PHOTOACCLIMATION IN THE PHOTOTROPHIC MARINE CILIATE MESODINIUM RUBRUM (CILIOPHORA)

Authors

  • Holly V. Moeller,

    1. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA
    Search for more papers by this author
    • Present address: Department of Biology, Stanford University, 371 Serra Mall, Stanford, California 94305, USA.

  • Matthew D. Johnson,

    1. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA
    Search for more papers by this author
    • Present address: Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA.

  • Paul G. Falkowski

    1. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA
      Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA
    Search for more papers by this author

  • Received 24 May 2010. Accepted 13 September 2010.

Abstract

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.

Ancillary