Distinct differences in photoacclimation potential between prokaryotic and eukaryotic oceanic phytoplankton

Six (pico)phytoplankton strains typical for the open oligotrophic oceans were acclimated to different irradiance regimes mimicking stable (ranging from 10 to 125 mu mol photons m(-2) s(-1)) and dynamic (averaging at 50 and 125 mu mol photons m(-2) S-1) water column conditions. Photoacclimation potential of the different phytoplankton species was assessed by analysis of specific growth rates, pigment composition, pigment absorption, elemental composition, and photosynthetic characteristics. Results showed distinct differences between the studied prokaryotic species, Prochlorococcus marinus and Synechococcus sp. (two strains), and the eukaryotic species. Ostreococcus sp., Emiliania huxleyi, and Thalassiosira oceanica. Based on growth and photosynthetic characteristics, the photoacclimation potential of the eukaryotic species was significantly higher compared to that of the prokaryotic species under high and dynamic irradiance conditions. Likewise, the eukaryotic species performed better than the prokaryotic species after photoacclimation to low irradiance conditions. No consistent differences between constant and dynamic irradiance treatments were found. Differences in pigment composition, for example the presence of a xanthophyll cycle, may have played an important role in the success of photoacclimation of the studied phytoplankton species. These results imply that the high photoacclimation potential of eukaryotic oceanic phytoplankton offers a selective advantage over prokaryotic phytoplankton, in both the upper mixed layer and the deep chlorophyll maximum. Thus, factors other than photoacclimation potential, for example low nutrient availability, are likely to explain the high abundance of prokaryotic picophytoplankton in the open oligotrophic oceans.