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Abstract

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 photosynthesis. First, we grew E. huxleyi under ZN₂+ limitation. The K-1/2 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 mumol 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 similar to0.4 pmol L-1, Zn limitation of E. huxleyi growth may indeed occur.