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Enhanced or reduced pCO₂ (partial pressure of CO₂) may affect the photosynthetic performance of marine microalgae since changes in pCO₂ can influence the activity of carbon concentrating mechanisms, modulate cellular RuBisCO levels or alter carbon uptake efficiency. In the present study we compared the photophysiology of the Antarctic diatom Chaetoceros brevis at two pCO₂ extremes: 750ppmv (2× ambient) and 190ppmv (0.5× ambient) CO₂. Cultures were acclimated to four irradiance regimes: two regimes simulating deep or shallow vertical mixing, and two regimes mimicking limiting and saturating stable water column conditions. Then, growth rate, pigmentation, RuBisCO large subunit expression, RuBisCO activity, photosynthesis vs irradiance curves, effective quantum yield of PSII (F_v/F_m), and POC were measured. The four irradiance regimes induced a suite of photophysiological responses, ranging …