Temperature modulates infection-associated metabolic responses in bloom-forming freshwater diatoms.

Warmer waters are reshaping unseen interactions at the base of aquatic food webs, with potential consequences for microalgal bloom dynamics and carbon transfer to higher trophic levels. Two major groups of aquatic parasites-chytrids and oomycetes-infect the same algal host, and changes in temperature can alter the dynamics of these infections and determine their success. Using laboratory-controlled synthetic communities, we investigated the effect of temperature on chytrid and oomycete infection dynamics in the bloom-forming freshwater diatom Ulnaria ulna. Our findings reveal complex dynamics, with temperature shaping infection prevalence and host growth differently between the two infection systems. Notably, chytrid infection prevalence increased at lower temperatures, challenging the general applicability of the cold-water refuge theory in this host-parasite system, whereas oomycete infection dynamics showed a contrasting temperature response. Using UHPLC-HRMS analysis, we identified significant metabolic changes associated with parasite infection, while temperature further modulated these responses differently in chytrid- and oomycete-infected cultures. Chytrid infections were associated with metabolites potentially linked to stress- or defense-related responses, whereas oomycete infections exhibited metabolic signatures potentially associated with nutrient scavenging and manipulation of host metabolism. These different metabolic responses suggest that warming not only alters parasite success, but also modifies the biochemical composition of infected host-parasite systems, with possible implications for trophic transfer efficiency and carbon cycling. Together, these findings establish a conceptual framework linking temperature with infection-associated metabolic changes and provide first insights into how temperature affects parasitic infections of microalgae and their biochemical consequences for aquatic food webs.