Central Carbon Metabolism and Electron Transport in Chlamydomonas reinhardtii: Metabolic Constraints for Carbon Partitioning between Oil and Starch

Abstract

eThe 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. Photosynthetic algae fix atmospheric carbon (CO 2 ) by using light energy to drive a series of chemical and redox reactions. This fixed carbon is transformed into reserve molecules that can be broken down at a later time to provide the cell with ATP, reducing power, and carbon skeletons. Starch, a polymer of glucose, is synthesized and degraded as a normal process in a light-dark cell cycle. Starch is also accumulated to prepare for gametogenesis, but it is not a prerequisite. The type and quantity of the reserve depend on environmental and cellular factors: stresses such as nutrient deprivation, salinity, temperature, and high light can be stimuli for starch accumulation and also for triacylglycerol (TAG) synthesis leading to lipid body accumulation in algae. Under nonstress conditions, starch would appear to be a preferential source of reserve in green algae as it is in plant leaves, whereas under stress conditions and in plant oil seeds, storage neutral lipids will accumulate to high levels.Photosynthesis is a tightly controlled process, from the capture of light energy to the conversion of this energy into ATP and reducing power (NADPH). ATP and NADPH feed the Calvin cycle, which is responsible for CO 2 fixation. In the absence of exogenous carbon supply, photosynthesis is the only source of energy, and CO 2 fixation provides carbon skeletons for all reactions in the cell. Tight control of the photosynthetic reactions is required to fit the downstream metabolic reactions in the chloroplast.We deliberately oriented our presentation and our choice of references toward Chlamydomonas reinhardtii, a unicellular green alga that has garnered much attention from the scientific community, primarily because it is haploid and the genetics have been extensively developed, but it has also proven to be a robust model system for the study of photosynthesis. No...

Two Glycerol-3-Phosphate Dehydrogenases from Chlamydomonas Have Distinct Roles in Lipid Metabolism

“…(Gee et al, 1993;Ansell et al, 1997;He et al, 2009). However, G3P also is a key precursor for the synthesis of glycerolipids within the chloroplast and the ER of photosynthetic organisms like microalgae (Johnson and Alric, 2013); thus, GPDH activity would be expected to be important for lipid synthesis in organisms like C. reinhardtii, as appears to be the case in higher plants (Shen et al, 2010). To date, all of the analysis of Dunaliella spp.…”

“…It also remains unknown whether the cytosolic and chloroplastic pools of G3P are distinct or are in equilibrium. DHAP but not G3P can be mobilized into the chloroplast via a triose phosphate/phosphate translocator (Linka et al, 2008;Johnson and Alric, 2013), while C. reinhardtii has an ortholog, as yet uncharacterized, of an Arabidopsis nutrient starvation-induced, plastidlocalized G3P transporter (Ramaiah et al, 2011;Kawai et al, 2014); thus, it is likely that the G3P pools are in equilibrium. The equivalent phenotypes for both gpd2 and gpd3 knockdown in response to P and N starvation may suggest that both glycerolipid pools are equally important for lipid induction in response to both of these nutrient starvation conditions.…”

“…1) and may act as a bridge for carbon transfer between carbohydrate and lipid metabolism (Johnson and Alric, 2013).…”

Genome-Scale Metabolic Model for the Green Alga Chlorella vulgaris UTEX 395 Accurately Predicts Phenotypes under Autotrophic, Heterotrophic, and Mixotrophic Growth Conditions

“…This metabolite can be incorporated into acetyl-CoA following two possible pathways that both require ATP: a direct conversion with acetyl-CoA synthetase or a two-step reaction involving acetate kinase and phosphate acetyltransferase (Johnson and Alric, 2013). Analyzing our predicted flux distribution under heterotrophy enabled Figure 5.…”

Induction of Photosynthetic Carbon Fixation in Anoxia Relies on Hydrogenase Activity and Proton-Gradient Regulation-Like1-Mediated Cyclic Electron Flow in Chlamydomonas reinhardtii

“…In C. reinhardtii, upstream glycolytic enzymes, including the reversible glyceraldehyde 3-P dehydrogenase, are located in the chloroplast (Johnson and Alric, 2012). This last enzyme is shared by the glycolysis (oxidative activity) and the CalvinBenson-Bassham (CBB) cycle (reductive activity; Johnson and Alric, 2013). In dark anoxic conditions, the CBB cycle is inactive, thus avoiding wasteful using up of available ATP and depletion of the required intermediates for glycolysis.…”