Assimilatory nitrate reductase (NR) was purified from the marine diatom Skeletonema costatum (clone Skel) using Cibacron blue-Sepharose affinity chromatography. The single-step purification scheme yielded a 103-fold purification of specific activity with an overall recovery of 40.8%. Only NADH-dependent NR activity (form EC 1.6.6.1) was observed in this species. Kinetic analysis revealed that this form had apparent Michaelis constants of 3.6 [mu]M for NADH and 295 [mu]M for NO3- when purified from cells grown in NO3--enriched seawater. The S. costatum NR exhibits a pH optimum of 7.4, a temperature optimum of 14[deg]C, and enzyme activity not sensitive to Mg2+ inhibition. The strong temperature dependence of NR activity in S. costatum may contribute to the seasonal and latitudinal distributions and abundances of this bloom-forming species. Chromatographically isolated NR was further purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, yielding a single polypeptide with an apparent molecular mass of 110 kD. The 110-kD polypeptide was used to generate polyclonal antibodies. The antiserum recognized a single 110-kD polypeptide in western blots of total proteins from S. costatum, as well as the native enzyme. Western blot analysis also revealed an antigenic similarity of NR from two additional diatom species, whereas no cross-reactivity was observed with NR from other phytoplankton taxa, including prymnesiophytes, dinoflagellate, cyanobacterium, and green alga. This result suggests a structural diversity of NR in phytoplankton and identifies the potential for development of taxon-specific NR antisera for ecological studies.