Elsewhere, this blog has noted efforts to generate petroleum using genetically engineered bacteria (Bell BioEnergy, Amyris and LS9). These methods use naturally-occurring abilities of certain bacteria to convert cellulose, a carbohydrate, to hydrocarbons, the chemicals that fuel our vehicles, heat our homes, and otherwise serve most of our energy needs.

Bacteria convert one type of chemical energy to another type, as explained here. Photosynthesizers such as green plants are needed to perform the first step of converting light energy into chemical energy for the bacteria to work on.

What if this middle step were eliminated? A new Bill Gates start-up, Sapphire Energy, begins and ends with the photosynthesizer — in their case, algae, single-celled plant-like organisms. The goal is to create "renewable gasoline," not ethanol, taking advantage of "non-arable land."

Another company, Solazyme, takes a more conservative approach, if anything about this field can be called conservative. It grows its oil-generating algae in the dark and feeds it sugar water, which must be derived from food sources. The fundamental principles are the same as with methods that use bacteria to produce renewable hydrocarbons.

Algae are aquatic organisms that require water to be circulated, a significant energy cost that currently keeps the price of algae-derived fuels high. This factor may limit algae's usefulness on an industrial scale. Algae also need a source of carbon dioxide. These fundamental differences may mean algae will turn out to be inferior to bacteria in the field of renewable fuels.

Or not. There are few certainties yet in this exciting, promising field. There is a lot of potential and a lot of hope, and I will be interested to see whether these new energy companies can bring down the costs enough to produce their products on a meaningful scale and compete with fossil fuels.

Read more at Scientific American.