Endosymbiotic apicomplexans of marine holobionts: microbial parasites in a warming ocean

Abstract Apicomplexans are a large group of protists, including several species of major medical importance, most notably those which cause malaria and toxoplasmosis in humans. They are obligate intracellular symbionts that have an interesting evolutionary history, having evolved from a free-living, phototrophic ancestor. Despite the extensive research interest in this group, novel apicomplexans with important ecological roles are still being discovered, particularly from the marine environment. These often-overlooked microbes infect a myriad of marine organisms, and recent research on this clade has expanded our knowledge of parasite evolution and symbiosis in the ocean. Corallicolids, for instance, have impacted our understanding of plastid evolution and have also been shown to play a role in coral thermal tolerance. Closely related are the ichthyocolids, intracellular fish blood symbionts that were mostly overlooked and misclassified until a phylogenomic investigation showed they are sister to the corallicolids, and incredibly widespread across marine fish diversity. Another recent phylogenomic study similarly resolved a new apicomplexan class, marosporidians, which have been implicated in marine heat wave-triggered mollusk mass mortality events. Given the pace of recent discoveries within this lineage, developing a cohesive framework for studying endosymbiotic apicomplexans is critical. Such an approach will illuminate their hidden biodiversity, clarify their impacts on host health and fitness, and provide the knowledge needed to predict how these symbionts and their hosts will respond to accelerating climate change.