PHOTOACCLIMATION IN THE PHOTOTROPHIC MARINE CILIATE MESODINIUM RUBRUM (CILIOPHORA)(1).

Mesodinium rubrum (=Myrionecta rubra), a marine ciliate, acquires plastids, mitochondria, and nuclei from cryptophyte algae. Using a strain of M. rubrum isolated from McMurdo Sound, Antarctica, we investigated the photoacclimation potential of this trophically unique organism at a range of low irradiance levels. The compensation growth irradiance for M. rubrum was 0.5 μmol quanta · m(-2)  · s(-1) , and growth rate saturated at ∼20 μmol quanta · m(-2)  · s(-1) . The strain displayed trends in photosynthetic efficiency and pigment content characteristic of marine phototrophs. Maximum chl a-specific photosynthetic rates were an order of magnitude slower than temperate strains, while growth rates were half as large, suggesting that a thermal limit to enzyme kinetics produces a fundamental limit to cell function. M. rubrum acclimates to light- and temperature-limited polar conditions and closely regulates photosynthesis in its cryptophyte organelles. By acquiring and maintaining physiologically viable, plastic plastids, M. rubrum establishes a selective advantage over purely heterotrophic ciliates but reduces competition with other phototrophs by exploiting a very low-light niche.