Interactions between nitrogen metabolism and photosynthesis in nitrogen-limited Monoraphidium falcatus

Abstract

Nitrogen and carbon metabolism of Monoraphidium falcatus were shown to compete for carbon skeletons under high PPFD and atmospheric levels of CO2. The uptake of was dependent upon the presence of a Ci-source and was inhibited in the presence of MSX with no detectable efflux of N H 4 + from the cells. These results indicate that either Ci or an intermediate of carbon metabolism was essential for the maintenance and stabilization of the NO 3 − uptake system. The addition of inorganic-N ( NO 3 − or N H 4 + ) to the N-limited cells resulted in a suppression in the rates of nett photosynthetic CO2 fixation and O2 evolution. The inhibition of CO2 fixation and O2 evolution by NO 3 − and N H 4 + was partially alleviated in the presence of MSX. The carbon skeleton demand of amino acid biosynthesis may be met by anaplerotic reactions which would drain off triose-phosphates from the RPP pathway via the TCA cycle resulting in reduced rates of RuBP regeneration and increased mitochondrial respiratory activity.