There have been a number of reports lately in the scientific literature about using bacteria to generate electricity. When bacteria respire they pull electrons off organic debris. Catch those electrons and you have a source of power. As described in the May 18 issue of Nature, microbiologist Peter Girguis of Harvard has bugs turning on a light with a bucket of cow manure as fuel.
It's not hard to imagine the day when sewage plants will be electrical generating plants. How about microbes in the family septic tank trickle-charging my electric car? Bacteria are the most numerous living things on the planet, having more total mass than all other creatures put together. Isn't it time they carried their own weight?
(Just kidding, of course. Bacteria play a crucial role in maintaining the ecosystem. They made the ecosystem!)