The other main "food" is concentrated sunlight.
Algae can be processed into ethanol and the carbon dioxide by-product of the distillation process can be piped back into the algae tanks.
One of the amazing things about the Age of Bio Technology is the sheer number of ways you can manipulate the processes. There is now a company setting up at a coal plant to take the CO2 from the stack, and sunlight and a proprietary process and produce industrial chemicals, which I understand are worth more than ethanol.
CO2 is a very valuable bi product of electrical generation that can be converted into any of thousands of chemical products. As the technologies grow, CO2 and the products made from it could become more profitable than the electricity from the coal combustion.
Exciting time we live in.