The effect of phosphate and flavin adenine dinucleotide on nitrate reductase activity of some unicellular marine algae

Nitrate reductase (NR) activity was measured in cell-free extracts from seven species of unicellular, marine algae. For most species Tris buffer was not a satisfactory assay buffer unless supplemented with phosphate. With some species (e.g. Brachiomonas submarina Bohlin, Nannochloropsis oculata Droop, Phaeodactylum tricomutum Bohlin) maximum NR activity required the addition of phosphate and flavin adenine dinucleotide (FAD) to assays; with extracts of P. tricornutum, the requirement for FAD increased after partial purification of extracts. Like phosphate, arsenate stimulated NR activity in extracts, but selenate, sulphate, and tungstate were less effective. Addition of manganese to partially purified extracts of P. tricomutum resulted in an inhibition of NR activity which was alleviated by phosphate or EDTA.