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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 …