Marine holobionts: a model arena for host–microbe biology, from ecosystem function to biotechnology

<jats:p>Microbial symbioses are the operating system of marine biology. Nearly every macroscopic organism in the ocean exists as a holobiont, the consortium composed of a eukaryotic host and its bacterial, archaeal, microeukaryotic and viral partners. Marine holobionts span a broader range of the metazoan tree of life than likely any other ecological setting on the planet. This breadth makes the marine realm a uniquely informative system to investigate how host-microbe partnerships shape organisms and ecosystems. In this review, we discuss marine holobionts around three integrative themes. First, marine selective regimes sustain a distinctive phylogenetic and metabolic diversity of holobionts. Second, despite their extraordinary diversity, most of the marine holobionts converge on a single carbon-fixing logic with two complementary branches: photosymbiosis in the photic zone and chemosymbiosis in the aphotic zone, both running through a microbial partner around which the host body is built. Third, each marine model system resolves distinctive mechanisms of host-microbe integration to a depth that no single system could achieve alone. Finally, we discuss the growing biotechnological potential of marine holobionts, including bioactive compounds, industrial enzymes and probiotic interventions. Across all these dimensions, the conservation of marine ecosystems is inseparable from the conservation of marine microbiomes.</jats:p>