UPTAKE OF NITRATE AND NITRITE BY DITYLUM BRIGHTWELLII‐KINETICS AND MECHANISMS12

Abstract

Ditylum brightwellii grown on NO₂- as a nitrogen source took up and assimilated NO₂- only in the light, apparently via a photosynthetic nitrite reductase. Assimilation was inhibited by dichlorophenyldimethylurea (DCMU), KCN, partially by 2,4 dinitrophenol, and by NO₃-. Kinetics of inhibition of NO₂- assimilation by NO₃- appeared to be “competitive.” D. brightwellii cells grown on NO₂- took up NO₃- in both light and dark and in both cases the uptake was inhibited by p-chloromercuribenzoate, but not by DCMU, KCN, or by NO₂-. Most of the NO₃- taken up in the dark was recovered unchanged from the cells. However only 40% of NO₃- taken up in light was recovered from the cells and no NO₂- was found. This suggests that a photosynthetic nitrate reduction mechanism was active in these cells. DCMU inhibited the light-induced NO₃- reduction. This mechanism of NO₃- reduction is distinct from that involving NADH nitrate reductase in D. brightwellii since the concentration of the latter enzyme is very low in cells grown on NO₂-. Saturation kinetics were observed for NO₂- and NO₃- uptake. Half-saturation concentrations (K_svalues) were 4 and 2 μM, respectively. These values are compared with those obtained for NO₂- and NO₃- assimilation by other unicellular algae. The comparisons show lower K_svalues in oceanic species compared with tide-pool or freshwater algae and they support the idea that K_svalues for NO₃- assimilation may provide a key to understanding species succession when this is due to declining: nitrate concentrations in the sea.