Hadal trench microbiomes as models of extreme adaptation and ecosystem function

Abstract Hadal trenches form where one tectonic plate descends beneath another, creating the deepest ocean habitats. Despite extreme pressure and limited energy supply, their microbiomes sustain biogeochemical cycling and support trench ecosystems. In this Review, we synthesize culture-based studies, single cell genomics and metagenomic surveys to (1) map the distribution of bacterial and archaeal lineages, (2) examine adaptations involving membranes, proteins, genome maintenance and nutrient use, (3) describe microbial roles across oxic and anoxic sediment zones, (4) assess partnerships between microbes and trench fauna, and (5) evaluate the biotechnological potential of extremozymes and bioactive compounds. Moreover, we compare patterns across studied trenches and identify major uncertainties caused by limited sampling, cultivation and in situ experimentation. By doing so, this Review provides a roadmap for targeted isolation, in situ experimentation and the discovery of uncommon extremozymes and bioactive compounds.