Some of the smaller Algae grow extraordinarily quickly. All of they need is light, some CarbonDioxide and a few other bits and bobs, and they will end up with a thick soup.
This soup could form an excellent form of BioFuel. You could take the soup directly, put it into a AnaerobicDigestionPlant and harvest the methane produced. Alternatively, you could dry the soup down into AlgaeCake.
One the advantages with Algae over other forms of BioFuel is that the raw structures are very small. A tree is made up of big chunks of wood, which Algae are small enough that, if dried, they turn into a fine powder. This will flow like a liquid. It's even small enough that if turned into an aerosol, they will combust explosively. In short, there is a reasonable chance that you could even feed them straight into a InternalCombustionEngine.