Ammonium Assimilation Requires Mitochondrial Respiration in the Light

Abstract

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

Purification and Characterization of NAD-Isocitrate Dehydrogenase from Chlamydomonas reinhardtii1

“…On the other hand, the participation of NAD-IDH in the 2-oxoglutarate supply for ammonium assimilation cannot be discounted. The TCA cycle is stimulated under conditions in which there is a high demand for this metabolite (Weger et al, 1988), and NAD-IDH activity increases under nitrogen starvation (Martínez-Rivas and Vega, 1993). Future studies, including the cloning of the gene, are required to understand the molecular basis of the structure and the regulatory mechanisms of NAD-IDH.…”

Net and gross productivity: weighing in with 14C

“…Both Peltier & Thibault (1985) (for Chlamydomonas reinhardtii) and Weger et al (1988) (for Selenastrum minutum) reported equivalence in light and dark respiration over time scales of minutes. A follow-on paper (Weger et al 1989), using Thalassiosira weisflogii, found an increase in light respiration by a factor of 2 (again for time scales of minutes).…”

Regulation of photosynthetic light harvesting by nitrogen assimilation in the green alga Selenastrum minutum

“…N resupply results in rates of N assimilation into protein which can exceed control rates of photosynthetic carbon fixation. The onset of these rapid rates of N assimilation causes a shut down of carbon fixation and the mobilization of starch to provide carbon skeletons for amino acid synthesis [8,9]. The reductant and ATP requirements associated with CO?, NI-G and NOS assimilation vary and are shown in Table I.…”