There is a video, running about 30 minutes on Samizdata with Craig Venter, talking about synthesizing life forms using digitally stored DNA segments put together in what might be the most efficient manner. His favorite form at this point is one that tales CO2 and sunlight, and poops out hydrocarbon fuels, such as Methane or even Octane.
He points out that such organisms exist in nature but are not particularly efficient, as CO2 does not make up much of the atmosphere. One imagines some really efficient ones tossed into a stagnant pond, in a low-wind area, where there's lots of CO2 bubbling up from decaying biomass at the bottom of the pond, until a flammable pool of methane has accumulated over the pond.
Weaponization of the technology was brought up, and quickly dismissed, as his organisms do not survive well outside the lab, and additionally, have a "destruct" gene that can be activated. No mention of whether other people's microbes will have the same features. I rather favor biological warfare as it's difficult for anyone to seriously oppose your advance when they're lying on the floor, projectile vomiting from both ends. Anyone with antidote is apt to be looked on with favor, at least initially.
His energy proposal offers the possibility of taking coal, which is pure carbon, burning it to produce electricity, then using the CO2 and water by-products to feed his bugs, who convert it to gasoline, which is burned in cars, producing CO2 and water, which is not readily captured. It's like getting to burn the coal twice.
It's a good video and worth the time.