The growth of terrestrial plants is limited by a variety of factors, depending on growth location and conditions. What is seldom limiting, however, is reductant, i. e., solar photons. Rather, plant growth is typically limited by water, nutrients, or CO2. Likewise, an Electrofuels approach to the production of liquid fuels will be limited by a variety of factors, depending on growth conditions. Ultimately, the two factors most likely to provide insurmountable limits to production are reducing equivalents and inorganic carbon.

The approaches under consideration in the Electrofuels program utilize various means to assimilate reducing equivalents, and the limiting factor to uptake will vary as the approach. In the case of hydrogen-utilizing organisms, the aqueous solubility of hydrogen is likely to be limiting, at least under some growth conditions. Incorporated into the program are multiple efforts aimed at novel bioreactors that, at least to some extent, ameliorate these concerns. The growth of electrotrophs will ultimately be limited by the ability of organisms to assimilate electrons. The mechanisms by which these unique species take up electrons remain unclear, and the limits to growth mandated by such processes are similarly opaque. Still, if organisms must be physically anchored to an electrode through, for example, conductive pili, then production will be limited by the accessible surface area of a highly porous elec­trode. On the other hand, if conduction can proceed through at least some thickness of a biofilm, the effective surface area could be multiplied manyfold, greatly increas­ing the potential for growth. In any event, the ability to assimilate reducing equiva­lents will ultimately provide a limit to fuel production.

The provision of inorganic carbon likewise presents challenges for an Electrofuels approach to fuel synthesis. Plants autonomously assimilate CO2 from the atmo­sphere; while the very low concentrations of CO2 in air limit growth, the carbon is at least free and essentially limitless. In an Electrofuels approach, inorganic carbon will be furnished at concentrations beyond that available directly from air. Such sources might include the effluent of conventional power plants, cement kilns and the like, direct mining of geologic CO2, carbonate or bicarbonate, or CO2 produced as a coproduct during the production of conventional biofuels from biomass, which necessarily release significant quantities of the total carbon as CO2. The extraction of inorganic carbon from seawater may be feasible as well. While any of these approaches would decrease the Nation’s dependence on foreign sources of oil, only some are carbon-neutral, while others would contribute to anthropogenic atmo­spheric carbon loads during combustion.