The uncontrolled use of fossil fuels and their impact on climate change has risen up a global energy crisis that has woken up interest on renewable sources as the call for sustainable fuels and electricity becomes louder. The concept of renewable sources of energy can be altered by the recent and exciting finding known as bioelectrogenesis, a process from which bacteria can directly transfer electrons to solid conductive surfaces as graphite, so clean electricity can be harvested. This suggests a new source of green energy and a big biotechnological challenge over the next years.

Which type of bacteria can generate electricity?

Electrogenic bacteria can be found in many anaerobic environments such as sediments of rivers, lakes or seas. Microorganisms such as Geobacter species have the ability to breathe iron and mineral compounds from soil and sediments in the same way we breathe oxygen. They have been acting this way since million years ago, when there was no oxygen over terrestrial surface. The capability of respiring iron oxides has great environmental relevance for those bacteria have influenced soil and subsoil composition. Nowadays we can use this capability for producing clean energy.

How do we convert chemical energy into electricity by means of bacteria?

Converting chemical energy into electricity is feasible through electro-chemical devices called Fuel Cells, wherein electricity is obtained from an external fuel like hydrogen or methanol. Some public transportation vehicles in our cities are already using this clean technology where steam water is the only waste product.

One recent variety is Microbial Fuel Cell (MFC), a device in which bacteria create electrical power by oxidizing organic matter in wastewater. MFCs do not only clean wastewater, but they also convert organics in these wastewaters into usable energy.

A MFC consists of anode and cathode compartments separated by a cation specific membrane. In the anode compartment, fuel is oxidized by microorganisms, generating electrons and protons. Hence, electrons are transferred to the cathode compartment through an external electric circuit and the protons are transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water.

In addition it is possible to lodge an MFC in a natural habitat wherein carbon cycles provide in situ an everlasting fuel source by means of microbial communities. This system is called a sedimentary MFC and requires burying the anode in anaerobic sediment, as the cathode is located in the overlying water.