Blue Light Induction of in situ Nitrate Reductase Activity in the Marine Green Alga Ulva rigida

Abstract

The regulation by light (especially blue light) and NO₃^- of the nitrate reductase (NR) activity was studied in the marine green macroalga Ulva rigida C. Agardh. NR activity was measured in situ as no available in vitro method exists for Ulva rigida. The effectiveness spectrum of NR-enhancement by light was determined. Blue light was clearly the most effective wavelength, being more effective than white or red light. Blue light enhancement of NR was strictly dependent on photosynthesis since it was inhibited by 95% in presence of 5 × 10^-5 M DCMU. The time course of NR enhancement by blue light was hyperbolic in shape, the semimaximal level of increase was reached in 24 min in N-sufficient algae. Blue light activation of NR is dependent upon photo flux density in a sigmoidal manner; 95% of the maximum increase was reached at 137 μmol m^-2 s^-1. Blue light enhancement of NR is linearly dependent on the amount of KNO³ available in the medium at the beginning of the experiment. Increase in NO₃^- by itself, in dark or without a concomitant increase in photon flux density, only increases NR activity by 12% (darkness) or 25% (white light, 4 μmol m^-2 s^-1) with respect to a blue light control. The blue light enhancement of NR was inhibited by actinomycin D (33%), rifampicin (32%) and cycloheximide (88%). Therefore, in Ulva rigida the blue light enhancement of NR occurs mainly by induction of de novo synthesis.