Functional relation between growth, photosynthetic rate and regulation in the coastal picoeukaryote Phaeomonas sp. RCC 503 (Pinguiophyceae, Stramenopiles)

Picoeukaryotes constitute good models for testing photophysiological hypotheses because of constraints as well as opportunities related to their minute size. This study was undertaken to investigate the relation between growth rate and regulation of photosynthesis in a coastal picoeukaryote, using the strain Phaeomonas sp. RCC 503 as a model. Here we address how photoacclimation responses affect photosynthetic capacity and how growth rate is modulated by photoacclimation and photosynthetic rate variations. Growth of the strain was followed over 5 days under five sinusoidal light regimes set to peak at maximal photon flux densities (PFDs) of 10, 50, 100, 250 and 500 μmol photons m - 2 s -1 . We measured growth rate, pigment composition, variable fluorescence, non-photochemical quenching, electron transport rate, absorption spectrum, cell carbon and nitrogen content one to three times per day under each of these five conditions. Results suggest that Phaeomonas sp. RCC 503 is a light-adapted species, showing a high physiological plasticity allowing it to grow under a broad range of PFD conditions. The PFD determined the growth rate, the latter being correlated significantly to photosynthetic capacity. Biochemical properties of cells, in term of carbon and nitrogen content were closely correlated to growth rate and PFD. Photoacclimation allows growth to be maintained at low irradiance except at 10 μmol photons m -2 s -1 , whereas photoprotection efficiency allows enhancement of growth at high PFDs.