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...