Stoichiometric modeling of photoautotrophic metabolism

Photosynthetic organisms are capable of producing a wide array of important organic compounds from freely available light and carbon dioxide. Roughly half of the annual rate of biological carbon fixation (105 petagrams) is performed by algae and other marine organisms. These organisms represent a large untapped potential for the production of proteins, secondary metabolites, organic acids, starch, lipids, etc. which can be converted into feed, specialty and bulk chemicals as well as biofuels. The main advantages of using algae for production of renewable feedstocks and fuels instead of terrestrial plants are that they have faster growth rates, they can be grown on non-arable land and they do not compete with food/feed supplies. In order to harness the potential of marine organisms to produce industrially relevant compounds at high levels, a basic understanding of cellular metabolism is required. We present the first metabolic network reconstruction and flux balance analysis of the model green alga, Chlamydomonas reinhardtii. The reconstructed network includes all pathways in primary metabolism (glycolysis, tricarboxylic acid cycle, reductive and oxidative pentose phosphate pathway) and the synthesis of amino acids, chlorophyll, starch, nucleotides and lipids. In total the network accounts for 484 enzymes, 729 reactions and 458 metabolites which are localized into three intracellular compartments (cytosol, mitochondria and chloroplast). Intracellular fluxes were estimated using flux balance analysis for optimal growth in three growth regimes: autotrophy, heterotrophy and mixotrophy and compared. Autotrophic fluxes were also compared to FBA results for Synechocystis, a cyanobacterium. Flux balance analysis was also used to compare fluxes and efficiencies of the five known carbon dioxide fixation pathways (Calvin Cycle, reductive TCA cycle, reductive acetyl-CoA pathway, 3-hydroxypropionate/malyl-CoA cycle and 3-hydroxypropionate/4-hydroxybutyrate cycle). Three of the pathways occur in photoautotrophic organisms, two in chemotrophic (hydrogen-utlizing) organisms. In contrast to light energy, hydrogen is not freely available; therefore the production of hydrogen from light is included in the overall energy demand calculation. This allows an even basis of comparison because all organisms are then using light as their primary energy source. Carbon fluxes, overall energy demand and efficiencies for each pathway are calculated and presented