Sea-ice microbial community succession and the potential role of parasitoids in the maintenance of diversity.

Sea ice is a crucial, yet declining, habitat in high latitude ecosystems. Here we present a high-temporal resolution amplicon sequence data set collected during the vernal ice-algal bloom near Utqiaġvik, Alaska in 2021 to study sea-ice microbial dynamics. The ice-algal bloom peaked on May 8th, reaching 46.6 mg chlorophyll a m-2, and thereafter became limited by nitrate availability. A massive bloom of the oil-degrading bacterium, Oleispira (>80% relative abundance), coincided with the algal bloom raising questions about hydrocarbon exposure. The sea-ice algal bloom was dominated by diatoms, particularly Nitzschia spp., and transitioned into a flagellate dominated post-bloom community which aligned with melt-associated irradiance increase and nutrient flushing. We explored the relationship between putative parasitoids, Chytridiomycetes, Thecofilosea (Cercozoa), Oomycetes, Syndiniales (Dinoflagellata), and Labyrinthulomycetes (Bigyra), and potential microalgal hosts. Chytrids peaked periodically suggesting synchronized infections and Cryothecomonas (Thecofilosea) was observed parasitizing Nitzschia spp. for the first time in Arctic sea ice. Co-occurrence analysis suggested that diatoms, especially Nitzschia, were the primary hosts of Arctic parasitoids, and that top-down parasitoid control may dramatically alter community composition over short timescales, such as days. These results provide important insights into the drivers of vernal bloom timing and maintenance of microalgal diversity in sea ice.