Changes in Net O₂ Exchange Induced by Inorganic Nitrogen in the Blue-Green Alga Anacystis nidulans

Abstract

The response of net 02 exchange to light intensity by intact Anacystis nidulans cells in the presence of saturating NaHCO3 concentrations followed a curve with an inflection near the light-compensation point. Addition of either KNO3 or NH4CI stimulated 02 uptake in the dark and at light intensities below the light-compensation point. This resulted in steeper slopes of the curve calculated below and above the light-compensation point. At 02 concentrations limiting dark respiration, addition of inorganic nitrogen had no effect on either dark respiration or 02 exchange in the light. The apparent changes in photosynthetic yield observed under normal 02 concentration disappeared when respiration was limited by 02 availability, indicating that the effects of inorganic nitrogen on 02 exchange at low light intensities are due to stimulation of respiration rather than to increases in photosyn-thetic yield. Studies with green and blue-green algae have provided evidence for the photosynthetic nature of nitrate assimilation, showing that illuminated suspensions of intact cells supplied with saturating concentrations of CO2 evolved 02 at a greater rate when nitrate was simultaneously present (7, 11, 16). In Anacystis nidulans, the extent of the stimulation of 02 evolution induced by nitrate was dependent on light intensity, being maximal at saturating photon flux densities. It seemed clear that nitrate enhances the maximum rate of noncyclic electron transport attained when CO2 is the only electron acceptor available (1 1). These data, obtained at relatively high photon flux densities, were, however, insufficient to determine whether nitrate assimilation also affected the yield of photo-synthesis, which should be determined at very low light intensities. This is not easy since at low photon fluxes, respiratory 02 uptake may interfere with net 02 exchange determinations, and further complications arise from the repeatedly observed fact that in algae nitrate and ammonium enhance the rates of respiration and carbohydrate degradation in the dark (2, 4, 9, 15, 19, 20). Actually, a controversy has been maintained for longer than 50 years when considering the validity of extraordinarily high values of photosynthetic yield obtained with ' Supported by the Agricultural and Food Research Council, U.K. 2 Visiting scientist under Acci6n Integrada UK-Spain No. 60/37 and a short-term FEBS fellowship (J. M. R.). Permanent address:

BIOENERGETIC PROCESSES ARE MODIFIED DURING NITROGEN STARVATION AND RECOVERY IN PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

“…These results contrast with those reported for N-sufficient cells ofA. tzidulans, in which neither NH4+ nor NO,modified the initial slope of the light-saturation curve (Romero et al 1989). Decrease of the initial slope after the addition of NH4+ to N-starved cells of P. la~ninosuin could be due to the decrease in the optical cross-section of PSII, as a result of a state 2 transition (Ochoa de Alda et al, unpubl.…”

“…Thus, NO,-assimilation by N-starved cells did not only induce a depression of CO, fixation but also stimulated degradation of stored carbohydrates (Tapia et al 1995b). T h e stimulation of respiration by NO,or NH4+ resupply was much higher than that observed in N-sufficient cells of A. nidulans (Romero et al 1989). This could be due to the higher rates of N assimilation of N-starved cells (Tapia et al 1996a, b) as well as to their greater availability of carbohydrates (Fresnedo and Serra 1992).…”

Nitrate-Dependent O2 Evolution in Intact Leaves

“…In previous work with intact cells of the cyanobacterium Anacystis nidulans, it was shown that, at saturating C02 concentrations, nitrate enhanced the rate of 02 evolution, the extent of this stimulation being maximal at saturating photon flux densities (16), although the apparent quantum yield of 02 evolution remained unchanged (17). In As Figure 1 shows, the rate of light-dependent O2 evolution by young barley leaves photosynthesizing in a saturating CO2 atmosphere was enhanced, for a wide range oflight intensities, when the leaves had previously been supplied with KNO3 through the cut end of the leaf.…”