Diet composition and environmental niche drive parasitic Syndiniales interactions with crustacean zooplankton

Abstract Syndiniales are a group of parasitic dinoflagellates belonging to marine alveolates that infect a wide range of planktonic taxa, including other protists and metazoans. Sequence-based correlation networks reveal that Syndiniales account for the majority of links associated with meso-zooplankton but do not differentiate whether the parasite-zooplankton interactions result from overlapping environmental niches or are the result of a direct, infection or an indirect transmission via intake of infected prey items. To address these open, questions, we identified associations between Syndiniales and zooplankton from selected, copepod and cladoceran taxa using DNA metabarcoding. We further identified the role of, prey consumption and environmental factors in driving Syndiniales-zooplankton, associations across a brackish-marine environmental gradient. Results show that Syndiniales, are highly associated with zooplankton, but their association varies and is shaped by diet, and environmental conditions. Syndiniales groups I and II interact with all zooplankton, species studied. The transmission to zooplankton is likely through secondary intake of, infected prey, although direct intake of dinospores cannot be excluded. However,, Syndiniales group IV shows more specific interactions, with copepods being most likely final, hosts, given the high read abundance associated with copepods and known parasitic, associations of some taxa in crustaceans. Low oxygen concentration enhances Syndiniales, group IV and copepod associations, while groups I and II show the opposite response,, indicating that environmental variables create unique niches for both the hosts and the, parasites, driving niche-specific associations. These findings suggest that parasitic, Syndiniales play a key role in planktonic food webs by redirecting carbon cycling. 22.