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. 2015 Mar 18:3:36.
doi: 10.3389/fbioe.2015.00036. eCollection 2015.

Photosynthetic constraints on fuel from microbes

Charles A R Cotton  1 , Jeffrey S Douglass  1 , Sven De Causmaecker  1 , Katharina Brinkert  1 , Tanai Cardona  1 , Andrea Fantuzzi  1 , A William Rutherford  1 , James W Murray  1
Affiliations

Photosynthetic constraints on fuel from microbes

Charles A R Cotton et al. Front Bioeng Biotechnol. .
No abstract available

Keywords: EROI; algal biofuel; biofuel; chlorophyll; light harvesting; photosynthesis; photosynthetic efficiency; rubisco.

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Figures

Figure 1
Figure 1
This diagram illustrates energy flow through a photosynthetic system without fossil fuel energy input (A) and another using fossil fuels (B). Horizontal arrows represent the flow of energy through the system, with labels on the arrows showing the type of stored energy at each stage. Theoretical maximum percentages of solar energy retention are written above the arrows for each stage, with yellow fill indicating that achievable levels are lower. Hatched arrows represent solar energy losses. Green circles represent cellular processes. Energy losses in the light reactions include: (1) photons outside the chlorophyll absorbance range, (2) the energy in absorbed photons in excess of that corresponding to the first excited state of the reaction center, and (3) antennae saturation and shading effects; those incurred in the dark reactions include: (1) mismatched substrate stoichiometries (e.g., NADPH, ATP), (2) Rubisco oxygenation reactions, and (3) thermodynamic losses; those in growth include: (1) protein turnover and (2) respiratory losses. (A) Biomass grows naturally with no fossil fuel input. The percentage of solar energy converted into the final product is very low (small yellow gradient). (B) Fossil fuel is used to control conditions such as temperature, light, CO2, sterility, mixing, and other factors. This system produces biofuel. The yield (arrow fill) is higher at each stage, partly due to an increased percentage of solar energy which is retained (yellow fill) and partly due to the energy from fossil fuel (gray fill). Synergistic interactions between engineered strains and controlled environments could increase the fossil-fuel-generated improvements to photosynthetic efficiency and the maximum energy retained. Such systems are only beneficial if the increase in yield is greater than the fossil fuel input. Currently, this condition is rarely met and generally ignored. Using energy from renewable sources such as hydro, photovoltaic, or wind instead of fossil fuels could make a similar conversion system sustainable.
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