Temperature and light drive physiological and transcriptional responses, modulating infection outcomes in a freshwater diatom–chytrid system

Abstract Stephanodiscus binderanus is a bloom‐forming diatom abundant in winter and persisting into spring in the Laurentian Great Lakes. Climate change impacts these blooms by altering ice cover, turbidity, light penetration, and water temperature. Fungal parasites, especially Chytridiomycota , can suppress phytoplankton growth and alter bloom succession. To address the effects of both biotic and abiotic factors on S. binderanus in the face of a changing climate, we tested a range of temperatures (9.4–24°C) and light intensities (15, 30, 50, 100 μ mol m −2 s −1 ) on infected and uninfected cultures. We also conducted an RNAseq analysis of both host and parasite across the described environmental parameters. Stephanodiscus binderanus can rapidly adapt to the above conditions, growing slowest at the lowest temperature and under low light, while adjusting its chlorophyll a (Chl a ) content in lower light treatments to be more efficient at light harvesting. Chytrid infections were more prevalent at either end of the tested temperature range, despite downregulation of zoospore mitotic cycle genes at elevated temperatures. Elevated temperatures also induced reproductive stress in S. binderanus , marked by downregulation of meiosis‐related genes. These effects, combined with nutrient depletion, likely contribute to seasonal declines in diatom populations as green algae and cyanobacteria emerge in late spring and early summer. It is anticipated that host response to lower light availability and the ability of the chytrid to infect under warming waters will contribute to a decline in filamentous diatom biomass in Lake Erie, especially as climate change increases the frequency of ice‐free winters.