NITRATE UPTAKE IN MARINE PHYTOPLANKTON: (NITRATE,CHILORIDE)-ACTIVATED ADENOSINE TRIPHOSPHATASE FROM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1,2

A membrane-bound (nitrate, chloride)-activated AT Pase was found in the neritie diatom, Skeletonema costatum (Grev.) Cleve. The enzyme is suggested to translocate NO₃^? across the plasmalemma, against a concentration gradient. The K_m of the enzyme with respect to NO₃^? is ca. 0.9 × 10^?6 M, and is in close agreement with the reported K₈ for NO₃^? uptake by the whole cell.