglect it will miss a large part of the story.

The research also has practical implications. Trench microbes are of growing interest for biotechnology and bioprospecting, because the enzymes and biochemical pathways that allow life under extreme pressure and cold could have applications in industry, medicine, and environmental remediation. Understanding which genes have been mobilized through horizontal transfer, and how they function in their new hosts, could help researchers identify promising molecules and predict how deep-sea microbial communities might respond to environmental change. As human activities, from deep-sea mining to plastic pollution, increasingly reach the ocean floor, knowing how these communities are wired, genetically and ecologically, becomes more urgent.

At the same time, the study underscores how much remains unknown about the hadal biosphere. The Kermadec Trench is only one of many trenches scattered along the margins of the Pacific and other oceans, and each may host its own distinct microbial assemblages shaped by local conditions and evolutionary history. Whether the patterns of extensive horizontal gene transfer observed in the Kermadec Trench hold true in other hadal settings is a question for future work, as is the precise identity of the mobile genetic elements that mediate the exchange. What is already clear, however, is that the deepest microbial communities on Earth are not evolutionary backwaters. They are dynamic, genetically interconnected systems in which the sharing of genes across lineages has become a central strategy for making a living at the bottom of the world, and in which adaptation and diversification proceed hand in hand through the constant circulation of genetic material among neighbors.

Subject of Research: Horizontal gene transfer in Kermadec Trench hadal zone microbiomes

Article Title: Extensive horizontal gene transfer drives microbial adaptation and diversification in Kermadec Trench hadal microbiomes

Article References: Liu, P., Lv, Y., Zhang, Y., Wang, Y., Zhu, K., Cao, Z., Dong, L., Xiao, X., Pan, Y., & Li, J. (2026). Extensive horizontal gene transfer drives microbial adaptation and diversification in Kermadec Trench hadal microbiomes. Nature Communications. https://doi.org/10.1038/s41467-026-77994-z

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77994-z

Keywords: Kermadec Trench, hadal zone, horizontal gene transfer, microbiome, deep-sea microbes, microbial adaptation, microbial diversification, Nature Communications, extreme environments, mobile genetic elements, deep ocean, microbial evolution