Environmental triggers of resting spore germination in the chytrid parasite Staurastromyces oculus

Many parasites produce resting spores to survive host absence or adverse conditions. Insights into the environmental triggers for switching between active and resting phases is crucial for understanding host-parasite dynamics in nature. Zoosporic fungi from the Chytridiomycota phylum, known as chytrids, are widespread parasites of phytoplankton, impacting bloom dynamics and influencing key processes in aquatic ecosystems. Though chytrids produce resting spores, the triggers for their germination remain poorly understood. A previous study found that germination could be induced in cold, dark-stored spores by exposing them to increased temperature and light. To disentangle the effects of temperature, light, and host presence, we conducted a microcosm experiment with the phytoplankton-chytrid model system Staurastrum sp. – Staurastromyces oculus . Resting spores were stored for 3 months at 4°C or 16°C in darkness or light. They were then exposed to combinations of temperature (4°C or 16°C), light (light or dark), and host presence (with or without host). Using automated fluorescence microscopy, we imaged and quantified resting spores, germination, and new infections daily for the first 5 days, and at days 7, 10, and 14. Our preliminary results indicate that an increase in temperature following 4°C storage is a primary trigger for germination, while light does not appear to significantly influence the process, as infections were observed even in dark conditions. Interestingly, the highest infection rate occurred in spores stored and incubated at 16°C with light. These findings suggest that multiple pathways can induce germination in chytrid resting spores, primarily through temperature shifts or host presence.