Diel patterns of photosynthate biosynthesis by phytoplankton in permanently ice-covered Antarctic lakes under continuous sunlight

Diel patterns of photosynthate biosynthesis by Antarctic freshwaterphytoplankton growing under the variable but continuous sunlightof summer were found to be similar in many respects to thosereported from other aquatic environments where light/dark periodsalternate. Lipid synthesis by freshwater phytoplankton in LakesVanda and Fryxell predominated during periods when solar radiationand photosynthesis were most intense; the inverse was generallytrue of the protein and polysaccharide fractions. The majorphotosynthetic end-products in both lakes were protein and polysaccharide,which together accounted for 60–81% of the total cellularcarbon incorporation. Less than 4% of the carbon was incorporatedinto lipid in Lake Vanda; >12% appeared in the lipid fractionin Lake Fryxell. The Lake Fryxell populations showed evidenceof photoinhibition of complete photosynthesis during ‘midday’when irradiance was most intense. I_k values, computed from thephotosynthesis irradiance relationships in Lake Fryxell, corroborateother studies suggesting that the phytoplankton populationsin permanently ice-capped Antarctic lakes are among the mostshade-adapted yet reported.