Photosynthesis-to-fuels: from sunlight to hydrogen, isoprene, and botryococcene production

Abstract

The concept of “Photosynthetic Biofuels” entails the direct application of photosynthesis for the generation of fuels and chemicals, in a process where a single organism acts both as catalyst and processor, synthesizing and secreting ready to use product. Examples of photosynthetic fuel and chemicals generation are offered in this perspective. Physiological and genetic manipulation of green microalgae enabled diverting the natural flow of photosynthetic electron transport toward sustained generation of hydrogen gas, instead of the normally produced oxygen. Heterologous expression of a plant isoprene synthase gene in cyanobacteria and microalgae enabled the renewable generation of volatile isoprene (C₅H₈) hydrocarbons, derived from sunlight, carbon dioxide (CO₂) and water (H₂O). Photobioreactor design concepts and conditions for capturing the isoprene are presented. The process of generating isoprene (C₅H₈) hydrocarbons serves as a case study in the development of technologies for the renewable generation of a multitude of other fuels and useful chemicals. Green microalgae of the genus Botryococcus afford naturally occurring examples on how to divert the flow of photosynthetic metabolites toward high-value long-chain hydrocarbon products instead of the normally produced sugars. Members of the genus Botryococcus direct photosynthate toward tri-terpenoidbotryococcene hydrocarbons or diene and triene alkanes, at the expense of investing photosynthate toward biomass accumulation. Lastly, methods are offered on how to improve the solar-to-biomass energy conversion efficiency of photosynthesis in high cell-density cultures, or canopies, under bright sunlight conditions. Goal of this effort is to improve efficiency, from the current-best of 2–3% up to a theoretical maximum of about 10%. Advances in each of the above-mentioned fields suggest blueprints of organism genetic and metabolic engineering, by which to improve the performance of photosynthesis and to divert cellular metabolic flux toward alternative high-value bio-products. Successful implementation of this R&D would lead to the generation of commercially viable fuels, chemicals, and other useful bio-products.