Functional and structural characteristics of the photosynthetic apparatus were studied in the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii which were grown semi-continuously under constant irradiance or under simulated natural light fluctuations. The light fluctuations consisted of 24 oscillations of exponentially increasing and decreasing irradiance over a 12-h light period. Maximum irradiance was 1100 mumol photons m(-2) s(-1) with the ratio of maximum to minimum intensities being 100, simulating Langmuir circulations with a ratio of euphotic to mixing depth of 1. S. neoastraea acclimated to the light fluctuations by doubling the number and halving the size of photosynthetic units (PS II) while the amount of chlorophylls and carotenoids remained unchanged. The chlorophyll-specific maximum photosynthetic rate was enhanced while the slope of the photosynthesis versus irradiance curves was not influenced by the light fluctuations. Acclimation of P. agardhii was mainly characterized by an increase in chlorophyll content. Both photosystems showed only little changes in number and size. Maximum photosynthetic rate, saturating irradiance and initial slope of the photosynthesis versus irradiance curves did not vary. Although both high and low light were contained in the fluctuating light, an analogy to low or high light acclimation was not found for the diatom nor for the cyanobacterium acclimated to light fluctuations. We suggest that the acclimation to fluctuating light is a response type outside the known scheme of low and high light acclimation.