Interactions between photosynthesis, respiration, and nitrogen assimilation in microalgae

Abstract

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].