Nitrate-reductase activity and in vivo nitrate-reduction rate in Ulva rigida illuminated by blue light

In the marine green alga Ulva rigida C. Agardh, nitrate reductase (NR) is synergetically induced by blue light and nitrate. The present study examines the effect of blue light and a large NO₃^–pulse (0.3 mM) on relevant variables of NO₃^–-assimilation such as NO₃^–-uptake, intracellular NO₃^–-storage, NR activity, in vivo NO₃^–-reduction rate and NO₂^–and NH₄⁺-accumulation. Nitrate uptake started immediately upon addition of NO₃^–, suggesting the presence of a constitutive carrier, however in the first 1.5 to 2 h, periods of net NO₃^–efflux were frequent. After this time, NO₃^–-uptake and intracellular NO₃^–-accumulation proceeded linearly with time, suggesting the existence of a different NO₃^–-uptake mechanism, which seems to be inducible. Our results indicate that in vivo NO₃^–-reduction is not exclusively dependent on the potential NR activity. In U. rigida, during the first 2 h after a NO₃^–pulse (300 M) there were clear indications that the induction state of the NO₃^–-carrier limits the reduction rate of NO₃^–. Once the induction of the NO₃^–-transporter had been completed (1.5 to 2 h), the NO₃^–-assimilation pathway reached a steady state, NO₃^–-uptake rate, NO₃^–-reduction rate and NO₂^–and NH₄⁺-accumulation being linear with time. Since the reduction of NO₃^–leads mainly to the accumulation of NH₄⁺, we conclude that, after the NO₃^–-reduction itself, NH₄⁺-fixation into carbon skeletons is the limiting step in the assimilation of NO₃^–by U. rigida under blue light.