MOLECULAR CHARACTERIZATION AND ANTIBODY DETECTION OF A NITROGEN‐REGULATED CELL‐SURFACE PROTEIN OF THE COCCOLITHOPHORE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE)1

Dissolved organic nitrogen (DON) can account for a significant portion of total nitrogen in some aquatic environments, and many species of phytoplankton are able to scavenge nitrogen from this pool especially when inorganic nitrogen is limiting. Emiliania huxleyi (Lohmann) H. W. Hay et H. Mohler is able to use various forms of DON for growth, including several amino acids, purines, and pyrimidines. A cell‐surface protein up‐regulated in the absence of inorganic nitrogen, NRP1, is hypothesized to play a role in the metabolism of one or more of these organic nitrogen forms. Here, the genomic and cDNA sequence of NRP1 is reported. Structural predictions based on the amino acid sequence suggest a pyridoxal‐5′‐phosphate‐dependent enzyme that may have a role in acquiring nitrogen from amino acids. Further evidence for the function of NRP1 is measured in spent media from nitrogen‐limited cultures, which contain NRP1 and have glutaminase and formamidase activity. Field studies using an antibody to NRP1 show that it is expressed in E. huxleyi during bloom conditions in a Norwegian fjord.