**1.2 Bioelectrochemistry**

Many microorganisms are exoelectrogenic. This means that they can generate an electrical current by connecting their electron transport system outside of their cell membranes either directly to an electrode or indirectly through added mediators or electron shuttles. Some microorganisms are electrotrophic, which means that they can harvest electrons to grow [12]. Bioelectrochemistry studies show how these microorganisms can be used in electrochemical cells to remove organic matter from wastewater and generate electricity.

There is a wide variety of bioelectrochemical systems. Some of the most studied, are the microbial fuel cells, illustrated in **Figure 2** (left). These devices generate electricity by using an exoelectrogenic biofilm-coated anode, also called bioanode, to oxidize organic matter and spontaneously generate a current that reduces oxygen at the cathode. [8]. On some occasions, electrotrophic biofilm-coated cathodes, also called biocathodes, can be implemented instead of regular counter electrodes to drive other reactions such as ammonia oxidation and nitrate reduction to nitrogen [13]. Another widely studied form of bioelectrochemical system is the microbial electrolysis cells. These devices have the same components as microbial fuel cells, with the difference that there is no oxygen in the cathodic side of the cell, which allows for other chemical species such as hydrogen ions to be reduced. The main drawback of these systems is that they need an applied voltage to drive the reduction reaction [8].

Exoelectrogenic and electrotrophic microorganisms are so diverse that they can be used to build bioelectrochemical systems with many applications for wastewater and

### **Figure 2.**

*Microbial fuel cell (left) and microbial electrolysis cell (right).*

saltwater treatment such as removal of organic compounds, water desalination, denitrification, and sulfate removal while generating electricity or storing chemical energy in the form of hydrogen or other high-value compounds such as methane [8, 12, 13].
