HARVARD COLLEGE, PRESIDENT & FELLOWS OF

Engineering a bacterial reverse fuel cell? ARPA-E ?Electrofuels? Award: To address the problem of global warming and the US?s strategic dependence on fossil fuels, many researchers are investigating ways to convert solar energy into fuel molecules. The primary biological approach is via photosynthesis, as carried out by green plants and cyanobacteria. There are several problems with this approach. First and most fundamental, the maximal efficiency of photosynthetic energy capture is about 11%, but in practice it is much lower: on a sunny day, light-harvesting pigments capture essentially all light energy, but the water-splitting, oxygen-generating enzyme active site has a low maximal reaction rate and cannot keep up with the influx of photonic energy. Thus, most of the light energy absorbed by plants is lost as heat. In practice, at maximal solar flux, only about 1% of light energy can be captured by the photosynthetic apparatus. In contrast, solar panels can absorb solar energy and convert about 15% of this energy into electricity, without reaching a maximum absorption as seen with biological systems. The goal of the proposed research is to engineer a bacterium to absorb electrical current as an input and convert this energy into chemical energy in the form of a biofuel. In this way, the system will take advantage of the higher efficiency of solar panels at converting sunlight into electricity. Work to be accomplished in the award: tasks broken down as follows. First, bacteria must be engineered to accept electrons in the form of current. None of the well-understood microbes used for genetic engineering can do this, but several engineerable microbes can produce electrons ? our strategy will be to reverse this process, using Shewanella oneidensis as the primary candidate bacterium to be engineered. Second, the bacteria will be engineered to fix carbon dioxide using either the Calvin cycle or the recently elaborated 3-hydroxypropionate pathway. Third, the bacteria will be engineered to produce a biofuel, specifically octanol, using a carbon chain length-specific reductase that we will engineer. Preliminary genetic engineering steps will be carried out in E. coli, which we will also test for the ability to accept input electrons via current. Finally, a device that combines features of an electochemical cell and a microbial fermenter must be constructed. This work will include development of peptide-based electrical current mediators that would be secreted from the bacterium. Some supporting genetic analysis into Shewanella oneidensis will be done in parallel, both to generate a better understanding of the organism?s metabolism and to obtain mutants that express the electrical conduction apparatus in aerobic conditions.

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