# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 3 Research Progress in Energy Harvesting from Microorganisms
### Principles of Electricity Generation with Microbial Fuel Cell (MFC)  
Structurally, an MFC is composed of a cathode, an anode, the microorganism and the oxidizing substrate, which in most cases, is composed of organic matter. The structure can be of single or dual chamber type depending on the absence or presence of a separator. In a classic configuration, the microorganisms decompose the organic substrate in the anaerobic anode chamber through catabolic processes to obtain energy and generate electrons and protons/cations as a by product. The generated protons flow to the cathode chamber through the cation (permeable) exchange membranes (CEMs) and thus create a potential difference between the electrodes. The excess electrons flow from the anode to cathode via external circuit and generate a current flow [18; 45; 46].  
A classic dual cell MFC is depicted in Figure 2. Bio-electrochemical systems started to gain attention as a possible green energy source in the early years of the 21 st century [29] since microorganisms possess the flexibility to generate energy from a very wide range of biomass varieties [47]. Bacterial electron transfer mechanisms and their dependence on mediators and biofilms are three major considerations of MFC performance.  
The intracellular electron transport (ET) mechanisms are of fundamental concern for MFC systems. The MFC compatible microorganisms can transfer the electrons to the anode either through direct contact or with the help of mobile electron shuttles or mediators [31; 45]. These mediators are compounds that act as biocatalysts and shuttle electrons from the intracellular space to the extracellular environment within the MFC and could either be externally added or produced by the living cells [46]. The ability of microorganisms to utilize the soluble mediators as electron shuttles was highly beneficial in the early stages of MFCs. In these first-generation or mediator-dependent MFCs, presence of a suitable electron shuttle or mediator was mandatory [31]. In addition, efficient MFCs  
Figure 2: A classic dual chamber microbial fuel cell (MFC) with cathode, cation exchange membrane and anode (from left to right).  
without requiring artificial mediators also started to emerge. However, not all bacteria are suitable for generating electricity in a mediator-less configuration MFC.  
Microorganisms that can effectively generate electricity in MFCs without additional mediators include a few classes of Proteobacteria in addition to some microalgae, yeast, and fungi species [48]. These microorganisms, capable of extracellular electron transfer are often referred to as exoelectrogens as well as electrochemically active bacteria, anode-respiring bacteria and electricigens [47]. They are capable of generating proteins or molecules, followed by oxidizing procedures, for transferring the electrons exogenously [48; 49; 50]. A sub group of these bacteria are nanowire generators. _Geobacter sulfurreducents_ was one of the first organisms shown to produce conductive nanowires. These nanowires were latter referred to conductive pilA [48] because of their composition with the pila protein. The bacterium _Shewanella oneidensis_ can also generate electrical nanowires [51; 52] under special condition and as extensions of their outer-membrane, allowing the transfer of electrons to the anode without requiring soluble electron shuttles.  
The presence of bacterial biofilm can be highly advantageous for MFCs because of their electroactive nature aids to generate electricity more efficiently. This biofilm is a complex, organic, polymeric matrix, produced by the bacteria themselves at any biotic or abiotic surface and these organic films can be formed by a single bacterial (pure-culture) or multiple bacterial species (mixed-culture) [48]. It has been proven that bacteria capable of forming thick anodic biofilms generate higher current densities than those who can not [53]. A pictorial summary of bacterial electron transport processes is given in Figure 3.