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Reaching up to 50% of the total biomass in oligotrophic waters and armed with a set of ecological and biological properties related to their small size, picophytoplankton (<3.0 µm) are a good model to address ecophysiological questions regarding phytoplankton biodiversity. Two picoplanktonic diatoms, one isolated from an upwelling ecosystem in the Pacific Ocean (Minutocellus sp., strain RCC967), and another from oceanic waters in the Indian Ocean (Minutocellus sp., strain RCC703) were used to test hypotheses on the functional relation between ecological niche adaptation and photosynthetic regulation capacity and efficiency. Cultures were subjected to five sine light climates, each one set to peak at a different photon flux density, respectively 10, 50, 100, 250 and 500 µmol photons m⁻²s⁻¹. Growth rate, photosynthesis, non-photochemical fluorescence quenching, pigment composition, and particulate …