Phytoplankton photoacclimation is a well-known response to changes in light and nutrients, with the intracellular Chlorophyll to Carbon ratio (Θ) increasing at low light and decreasing under high light to optimize growth rate. Accurate quantification of phytoplankton growth rates, Net Primary Production (NPP), and carbon export from space requires careful consideration of Θ. The scientific community needs gap-free estimates of θ for precise global NPP quantification. Current NPP models interpolate Chlorophyll concentration (and other inputs) from clear-sky pixels under clouds, assuming no photoacclimation—an assumption we challenge.   Using data from ≈14,000 BGC-Argo floats and MODIS-derived cloud cover, we compared the response of Θ to various irradiances under both cloudy and clear skies. We found that phytoplankton photoacclimate similarly regardless of sky conditions, at the global, local, and seasonal scales. This study highlights an incorrect assumption in current NPP estimates and suggests ways to improve global assessments.