CYANOBACTERIAL ASSEMBLAGES IN PERMANENT ICE COVERS ON ANTARCTIC LAKES- DISTRIBUTION GROWTH-RATE, AND TEMPERATURE RESPONSE OF PHOTOSYNTHESIS

The proliferation of microalgae in the McMurdo Dry Valleys of Antarcti ca is intricately linked to the seasonal cycle involving the freezing and melting of water. Anecdotal observations and preliminary sampling have found cyanobacterial cells in ice covers on lakes in the McMurdo Dry Valleys, and several of these ice covers are known to undergo seas onal freeze-thaw cycles. Therefore, we sought to determine the distrib ution and abundance of cyanobacterial assemblages in several permanent ice covers throughout the McMurdo Dry Valleys and to determine their rates of growth and their photosynthetic physiologies upon encounterin g liquid water. We found that the majority of the permanent ice covers contained cyanobacterial assemblages in close association with sedime ntary material. Cyanobacterial biomass was conspicuously absent in sed iment-free ice covers, suggesting that the seasonal interaction betwee n the sediments, ice, and solar radiation present the necessary liquid water environment for cyanobacterial growth. All assemblages exhibite d extremely low rates of photosynthesis when first exposed to liquid w ater. Despite the low rates of photosynthesis, a large proportion (41 %) of the photosynthate was incorporated into protein, indicating that the cells were undergoing efficient net cellular growth. The short-te rm response (24 h) of photosynthesis to a range of temperatures showed optimum rates occurring at temperatures >15 degrees C, which is simil ar to those of psychrotrophic cyanobacteria isolates from soil and str eam habitats, which we believe provides the inoculum for the in- ice h abitats.