Abstract
Marine zooplankton are a link between primary producers and higher trophic levels and play a pivotal role in organic matter export via diel vertical migration and faecal-pellet production. Molecular surveys have revealed each individual as a holobiont hosting dense, taxonomically structured microbiomes in the gut, on the cuticle, and within feeding structures. These microbial partners expand dietary breadth through carbohydrate-active enzymes, supply limiting vitamins, detoxify xenobiotics, and may buffer thermal and oxidative stress, thereby influencing host fitness and the fate of particulate organic carbon. Experimental studies show intact microbiomes often enhance growth or fecundity, with effects amplified under vitamin limitation or toxin exposure. In contrast, gut and pellet-associated respiration can substantially reduce the carbon content of pellets within hours to days, depending on temperature and microbial composition. Vertical migrators also transport live bacteria and viruses below the thermocline, potentially seeding mesopelagic niches and affecting remineralization patterns. Despite these influences, zooplankton holobionts remain largely absent from biogeochemical models. This minireview synthesises current understanding of microbiome assembly and plasticity, their contributions to host performance and carbon export, and their role in microbial dispersal, underscoring the need to integrate holobiont traits into Earth-system models to better predict ecosystem responses to warming and deoxygenation.