Description: To test the influence of calcification on the exceptional high light tolerance of Emiliania huxleyi calcifying and non-calcifying cells were exposed to light stress and investigated regarding growth, production rates, pigment composition and photosynthetic capacity. For this purpose cells of a calcifying strain of E.huxleyi were acclimated to two different Calciumconcentrations (0.1 mmol 1-1, 10 mmol 1-1), which led to a complete inhibition of calcification at low calcium concentration and common calcification rates at replete calcium concentration. Calcifying and non-calcifying cells were exposed to two different light regimes in two separate experiments. Tue total integrated amount of quanta was equal in both experiments but supplied in a different manner. Tue cells of one experiment were grown in constant light (184 μmol m-2 s-1) whereas the cells of the other experiment were exposed to fluctuating light with low and high light periods (20 and 507 μmol m-2 s-1). lt could be shown that calci:fication does not serve as photoprotective mechanism in E.huxleyi. Neither a physiological effect due to the dissipation of excess energy nor a physical effect by coccoliths shading the cell from light was observable. Non-calcifying cells showed a similar high light tolerance like calcifying cells. Photosynthetic capacity, pigment composition and particulate matter-production in response to light did not show a dependence on the ability to calcify. Tue lowered Ca2+-concentration only had an inhibiting effect on the growth rate. Moreover it could be observed that the modality the light was supplied to the cells had strong impact on the measured parameters although the integrated total amount of quanta was the same. Fluctuating light could be utilized less efficiently by E.huxleyi than constant light regarding growth and photosynthesis. Furthermore cells from fluctuating light showed a tendency to low light acclimation compared to cells from constant light.