The effects of light and temperature on cell size and cellular composition (chlorophyll, protein, carbohydrate) of two freshwater cryptophytes were studied with batch cultures. Neither of the species had a constant cell size but the size varied with growth conditions. At each temperature the smallest cells were recorded at the lowest experimental photon flux density. The smallest cells ofCryptomonas979/67 had an average volume of 232 μm3and the largest ones 1 020 μm3. InCryptomonas979/62 the smallest and largest cells measured 4 306 μm3and 12 450 μm3. Both species increased their cellular chlorophyll content when PFB dropped below 110–120 μmol m‐2s‐1. The highest and lowest chlorophyll contents of 979/67 were 7.45 fg μm‐3and 0.55 fg μm‐2respectively. For 979/62 the corresponding values were 10.23 fg μm‐3and 0.93 fg μm‐3. In both species the protein content remained stable at PFDs higher than 110–120 μmol m‐2S‐1. The highest content of protein measured in 979/67 was 638 fg μm‐3and the lowest 147 fg μm‐3. For 979/62 these values were 1 036 fg μm‐3and 148 fg μm‐3respectively. The carbohydrate results were less clear and no pattern either in response to photon flux density or temperature was obvious. The lowest and highest contents recorded for 979/67 were 62 fg μm‐3and 409 fg
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