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 NO3- 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 mu 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, NO-3-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.