In the World of Aquatic Parasites, Microsporidia Matter!

Microsporidia are highly diverse, widespread parasites related to fungi and several groups of zoosporic protistan parasites. They infect a broad range of hosts, from protists to vertebrates, in many environments including most, if not all, aquatic habitats. Their genomes range from relatively conventional to highly reduced, yet they share key cellular structures and traits underlying infection and host association. Overall microsporidian diversity, including both 'long-branched' and 'short-branched' lineages, is poorly sampled; their environmental diversity and host range remain therefore largely unexplored. Although short sequences are often insufficient to distinguish between species, recent targeted metabarcoding studies have revealed unexpectedly high microsporidian diversity in both oceanic and lacustrine ecosystems, along with significant spatial and temporal variation. These studies have also identified several previously unrecognized hosts, including heterotrophic protists, phytoplankton and microzooplankton, all key components of the base of aquatic food webs. These findings suggest that Microsporidia, like other parasitic groups, may strongly influence the structure and functioning of aquatic ecosystems. We therefore advocate increased research on Microsporidia across aquatic habitats. Integrative research combining phylogenomics, long-read sequencing and ecology studies will be essential to fully characterize their diversity, host associations and ecological impacts.