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

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 have revealed that although Syndiniales account for the majority of links associated with mesozooplankton. However, these studies have not determined whether these parasite-zooplankton interactions result from overlapping environmental niches, direct infection, or 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 shaping Syndiniales-zooplankton associations across a brackish-to-marine environmental gradient. Our 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 were associated with all zooplankton species studied. The transmission to zooplankton likely occurs through secondary intake of infected prey, although direct ingestion of dinospores cannot be excluded. In contrast, Syndiniales group IV showed more specific associations, with copepods being the most likely final hosts given the high read abundance detected in copepods and previously described parasitic associations with crustaceans. Low oxygen concentration enhanced associations between Syndiniales group IV and copepods, whereas groups I and II showed the opposite pattern, indicating that environmental variables create distinct ecological 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.