Asymmetrical interactions between Ulva prolifera with different health status and its epiphytic diatoms and ciliates

The interactions between epiphytic organisms and Ulva prolifera play a crucial role in regulating species communities within pelagic ecosystems and in the dynamics of U. prolifera-dominated green tides. Despite diatoms and ciliates being common epiphytic microorganisms, their relationships with U. prolifera remain poorly understood. In this study, we conducted two experiments to investigate these relationships. Our study identified the diatoms Cylindrotheca closterium, Nitzschia sp., small pennate diatoms (<20 μm), and the euplotid ciliates (Euplotes spp.) as epiphytic microorganisms associated with U. prolifera. Conversely, scuticociliates, although previously reported to increase in abundance during U. prolifera blooms, were not directly associated with U. prolifera. The presence of these diatoms and ciliates did not affect the growth of healthy U. prolifera. However, the health status of U. prolifera differentially affected the diatoms. Healthy U. prolifera promoted the growth of C. closterium by releasing phosphate, but likely suppressed the growth of Nitzschia sp. and small pennate diatoms (<20 μm) through allelopathy. In contrast, decayed U. prolifera provided adhesion substrates and benefited the growth of Nitzschia sp. and small pennate diatoms. Euplotid ciliates likely consumed propagules released by U. prolifera, suggesting a trophic link to the algal reproductive process.