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Metabolism and Enzymology

Mitochondrial Respiration Can Support NO₃^– and NO₂^– Reduction during Photosynthesis ¹

Interactions between Photosynthesis, Respiration, and N Assimilation in the N-Limited Green Alga Selenastrum minutum

Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6

Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH₄⁺, NO₂^–, NO₃^–) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO₂ release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH₄⁺ assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O₂ consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO₃^– and NO₂^– assimilation resulted in a larger stimulation of TCA cycle CO₂ release than did NH₄⁺, but a much smaller stimulation of mitochondrial O₂ consumption. NH₄⁺ assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO₃^– and NO₂^– assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH₄⁺, NO₃^– and NO₂^– assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO₃^– and NO₂^– assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O₂ consumption during NO₃^– and NO₂^– assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO₃^– and NO₂^– reduction.