Introduction

The importance of microbial electricity generation

Energy is an important material basis of the advancement of human society and economic development, the lifeblood of economic development of a country and region. However, with the gradual depletion of oil, natural gas and other fossil fuels, renewable energies like solar energy, bio-energy, wind energy, geothermal energy and marine energy are becoming more and more essential for development and are playing an increasingly important role in our daily life.
As a protagonist in the development of renewable energy, microorganisms play a significant role in sustainable energy exploitation in which biomass generation is of promising future development. Bioelectrogensis is a non-polluting, highly-efficient power generation with mild reaction conditions. The history of this kind of power generation can date back to as early as 19 century when the British botanist Potter discovered that some types of bacteria culture medium can produce electric current, and successfully made the world's first bacterial battery after a lot of trials and errors. From then on, especially in recent years, many countries have begun to conduct researches on environmentally friendly and benign batteries, and have already made great progress. Characteristics of biomass generation are of great significance in solving environmental pollution problems and alleviating energy crisis. The large gross of accessible microorganisms becomes a major advantage of bioelectrogensis over other kinds of energy generation.
In recent years, scientists have been constantly creating new microbial cells and improving the efficiency of the bio-batteries. Exciting news comes one by one, which gives us reasons to believe that microbial fuel cells as well as correlational researches will be of greater significance in the near future.

Microbial Fuel Cells

According to the study, microbial fuel cells (MFCs) are microorganisms which can convert chemical energies into electricity by the catalytic degradation of complex organic substrates via its self oxidation-reduction reaction. The functions of MFCs are based on the anaerobic fermentation of the cell, and here is its working-mechanism in general: during the anaerobic fermentation procedure, substrates around the negative pole are broken down into water and carbon dioxide, producing electrons and protons as well. Then, electronics transfer to the positive electrode through the external circuit, protons move to the other electrode through the solution and eventually electrons, protons and oxygen will react to form water around the positive pole. We can achieve supplying electricity to the external environment with the electron mobility in the course. MFCs can be divided into several kinds including medium auxiliary MFCs, free medium auxiliary MFCs, microbial electrolysis cells, soil microbial energy cells. MFCs can be applied in many suspects, such as generating electricity through the degradation of biomass or organics in sewage and detecting substances in solutions as a biosensor. Therefore, in recent years the MFCs have attracted the attention of a growing number of researchers.

Using biofilm to construct primary batteries

In the research that treating sewage by means of MFCs to generate electricity, researchers found that the capacity of electricity generationand conduction greatly enhanced after the formation of biofilm, compared to a single free bacterial microorganism, which inspires us to generate electricity after making MFCs to produce biofilm.So far,a lot of researches have been done on the machanism of the formation of microoganism's biofilm.And gene CsgD is the rendezvous point of all kinds of regulatory signals in the fomation process of the biofilm,whose production acts on the downstream promoter to activate the expression of downstream genes as a regulon,thereby inhibiting the formation of flagella and promoting the formation of pilus and the secretion of extracellular polysaccharide, ultimately promoting the formation of biofilm.
Researches about the process of biomass generation indicate that electron generation is critical in the process. Currently, NADH as an electron donor is facing the challenge of NAD+ regeneration, namely how to consume NADH formed in the metabolic process effectively without excessive harmful NADH accumulation in the cell. NADH oxidase (NOX2) used in this project is an oxidoreductase which can catalyze the transformation from NADH to NAD+ and consumes oxygen. The final product of this process is water, avoiding the influence of H2O2(another kind of NOX(NOX1)’s end product).
In our project, NOX2 can only function properly under anaerobic condition. Only in this way can electrons that generated in the process of the oxidation of NADH be transferred to the other electrode in an external circuit effectively, then, the function of the two electrodes of the battery can be achieved.
So, the entire design of the project is built around how to produce oxygen-free environment, that is, how to form biofilm effectively as well as how to use the biofilm to generate and transmit electrons.