INORGANIC NITROGEN-METABOLISM IN ULVA-RIGIDA ILLUMINATED WITH BLUE-LIGHT

Inorganic nitrogen metabolism in blue light was studied for the green alga Ulva rigida C. Agardh collected in the south of Spain (Punta Carnero, Algeciras) in the winter of 1987. NH4+ has been reported to inhibit NO3- uptake; however, U. rigida showed a net NO3- uptake even when the NH4+ concentration of the external medium was three or four times greater than the concentration of NO3-.NO3- uptake rates were similar in both darkness and in blue light of various photon fluence rates (PFR) ranging from 17 to 160-mu-mol m-2 s-1. Since NO3- uptake is an active mechanism involving the consumption of ATP, respiratory metabolism can provide enough ATP to maintain the energetic requirement of NO3- transport even in darkness. In contrast, NO3 reduction in U. rigida was highly dependent on the net photosynthetic rate. After 7 h in blue light, intracellular NO3- concentrations ([NO3-]i) were higher in specimens exposed to intensities below the light compensation-point (LCP) than in those incubated at a PFR above the LCP. When PFR is below the light compensation point, NO3- reduction is low, probably because all the NADH produced by the cells is oxidized in the respiratory chain in order to produce ATP to maintain a steady NO3- transport rate. The total nitrogen (TN) and carbon (TC) contents decreased from darkness to 33-mu-mol m-2 s-1 in blue light. In this range, catabolic processes prevailed over anabolic ones. In contrast, increases m N sd TC contents were observed above the light compensation point. The C: N ratio increased with light intensity, reaching a stable value of 17 at 78-mu-mol m-2 s-1 in blue light. Intracellular NO3- concentration and NO3- reduction appear to be directly controlled by light intensity. This external control of [NO3-]i and the small capacity of U. rigida to retain incorporated NO3-, NO2- and NH4+ ions may explain its nitrophilic character.