Central carbon metabolism and electron transport in Chlamydomonas reinhardtii: metabolic constraints for carbon partitioning between oil and starch

Abstract

The metabolism of microalgae is so flexible that it is not an easy task to give a comprehensive description of the interplay between the various metabolic pathways. There are, however, constraints that govern central carbon metabolism in Chlamydomonas reinhardtii that are revealed by the compartmentalization and regulation of the pathways and their relation to key cellular processes such as cell motility, division, carbon uptake and partitioning, external and internal rhythms, and nutrient stress. Both photosynthetic and mitochondrial electron transfer provide energy for metabolic processes and how energy transfer impacts metabolism and vice versa is a means of exploring the regulation and function of these pathways. A key example is the specific chloroplast localization of glycolysis/gluconeogenesis and how it impacts the redox poise and ATP budget of the plastid in the dark. To compare starch and lipids as carbon reserves, their value can be calculated in terms of NAD(P)H and ATP. As microalgae are now considered a potential renewable feedstock, we examine current work on the subject and also explore the possibility of rerouting metabolism toward lipid production.

Fig 1

Energetic organelles of Chlamydomonas. In the chloroplast, the light reactions and electron transfer chain feed NADPH and ATP toward the carbon fixation pathway (Calvin cycle). In the mitochondria, the major components of the respiratory chain produce ATP at the expense of NADH. Chlamydomonas reinhardtii has one chloroplast and multiple mitochondria which appear tightly packed together in electron micrographs. A possible pathway for the transfer of reductants from one organelle to the other, the malate shunt, is shown here. Black arrows denote electron and proton transfer pathways, green and red arrows denote alternative electron transfer pathways, and blue arrows denote ATP synthesis. P-phase, positive phase; N-phase, negative phase; UQH₂, ubiquinol; c, soluble cytochrome; hυ, photon energy; MDH, malate dehydrogenase; OMT, oxoglutarate/malate transporter.

Fig 2

Compartmentalization of central carbon metabolism in Chlamydomonas reinhardtii. The abbreviations for enzymes, also specified in Table 1, are written in gray. Glycolysis is localized in the chloroplast (from hexose-phosphates to 3-phosphoglycerate) and the cytosol (from triose-phosphates to pyruvate) (see Table 1 and references in Table 1 for the localization data). Red arrows represent reducing power, NAD(P)H, and blue arrows represent ATP. Acetate uptake (green arrows) can feed into the glyoxylate cycle for carbon assimilation via gluconeogenesis (purple arrows) or into the TCA cycle to sustain respiration and mitochondrial ATP production. Gray arrows represent alternative pathways (chlororespiration and fermentation). Mito, mitochondrion; Chloro, chloroplast.

Fig 3

Schematic representation of the reversible action of the TPT. (A) In the light, when photosynthesis produces NADPH and ATP in the chloroplast, G3P is formed from CO₂ and directed toward starch synthesis or exported toward the cytosol. Cytosolic G3P is oxidized into 1,3-BPG by GAPDH, and PGK dephosphorylates 1,3-BPG into 3-PGA. While a fraction of 3-PGA can feed into glycolysis, the rest can be retrieved into the chloroplast where it further accepts ATP and electrons from photosynthesis. The thickness of the arrow represents the amount of flux. (B) In darkness, when ATP levels are higher in the cytosol than they are in the chloroplast, the TPT can operate in the reverse mode. 3-PGA formed from acetate assimilation, glyoxylate cycle, and the first steps of gluconeogenesis, is phosphorylated and reduced into G3P. While part of the G3P translocated into the chloroplast can be further directed toward gluconeogenesis, the rest can be used for ATP production in the chloroplast. NADPH can be reoxidized via chlororespiration to sustain the flux of ATP production.