New Method of Microbial Energy Production Discovered

> Until recently it was thought that in all of biology, from microbes to humans, there were only two methods to generate and conserve the energy required for cellular metabolism and survival.

What are the existing two methods? ATP & something else?

something wrong with nrel's website. I think this is the same article: https://phys.org/news/2017-05-method-microbial-energy-produc...