Creating microbial fuel cells using bacteria in your garden

it doesn’t really say how much power you can get out of it, but it sounds interesting!

Not much. The manual says you can run one LED off 5 mud cells. That would imply about 3 volts and maybe 10 milliamps. Enough to show that there's a gradient of potential, not enough to accomplish much of anything. :)

The handbook in the article says .3 - .8 volts per cell