Electric-eating bacteria!
 

These special bacteria, geobacter metallireducens bacterium, doesn't need sugars but their life process by definition, eat AND excrete electricity or electrons. With advanced genetic engineering, they could be bred into nano wires, nano-sized electrical circuits or even fully operational nano-machines!

Biologists discover electric bacteria that eat pure electrons rather than sugar, redefining the tenacity of life | ExtremeTech

Scientists Reveal How Microbe 'Eats' Electricity | IFLScience

Cool. Also reminds me of the premise of that kinda bad TV series "Revolution." Revolution (TV Series 2012

Spoiler alert, a microscopic robot multiplies like bactreria and "eats" all the electricity in the world.

Oh that's cool!

"breathe" might be a better term, they still need nutrients.
Scientists Trick Iron-Eating Bacteria into Breathing Electrons Instead

So, rust has been bacteria poop all along? Who knew.

It's interesting to note that most iron ore deposits are also the result of biological action. When cyanobacteria evolved photosynthesis, about 2.5 billion years ago, the oxygen at first reacted with dissolved iron &c in the oceans. Only when most of that iron had been oxidized and precipitated out could O2 build up in the atmosphere. See e.g. 'The Great Oxygenation Event'.

Since we are talking about iron. Just read about this geo-engineering, an intentional 2012 iron-dust 'fertilization' was closely monitored and indicated an enormous plankton bloom, carbon dioxide absorption/scrubbing, salmon/fish population increase... /http://www.popsci.com/article/science/rogue-geoengineering-project-may-have-increased-salmon-numbers

The entrepreneur counters that strong oceanic underwater currents stir iron sediments and occasional 'natural' volcanic eruptions sow airborne iron dust and the trigger the same plankton explosions.
There are other oceanic experiments but is this environmental stewardship or 'pollution'?
Russ George's blog on how to bring back the fish.