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A Comparison of In Vitro and In Vivo Nitrate Reductase Assays in Three Intertidal Seaweeds

In vitro and in vivo nitrate reductase (NR) activities of three species of intertidal macrophytes (Fucus gardneri, Enteromorpha intestinalis, and Porphyra perforata) were compared. Optimum assay conditions and in vitro enzyme kinetics were also determined. The optimum pH and MgSO4 concentrations for in vitro nitrite production and the optimum n-propanol concentration in the in vivo assay medium were different in all three species. PVP (polyvinyl pyrrolidone) inhibited in vitro NR activity in E. intestinalis but it was required for activity in F. gardneri. In vitro nitrate production was constant over the 45 min assay time but the measured in vivo activity was dependent on the amount of time the thallus had been immersed in the in vivo n-propanol medium. The NR activity of P. perforata showed a much hither affinity for nitrate than the other two species. Growth on ammonium inhibited NR activity in all three species. Freshly collected F. gardneri had no measurable NR activity. However, activity could be induced by incubating the thalli for 36—48 h in ammonium-free nitrate enriched seawater.