Inorganic phosphorus (Pi) and carbon (here, CO2) potentially limit the photosynthesis of phytoplankton simultaneously (colimitation). A single Pi limitation generally reduces photosynthesis, but the effect of a colimitation is not known. Therefore, photosynthesis was measured under Pi-limited conditions and high and low CO2, and osmo-mixotrophic (i.e., growth in the presence of glucose) conditions that result in colimiting conditions in some cases. The green alga Chlamydomonas acidophila Negoro was used as a model organism because low Pi and CO2 concentrations likely influence its photosynthetic rates in its natural environment. Results showed a decreasing maximum photosynthetic rate (Pmax) and maximum quantum yield (ΦII) with increasing Pi limitation. In addition, a Pi limitation enhanced the relative contribution of dark respiration to Pmax (Rd:Pmax) but did not influence the compensation light intensity. Pmax positively correlated with the cellular RUBISCO content. Osmo-mixotrophic conditions resulted in similar Pmax, ΦII, and RUBISCO content as in high-CO2 cultures. The low-CO2 cultures were colimited by Pi and CO2 and had the highest Pmax, ΦII, and RUBISCO content. Colimiting conditions for Pi and CO2 in C. acidophila resulted in an enhanced mismatch between photosynthesis and growth rates compared to the effect of a single Pi limitation. Primary productivity of colimited phytoplankton could thus be misinterpreted.