Marine Biology(1994) 120:17-23 9 Springer-Verlag 1994 A. Corzo. F. X. Niell Nitrate.reductase activity and in vivo nitrate-reduction rate in Ulva rigida illuminated by blue light Received: 10 December 1993 / Accepted: 25 February 1994 Abstract 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 NO 3 pulse (0.3 raM) on relevant var- iables of NO3-assimilation such as NO3-uptake, intracel- lular NO3-storage, NR activity, in vivo NO;--reduction rate and NO 2 and NH~--accumulation. Nitrate uptake started immediately upon addition of NO3, suggesting the pres- ence of a constitutive carrier, however in the first 1.5 to 2 h, periods of net NO 3 effiux 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 in- ducible. 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 NO 3 pulse (300 ~tM) there were clear indications that the induction state of the N03-carrier limits the reduction rate of NO3. Once the in- duction of the N03-transporter had been completed (1.5 to 2 h), the NO3-assimilation pathway reached a steady state, N0y-uptake rate, NO3-reduction rate and N02 and NH~-- accumulation being linear with time. Since the reduction of NOy leads mainly to the accumulation of NHJ, we con- clude that, after the NO~--reduction itself, NH~--fixation into carbon skeletons is the limiting step in the assimila- tion of NO~- by U. rigida under blue light. Communicatedby J. M. Pdr~s,Marseille A. Corzo ([]) Departamento de Biologfay Ecologia, Facultad de Cienciasdel Mar, Universidad de Cfidiz, E- 11510 Puerto Real, Spain F. X. Niell Departamento de Ecologia, Universidad de Mfilaga, Campus Universitariode Teatinos s/n, E-2907 ] M~ilaga, Spain Introduction Blue light acts on a number of physiological processes, triggering different morphogenetic responses in fungi, higher plants and algae, e.g. the development of function- ally active chloroplasts, the formation of whorls and caps in the genus Acetabutaria, and many other processes (Sen- get 1984; Galland and Senger 1988). As protein synthesis is required for the completion of all photomorphogenic re- sponses, it is of interest to study the assimilation of NO 3 under blue light. In addition, nitrate reductase (NR), a key enzyme in the inorganic nitrogen metabolism, is inducible by blue light, involving de novo NR synthesis in higher plants (Rao et al. 1982). Additionally, NR from green al- gae, fungi and higher plants may be activated by blue light in vitro (Aparicio et al. 1976; Rold~n and Butler 1980; Ar- yan et al. 1983; Fritz and Ninnemann 1985). The enzyme- bound FAD (flavin adenine dinucleotide) appears to be the photoreceptor, photoreactivation being caused by the ex- cited triplet flavins (Fritz and Ninnemann 1985; Maldo- nado and Aparicio 1987). The same mechanism has been suggested to exist in vivo (Maldonado and Aparicio 1987). Despite the economical importance of marine macroalgae both as food for human consumption and as a source of phycocolloids, the information available on the regulation of the key enzymes of inorganic nitrogen metabolism such as nitrate reductase is so far very limited. According to our previous work (Corzo and Niell 1992b), in Ulva rigida, blue light-enhancement of NR occurs mainly through the induction of de novo NR synthesis, since the response is inhibited by cycloheximide. Actinomycin D and rifampi- cin are considerably less effective, suggesting that a mRNA pool coding for NR is already present. The accomplish- ment of NR-enhancement by blue light is dependent on ei- ther newly synthesized carbohydrates, reducing power, or ATP-derived from photosynthesis, since it is inhibited by 3-(3,4-dichlorophenyl)- 1,1-dimethylurea (Corzo and Niell 1992b). Nitrate assimilation involves the uptake of NO 3 from the medium, the reduction of NO 3 to NH~-Catalyzed by the