Macroalgal Biofilm Harbours a Wide Diversity of Parasitic Protists With Distinct Temporal Dynamics

Marine macroalgae surfaces create a nutrient-rich environment that promotes the formation of epiphyte biofilms. Biofilms are complex systems that facilitate ecological interactions within the community, yet parasitism remains largely unexplored. This study describes the diversity and temporal dynamics of the microeukaryotic community in the biofilm of Mediterranean macroalgae during summer, focusing on parasitic groups. Protist diversity was assessed using metabarcoding sequencing of the V4 region of the 18S rDNA gene using primers biased against metazoans. The macroalgal biofilm exhibited dynamic shifts in the microeukaryotic community structure associated to three phases of biofilm formation. Each phase was characterised by the dominance of specific eukaryotic and parasitic groups with clear successions between them. Our study revealed a high diversity of parasitic protists from different lineages in the macroalgal biofilm. These parasites can infect a wide variety of hosts, including the basibiont, species within the biofilm (micro- and macrocolonizers), nearby marine hosts and terrestrial organisms. The highest diversity and abundance of parasites were found in the mature phase of the biofilm, where the complexity and stability of the system seem to favour parasitism. The parasite assemblage was dominated by Apicomplexa, with many corresponding to unknown diversity, demonstrating that biofilms are a hotspot of unknown parasitic interactions. These parasites could potentially affect the dynamics of these communities and facilitate ecological interactions between the biofilm and surrounding organisms, suggesting that parasitism play a key, but still unexplored role, in shaping complex marine biofilms network.