Solar-powered production of biofuels and other petroleum substitutes by cyanobacteria: Stoichiometries of reducing equivalents and chemical energy, and energy conversion efficiency

Abstract

Cyanobacteria are a promising platform for solar-powered, CO2-consuming production of biofuels, petroleum substitutes, and other useful products using photosynthesis. Efficient production of such compounds requires that the stoichiometry of reducing equivalents (NADPH) and chemical energy (ATP) produced as a result of photosynthetic electron transport is well-matched by the stoichiometry of reducing equivalents and chemical energy required for production of the desired compounds. Here it is shown that stoichiometry requirements are met when producing compounds generated via the fatty acid or isoprenoid biosynthesis pathways. In the case of fatty acid production, the amount of energy stored in the fatty acid can be up to 28% of the energy of the light if one were to excite with 680 nm light and all absorbed light was used for fatty acid production. Making adjustments for solar illumination (only ∼50% of the energy can be used for photosynthesis), blue-photon utilization, and losses due to non-photochemical quenching and the requirements for maintenance energy, the solar energy conversion efficiency may still be in the range of ∼7%, which is superior to most other bio-based approaches. However, photohydrogen production that directly uses reducing equivalents from photosynthetic electron transfer for H2 production does not require ATP and thereby is not properly stoichiometrically balanced. An additional complexity of H2 production in relatively small cyanobacterial cells at somewhat alkaline pH is that the number of free protons in a cell is extremely limited (a few protons per cyanobacterial cell of 1 fL at pH 8.0). However, regardless the inherent difficulties of light-driven H2 production in cyanobacteria, the utilization of cyanobacteria for light-driven generation of carbon-based biofuels and related products can be efficient and is very promising.