Species-Specific Exudates From the Harmful Algae Alexandrium minutum and Prymnesium parvum Drive the Structure of a Synthetic Microbial Community

In this study, we examined the role of species-specific algal exudates in shaping bacterial community structure and interactions using synthetic microbial communities co-cultured with Alexandrium minutum and Prymnesium parvum. Cell-free algal exudates supported bacterial growth as the sole carbon source and revealed distinct exometabolomes unique to each algal species. These exometabolomes selectively influenced bacterial community composition, even among common copiotrophic taxa. Furthermore, co-culture experiments highlighted that the presence of algal cells drove further bacterial assembly, particularly within particle-attached communities, emphasising the role of close algal–bacterial interactions in structuring microbial consortia. Metabolomic analyses showed significant modulation of algal exudates by bacteria, with axenic cultures containing a broader and more diverse range of metabolites. This suggests microbial consumption, degradation or suppression of metabolite production under xenic conditions. Importantly, we observed a bacterial-mediated increase in extracellular paralytic shellfish toxins (PSTs) in A. minutum cultures. Exudates of both algal species displayed hemolytic activity, which was not affected by the presence of bacteria, suggesting bioactive extracellular compounds (BECs) production does not rely on bacteria. These findings underscore the complex and dynamic nature of algal–bacterial interactions, with implications for nutrient cycling, toxin dynamics and harmful algal bloom ecology.