Zoosporic parasites infecting marine diatoms – A black box that needs to be opened

• Zoosporic parasites from at least seven different phyla infect marine diatom hosts.
• Zoospores encyst on the surface of the host and produce specialized infection structures.
• Multiple infection of one host by many zoosporic parasites frequently occurs.
• Zoosporic parasites may impact host population sizes and species composition.
• Species of zoosporic parasites have been very poorly studied in marine ecosystems.

Living organisms in aquatic ecosystems are almost constantly confronted by pathogens. Nevertheless, very little is known about diseases of marine diatoms, the main primary producers of the oceans. Only a few examples of marine diatoms infected by zoosporic parasites are published, yet these studies suggest that diseases may have significant impacts on the ecology of individual diatom hosts and the composition of communities at both the producer and consumer trophic levels of food webs. Here we summarize available ecological and morphological data on chytrids, aphelids, stramenopiles (including oomycetes, labyrinthuloids, and hyphochytrids), parasitic dinoflagellates, cercozoans and phytomyxids, all of which are known zoosporic parasites of marine diatoms. Difficulties in identification of host and pathogen species and possible effects of environmental parameters on the prevalence of zoosporic parasites are discussed. Based on published data, we conclude that zoosporic parasites are much more abundant in marine ecosystems than the available literature reports, and that, at present, both the diversity and the prevalence of such pathogens are underestimated.

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