Selenastrum minutum (Naeg.) Collins was grown over a wide range of growth rates under phosphate or nitrate limitation with non‐limiting nutrients added to great excess. This resulted in saturated luxury consumption. The relationships between growth rate and cell quota for the limiting nutrients were well described by the Droop relationship. The observed variability in N cell quota under N limitation as reflected in kQ·Qmax−1*, was similar in magnitude to previously reported values but kQ·Qmax−1* for P under P limitation was greater than previously reported for other species. These results were evaluated in light of the optimum ratio hypothesis. Our findings support previous work suggesting that the use of a single optimum ratio (kQi·KQj−1) is inappropriate for dealing with a species growing under steady‐state nutrient limitation. Under these conditions the optimum ratio should be viewed as a growth rate dependent variable. Two approaches for testing the growth rate dependency of optimum ratios are proposed. The capacity for luxury consumption differed between nutrients and was growth rate dependent. At low growth rates, the coefficient of luxury consumption (Rsat) for P was ca. four times that for N. The set of all possible relationships between N and P cell quota under these conditions was reported and these values were then used to establish the cellular N:P niche boundaries for S. minutum. Cell quotas of non‐limiting nutrients were not described by the Droop equation. Analysis showed that as the cellular N:P ratio deviates from the optimum ratio, the ability of the Droop equation to describe the relationship between growth rate and non‐limiting cell quotas decreases. When non‐limiting nutrient cell quotas are saturated, the Droop equation appears to be invalid. Previously reported patterns of non‐limiting nutrient utilization are summarized in support of this conclusion. The physiological and ecological consequences of luxury consumption and growth rate dependent optimum ratios are considered.

The assimilation of nitrogen by N-limited microalgae has profound effects on respiratory and photosynthetic metabolism. The addition of inorganic nitrogen causes a rapid increase in the rate of amino acid synthesis, which increases the requirements for keto-acids. This results in a large increase in the demand for tricarboxylic acid cycle intermediates. To meet this demand, tricarboxylic acid cycle activity increases, resulting in high rates of respiratory CO2 release during photosynthesis. Tricarboxylic acid cycle reductant, produced during ammonium assimilation, is oxidized via the mitochondrial electron-transport chain, resulting in a substantial increase in the rate of O2 consumption during photosynthesis. When [Formula: see text] is assimilated, tricarboxylic acid cycle activity increases, but there is little effect on mitochondrial O2 consumption. This implies that the tricarboxylic acid cycle reductant produced during [Formula: see text] assimilation is oxidized by some mechanism other than the mitochondrial electron-transport chain, possibly through the reduction of [Formula: see text].These results show that both the tricarboxylic acid cycle and the mitochondrial electron-transport chain are capable of operation during photosynthesis and that a major role of mitochondrial respiration during photosynthesis is the provision of carbon skeletons for biosynthetic reactions. The increase in tricarboxylic acid cycle activity during nitrogen assimilation is supported by anaplerotic reactions. The requirement for substrates by these reactions causes a redirection of recent photosynthate from the synthesis of starch to glycolysis and the tricarboxylic acid cycle. This corresponds with a decrease in the concentration of ribulose bisphosphate in the chloroplast. Under some conditions the concentration of ribulose bisphosphate drops below the ribulose bisphosphate binding site density of ribulose bisphosphate carboxylase:oxygenase resulting in ribulose bisphosphate limitation of photosynthetic carbon fixation. When ammonium is the added N source, there is a corresponding decrease in gross photosynthetic oxygen evolution. When [Formula: see text] is added, the decreased demand for photogenerated reductant brought about by a decrease in Calvin cycle activity is offset by an increase in electron flow to [Formula: see text].

The effects of phosphorus nutrition on several physiological and biochemical parameters of the green alga, Selenastrum

Nitrate-limited chemostat cultures of Selenastrum minutum Naeg. Collins (Chlorophyta) were used to determine the effects of nitrogen addition on photosynthesis, dark respiration, and dark carbon fixation. Addition of N03-or NH4' induced a transient suppression of photosynthetic carbon fixation (70 and 40% respectively). Intracellular ribulose bisphosphate levels decreased during suppression and recovered in parallel with photosynthesis. Photosynthetic oxygen evolution was decreased by N-pulsing under saturating light (650 microeinsteins per square meter per second). Under subsaturating light intensities (<165 microeinsteins per square meter per second) NH4W addition resulted in 02 consumption in the light which was alleviated by the presence of the tricarboxylic acid cycle inhibitor fluoroacetate. Addition of N03-or NH4W resulted in a large stimulation of dark respiration (67 and 129%, respectively) and dark carbon fixation (360 and 2080%, respectively). The duration of Ninduced perturbations was dependent on the concentration of added N. Inhibition of glutamine 2-oxoglutarate aminotransferase by azaserine alleviated all these effects. It is proposed that suppression of photosynthetic carbon fixation in response to N pulsing was the result of a competition for metabolites between the Calvin cycle and nitrogen assimilation. Carbon skeletons required for nitrogen assimilation would be derived from tricarboxylic acid cycle intermediates. To maintain tricarboxylic acid cycle activity triose phosphates would be exported from the chloroplast. This would decrease the rate of ribulose bisphosphate regeneration and consequently decrease net photosynthetic carbon accumulation. Stoichiometric calculations indicate that the Calvin cycle is one source of triose phosphates for N assimilation; however, during transient N resupply the major demand for triose phosphates must be met by starch or sucrose breakdown. The effects of N-pulsing on 02 evolution, dark respiration, and dark C-fixation are shown to be consistent with this model.

Mass spectrometric analysis of 02 and CO2 exchange in the green alga Selenastrum minutum (Naeg. Collins) provides evidence for the occurrence of mitochondrial respiration in light. Stimulation of amino acid synthesis by the addition of NH4Cl resulted in nearly a 250% increase in the rate of TCA cycle CO2 efflux in both light and dark. Ammonium addition caused a similar increase in cyanide sensitive 02 consumption in both light and dark. Anaerobiosis inhibited the CO2 release caused by NH4Cl. These results indicated that the cytochrome pathway of the mitochondrial electron transport chain was operative and responsible for the oxidation of a large portion of the NADH generated during the ammonium induced increase in TCA cycle activity. In the presence of DCMU, ammonium addition also stimulated net 02 consumption in the light. This implied that the Mehler reaction did not play a significant role in 02 consumption under our conditions. These results show that both the TCA cycle and the mitochondrial electron transport chain are capable of operation in the light and that an important role of mitochondrial respiration in photosynthesizing cells is the provision of carbon skeletons for biosynthetic reactions.Mitochondrial respiration consists of two distinct but related processes. The first is the oxidation of organic acids by the TCA cycle2 and the production of NADH and FADH2, C02, and carbon skeletons which are then available for subsequent oxidation or use in biosynthetic reactions. The second process is the oxidation of NADH and FADH2 via the cytochrome or alternate electron transport chains of the mitochondrion. This results in 02 consumption and the production of ATP. In photosynthetic organisms it has been thought that the light reactions of photosynthesis provide ample ATP and reducing equivalents for metabolism in the light. As a result, a major role of mitochondrial respiration in the light should be the provision of TCA cycle intermediates for biosynthetic reactions (10).The occurrence of mitochondrial respiration in photosynthesizing cells has been the subject of much debate. Biochemical evidence suggests that TCA cycle carbon flow is maintained in the light (8,10,12,18), but studies in which net CO2 exchange