| Summary: | Sea ice microalgal communities are dominated by diatoms and they play important roles in primary production at high latitudes. Growth of microalgae in ice-covered areas is primarily controlled by the seasonal light climate. In Arctic and subarctic seas, more light penetrates through sea ice in early spring than that during the winter time. This increase in light availability is a key driver of bloom in ice-covered seawaters. During ice melt in spring, ice algae are released from sea ice and could be exposed to changeable temperature, salinity and irradiance levels in surface water. Large variations of those environmental factors are likely to influence photophysiological performance of ice algae. Species with greater flexibility in photoacclimation are afforded a higher chance of survival and seed the following phytoplankton bloom. The Sea of Okhotsk is the southernmost sea ice zone in the northern hemisphere with a sizeable seasonal ice cover, thus ice algae of the Sea of Okhotsk have a large potential to seed the early spring diatom bloom in the water column. However, little is known about the Okhotsk ice algal communities and their seeding effects. We investigated the dynamics of the composition and the photophysiological performances of an ice algal community in a 6-day laboratory incubation experiment that simulated the natural ice melt conditions. Centric diatoms, especially Thalassiosira spp., overwhelmingly dominated the ice algal community throughout the incubation, whereas pennate diatoms, mostly Navicula and Nitzschia, showed little growth with much higher mortality. The maximum photochemical efficiency of Photosystem II (Fv/Fm) was the lowest at the beginning of the ice melt, suggesting a suppressed photosynthetic functioning by changes in salinity. The cellular pigment contents decreased by 30% due to cellular damage, evidenced by deformed plastids under a microscope. The transcript level of the rbcL gene that encodes the large subunit of RubisCO was significantly higher during the ice melt and ...
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