Subcellular Energetics and Carbon Storage in Chlamydomonas

Abstract

Microalgae have emerged as a promising platform for production of carbon- and energy- rich molecules, notably starch and oil. Establishing an economically viable algal biotechnology sector requires a holistic understanding of algal photosynthesis, physiology, cell cycle and metabolism. Starch/oil productivity is a combined effect of their cellular content and cell division activities. Cell growth, starch and fatty acid synthesis all require carbon building blocks and a source of energy in the form of ATP and NADPH, but with a different requirement in ATP/NADPH ratio. Thus, several cellular mechanisms have been developed by microalgae to balance ATP and NADPH supply which are essentially produced by photosynthesis. Major energy management mechanisms include ATP production by the chloroplast-based cyclic electron flow and NADPH removal by water-water cycles. Furthermore, energetic coupling between chloroplast and other cellular compartments, mitochondria and peroxisome, is increasingly recognized as an important process involved in the chloroplast redox poise. Emerging literature suggests that alterations of energy management pathways affect not only cell fitness and survival, but also influence biomass content and composition. These emerging discoveries are important steps towards diverting algal photosynthetic energy to useful products for biotechnological applications.

Keywords: chloroplast; metabolic shuttles; mitochondria; oil; organelle; peroxisome; phosphorylating power; photosynthesis; reductant; starch.

Figure 1

Source and sink relationships in a photoautotrophically grown algal cell. A simple flow is drawn here, but it should bear in mind that catabolisms of «sink» compounds could influence the flux or repartition of «metabolic intermediates». Note: the blue numbers, taken from [2], denote the ratio of ATP/NADPH needed for the production of respective molecules. Abbreviations: ATP, adenosine triphosphate; CBB, Calvin-Benson Bassham; CoA, coenzyme A; FA, fatty acid; GA3P, glyceraldehyde-3-phosphate; NAD(P)H, nicotinamide adenine dinucleotide (phosphate).

Figure 2

A pathway map showing energy production, management and carbon storage in a Chlamydomonas reinhardtii cell. Abbreviations: AOX, alternative oxidase; ATP, adenosine triphosphate; β-oxi., β-oxidation; CBB, Calvin Benson Bassham; C1, mitochondria respiratory complex 1; FA, fatty acid; FAS, fatty acid synthase; Fd, ferredoxin; FLV, flavodiiron protein; FNR, ferredoxin-NADP⁺ reductase; Hase, hydrogenase; MDH2, malate dehydrogenase 2; NAD(P)H, nicotinamide adenine dinucleotide (phosphate); NDA2, NAD(P)H dehydrogenase 2; NiR, nitrite reductase; PQ/PQH₂, plastoquinone/plastoquinol; PTOX, plastoquinone terminal oxidase; PS, photosystem; TAG, triacylglycerol; TCA, tricarboxylic acid cycle; UQ/UQH₂, ubiquinone/ubiquinol.