Abstract
Microbial production of chemicals using renewable feedstocks such as glucose has emerged as a green alternative to conventional chemical production processes that rely primarily on petroleum-based feedstocks. The carbon footprint of such processes can further be reduced by using engineered cells that harness solar energy to consume feedstocks traditionally considered to be wastes as their carbon sources. Photosynthetic bacteria utilize sophisticated photosystems to capture the energy from photons to generate reduction potential with such rapidity and abundance that cells often cannot use it fast enough and much of it is lost as heat and light. Engineering photosynthetic organisms could enable us to take advantage of this energy surplus by redirecting it toward the synthesis of commercially important products such as biofuels, bioplastics, commodity chemicals, and terpenoids. In this work, we review photosynthetic pathways in aerobic and anaerobic bacteria to better understand how these organisms have naturally evolved to harness solar energy. We also discuss more recent attempts at engineering both the photosystems and downstream reactions that transfer reducing power to improve target chemical production. Further, we discuss different methods for the optimization of photosynthetic bioprocess including the immobilization of cells and the optimization of light delivery. We anticipate this review will serve as an important resource for future efforts to engineer and harness photosynthetic bacteria for chemical production.
Highlights
Societal dependence on fossil fuels is extensive and commonplace
The world’s oldest photovoltaic cells evolved within living organisms— the photosystems of photosynthetic bacteria capture photons to provide themselves the energy required for survival
Harnessing their ability to harvest solar energy and using it to produce the chemicals humans use daily could turn these bacteria into solarpowered microbial cell factories
Summary
Societal dependence on fossil fuels is extensive and commonplace. This can be inferred from their widespread use in diverse applications including power, transportation, heating, and in the manufacture of plastics and beauty products. The world’s oldest photovoltaic cells evolved within living organisms— the photosystems of photosynthetic bacteria capture photons to provide themselves the energy required for survival Harnessing their ability to harvest solar energy and using it to produce the chemicals humans use daily could turn these bacteria into solarpowered microbial cell factories. This review will look at both oxygenic and anoxygenic photosynthesis focusing on cyanobacteria and PNSB and consider how the different photosynthetic systems are being used and engineered both on a genetic level and a bioprocess level to overproduce native chemicals or to divert electrons toward heterologous products. The complexes that drive photosynthesis can be engineered for increased electron flux to downstream cellular processes such as for the production of chemicals through the introduction of heterologous pathways External factors such as photobioreactor design can affect production as cells react to their environment and, as such, is a significant consideration. This review will examine each of these areas with the overarching goal of bioprocess design for solar-powered cell factories
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