[rcthompson]

They use electricity as their energy source, but they still need a source of matter from which to synthesize their biomass. Just like plants derive energy from light, but they still need to take up water and CO2 to synthesize glucose with that energy.

Separating the concept of an energy source from a source of biomass is difficult for us humans because everything we eat supplies both in roughly equal measure, but there are many examples of organisms that use different sources for their energy and their biomass.