In analogy to the Fe hypothesis, the Zn hypothesis states that Zn may limit primary production in some regions of the world oceans and therefore influence the global carbon cycle. The proposed mechanism is via carbon limitation due to a lack of the cofactor Zn in carbonic anhydrase. In the current conceptual model for the use of inorganic carbon by E. huxleyi, carbonic anhydrase in the chloroplast generates CO2 from HCO3− at the site where CO2 is fixed by ribulose bisphosphate carboxylase oxygenase (Rubisco). The H+ that is required in this reaction comes from calcification. From this it can be expected that carbonic anhydrase affects the use of HCO3− in photo-synthesis. First, we grew E. huxleyi under Zn2+ limitation. The K1/2 for growth of E. huxleyi is 19 ± 8 pmol L−1 Zn2+ with a minimum requirement of 9 ± 3 pmol L−1. Additions of both ethylenediaminetetraacetic acid (EDTA) and ZnCl2 show that EDTA is not detrimental to E. huxleyi up to a concentration of 200 µmol L−1. Then we grew E. huxleyi under Zn2+-HCO3− colimitation to test the conceptual model outlined above. The results were partly inconsistent with the model. Contrary to what was expected from the conceptual model, the efficiency of CO2 use decreased when both Zn2+ and HCO3− concentrations were low, even though the experiment was conducted at a constant high concentration of CO2. This shows that Zn2+, and possibly carbonic anhydrase activity, are needed for CO2 fixation also. In accordance with the model, we found that Zn2+ affects the efficiency of HCO3− use by E. huxleyi. Since the lowest Zn2+ concentration in the Northeast Pacific is ~0.4 pmol L−1, Zn limitation of E. huxleyi growth may indeed occur.