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blast searches of expressed sequence tag libraries have revealed putative homologs of the archetypal diatom δ-carbonic anhydrase (CA) TWCA1 (for Thalassiosira weissflogii CA) in a broad range of eukaryotic phytoplankton including haptophytes, prasinophytes and dinoflagellates. Four putative homologs of TWCA1 are also reported and described from a search of the genomic sequence of Thalassiosira pseudonana and designated TWCATp1–4. The δ-CA class is therefore more widely distributed in marine phytoplankton than previously thought. In particular, it is not restricted to the diatoms like the cadmium-containing enzyme, CDCA, seems to be (ζ-CA class). Zinc status strongly influences growth rate in marine diatoms. We observed a decrease in the specific growth rate of T. pseudonana from 2.1 to 1.3 day−1 when the unchelated Zn concentration (Zn′) was lowered from 20 to 5 pM. In the same cultures, we detected a drop in whole-cell CA activity of about 50% in the most Zn-limited cells that occurred simultaneously with a large downregulation of TWCATp1, TWCATp2 and CDCATp gene transcripts and protein. These three genes were also found to be strongly upregulated by low pCO2 in a manner typical of many algal CA enzymes. We have also conducted field experiments in diatom-dominated natural phytoplankton assemblages sampled from Great Bay of the coast of New Jersey. We observed increased total CA activity in parallel with increased expression of homologous twca and cdca gene transcripts in water incubated at increasingly higher pH values. Immunoblot and transcript expression analyses demonstrated a clear upregulation of CDCA transcript and protein as incubation pH increased both in the lab and in the field indicating that this CA is expressed under a broad range of environmental conditions and not restricted to low pCO2 and low Zn.