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].

Mass spectrometric analysis of gas exchange by the diatom Thalassiosira weisflogii grown under at the end of scotophase. When cells were poised at the DIC (dissolved inorganic carbon) compensation point in the light (DIC concentration where net photosynthesis equals zero), mitochondrial 0, consumption was only slightly higher than in the dark. Adding DIC to cells at the compensation point resulted in a rapid increase in both photosynthetic 0, evolution and mitochondrial 0, consumption, indicating that the higher mitochondrial respiration rates observed in the light are probably due to an increase in substrate supply from photosynthesis. If estimates of primary production are based on oxygen exchange, this effect is accounted for; however if radiocarbon is used to measure production, especially in short-term measurements, net production may be significantly overestimated.Phytoplankton respiration is used in aquatic ecology to quantify at least three fundamental parameters. First, knowledge of the rate of respiratory losses is required ' To whom reprint requests should be addressed.

Cells of the green alga Chlamydomonas reinhardtii Dangeard were grown in Fe-limited chemostat culture over a range of growth rates (0.15±1.5 d A1 ). Greater cell densities and culture chlorophyll levels were achieved using an excess of chelator [ethylenediamine di-(o-hydroxyphenylacetic acid)] relative to FeCl 3 (80:1), compared to growth using a 1:1 chelator:FeCl 3 ratio. The C. reinhardtii cells reduced extracellular ferric chelates, and ferric chelate reductase activity increased with increasing Fe-limited growth rates. However Fesucient cells exhibited a low rate of ferric chelate reductase activity, similar to severely Fe-limited cells. Iron-limited cells were capable of reducing a wide variety of ferric chelates, representing a wide range of stability constants, at similar rates, suggesting that the stability constants of ferric complexes are not important determinants of ferric reducing activity. Cupric reductase activity also increased with increasing Fe-limited growth rates, and Cu(II) was preferentially reduced compared to Fe(III). These results suggest that both reductase activities may represent the same plasmamembrane enzyme. The rate of cupric reduction was a function of the free [Cu 2+ ], not the total [Cu(II)], suggesting that free Cu 2+ is the actual substrate for cupric reductase activity.

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

The rate of respiratory O , consumption by Chlamydomonas reinhardtii cell suspensions was greater after a period of photosynthesis than i n the preceding dark period. This "light-enhanced dark respiration" (LEDR) was a function of both the duration of illumination and the photon fluence rate. Mass spectrometric measurements of gas exchange indicated that the rate of gross respiratory O , consumption increased during photosynthesis, whereas gross respiratory CO, production decreased in a photon fluence ratedependent manner. The rate of postillumination O , consumption provided a good measure of the O, consumption rate in the light.LEDR was substantially decreased by the presence of 3-(3,4-dichloropheny1)-1,l-dimethylurea or glycolaldehyde, suggesting that LEDR was photosynthesis-dependent. The onset of photosynthesis resulted in an increase in the cellular levels of phosphoglycerate, malate, and phosphoenolpyruvate, and a decrease in whole-cell ATP and citrate levels; all of these changes were rapidly reversed upon darkening. These results are consistent with a decrease i n the rate of respiratory carbon flow during photosynthesis, whereas the increase in respiratory O, consumption during photosynthesis may be mediated by the export of photogenerated reductant from the chloroplast. We suggest that photosynthesis interacts with respiration at more than one level, simultaneously decreasing the rate of respiratory carbon flow while increasing the rate of respiratory O , consumption.