The parasite Parvilucifera infectans kills the harmful alga Alexandrium minutum under different salinities and...

Harmful algal blooms (HABs) can be caused by dinoflagellates like Alexandrium minutum, which produces Paralytic Shellfish Poison toxins derived from saxitoxins. The abundance of these toxic algae is shaped not only by abiotic factors, but also by microbial interactions such as parasitism by the alveolate Parvilucifera infectans. These parasites can infect algal cells, inhibit growth, and induce death, thereby contributing to the termination of HABs. Salinity can affect algal physiology and may modulate host-parasite dynamics by altering the parasite's ability to infect and kill HAB-forming algae. Despite their ecological relevance, the interactions between algae and parasites, and the modulation of algal toxins during cell infection at varying salinity, remain poorly understood. Here, we investigated the impact of the parasite P. infectans, a well-recognized model organism that preys on A. minutum under different salinity conditions. We monitored infection success in cultures and recorded metabolome changes during parasite cell invasion using ultra-high-performance liquid chromatography-high-resolution mass spectrometry (UHPLCHRMS). P. infectans tolerated salinity from 20 PSU to 32 PSU and induced the death of whole algal cultures within 3 weeks in laboratory-controlled experiments. Comparative metabolomics revealed that parasite infection decreased the abundance of algal toxins in A. minutum. The parasite infection, rather than salinity, induced significant shifts in the host cell metabolome. Several polar metabolites and osmolytes, such as ectoine, dimethylsulfoniopropionate, glycine betaine, choline, and carnitine, were identified at elevated levels in parasite-infected cells, suggesting potential roles in parasite development and reproduction.