Light affects the cellular iron (Fe) requirement of phytoplankton because of its presence in major photosynthetic proteins. Thus, interactions between variable Fe concentrations and light intensities could restrict photosynthetic carbon fixation in the ocean. Additionally, CO₂-driven climate change affects light intensity and iron (Fe) availability in surface seawaters, two critical factors for marine phytoplankton carbon fixation and growth, as they play a central role in regulating photosynthesis. Here we show the interactive effects of Fe, light, and CO₂ on a key marine photosynthetic picoeukaryote, Ostreococcus lucimarinus. We cultured O. lucimarinus under various Fe, light, and CO₂ treatments to examine photosynthesis, growth, and protein expression. We find that O. lucimarinus is a “low iron requiring strain”, which acclimates efficiently to low iron conditions. Meanwhile, light and iron regulate its positive effect on elevated CO₂ levels. Increased partial pressure of CO₂ from its current value (400 μatm) to 750 μatm had no significant effect at low-light intensity, but carbon fixation and growth rates increased with light intensity, and had an even greater positive effect in the Fe-limited and high-light intensity.