Expression of biomineralization-related ion transport genes in Emiliania huxleyi

Biomineralization in the marine phytoplankton Emiliania huxleyi is a stringently controlled intracellular process. The molecular basis of coccolith production is still relatively unknown although its importance in global biogeochemical cycles and varying sensitivity to increased pCO2 levels has been well documented. This study looks into the role of several candidate Ca2+, H+ and inorganic carbon transport genes in E. huxleyi, using quantitative reverse transcriptase PCR. Differential gene expression analysis was investigated in two isogenic pairs of calcifying and non-calcifying strains of E. huxleyi and cultures grown at various Ca2+ concentrations to alter calcite production. We show that calcification correlated to the consistent upregulation of a putative HCO3- transporter belonging to the solute carrier 4 (SLC4) family, a Ca2+/H+ exchanger belonging to the CAX family of exchangers and a vacuolar H+-ATPase. We also show that the coccolith-associated protein, GPA is downregulated in calcifying cells. The data provide strong evidence that these genes play key roles in E. huxleyi biomineralization. Based on the gene expression data and the current literature a working model for biomineralization-related ion transport in coccolithophores is presented.