Possibilities for extremophilic microorganisms in microbial electrochemical systems

Abstract

Microbial electrochemical systems exploit the metabolism of microorganisms to generate electrical energy or a useful product. In the past couple of decades, the application of microbial electrochemical systems has increased from the use of wastewaters to produce electricity to a versatile technology that can use numerous sources for the extraction of electrons on the one hand, while on the other hand these electrons can be used to serve an ever increasing number of functions. Extremophilic microorganisms grow in environments that are hostile to most forms of life and their utilization in microbial electrochemical systems has opened new possibilities to oxidize substrates in the anode and produce novel products in the cathode. For example, extremophiles can be used to oxidize sulfur compounds in acidic pH to remediate wastewaters, generate electrical energy from marine sediment microbial fuel cells at low temperatures, desalinate wastewaters and act as biosensors of low amounts of organic carbon. In this review, we will discuss the recent advances that have been made in using microbial catalysts under extreme conditions and show possible new routes that extremophilic microorganisms open for microbial electrochemical systems.

Keywords: MEC; MFC; anode-respiring bacteria; bioanode; bioelectrochemical systems; electricity generation; extremophiles; microbial electrolysis cells; microbial fuel cells.

Figure 1.

Schematic of a bioelectrochemical system. The system typically consists of anode (where substrate is oxidized (a)) and cathode (where the product is formed (b)) compartments separated by a membrane. Energy is applied by means of a power supply in MECs (c) or harvested from the system in MFCs (d). The anode and cathode chambers can be separated by various types of membranes (e–g) or be a single chamber without an ion exchange membrane (h).

Figure 2.

Schematic representation of the electron flow from microorganism to the electrode. The microorganisms oxidize a substrate (S) to a product (P) from which electrons are released into the electron transfer chain. The energy level of these electrons is determined by the Gibbs free energy change of this oxidation reaction (E_P/S). Finally, the electrons are transferred to the electrode surface by a redox active enzyme. The energy at which the electrons are released is determined by the final step in the electron transfer chain and the enzyme that catalyzes this final step (E_an). Figure adapted from Hamelers et al. (2011).

Figure 3.

Classes of extremophilic microorganisms that have been utilized in MESs, their defining characteristic and an example of an environment where they grow. Photographs are by the authors except of the chloride and soda lakes (courtesy Dimitry Sorokin) and the Antarctic Sea (courtesy Daniel Bengtsson).

Figure 4.

Example of the change in anode potential according to changes in the pH based on thermodynamics. Potentials at standard conditions are calculated with a concentration of 1 M, while the practical conditions are calculated with a concentration of 0.005 M. All potentials are reported vs NHE.