## 4 The Role of Macrophyte in Bioelectricity Generation  
The production of electricity in CWMFC is essentially the result of oxidation-reduction (redox) reactions through electrochemical or biochemical reactions at the anode and cathode chambers, respectively, as shown in Figure 3[18; 21]. The cathode electrode in the cathodic chamber features as an electron donor, whereas the anode fundamentally aids as an electron acceptor [49]. The acceptance of electrons from the cathode electrode for the reduction reaction in the cathode chamber is the responsibility of certain chemical compounds known as terminal electron acceptors (TEA). Transitional metals (e.g., platinum (Pt)) and nitrate (NO\({}_{3}\)) compounds are materials often used as TEEs [50; 51]. Nevertheless, oxygen (O\({}_{2}\)) is generally recommended due to its sustainable characteristic and relatively high redox potential [51; 52], for example, organic matter degradation using glucose as the substrate and oxygen as the electron acceptor. Therefore, using DO at the system's air-water interface is considered ideal to make oxygen available for this reaction. Given that, some researchers employed air-cathodes to increase the cathodic reduction reaction by engaging the cathode material at the air-water interface. However, this arrangement was affected by huge ohmic losses because such an arrangement widens the distance between electrodes (the anode and the cathode), which increases the internal resistance of the cell [22; 23]. In addition to the air-cathode that suffered setbacks of increased internal resistance, some other researchers also employed supplementary artificial aeration. According to [PERSON] et al. [53], the supplemental artificial aeration was recognized as a form of energy wastage and inhibited the denitrification process. The use of macrophytes to meet this major requirement became a huge breakthrough for the optimal functioning of the system. The prerequisite for oxygen without any setbacks in the upper cathodic region for the mentioned reduction reaction makes the macrophyte component a significant part of the system to guarantee optimum performance and efficiency [14]. The macrophyte plants use sunlight as an energy source during photosynthesis to turn water and carbon dioxide into chemical energy (glucose), and the by-product is gaseous O2 in their rhizosphere, which is then made available as TEAs to complete the energy production cycle in the system [54] as shown in Figure 4. Oxygen is made available for the reduction reaction by utilizing the oxygen excreted by the macrophyte roots within the cathode zone. Research has shown that with such incorporation, the wetland plant improves the cathode potential of the CW-MFC for optimum energy production. In an experiment conducted by [PERSON] et al. [48], they compared CW-MFC planted with _Ipomoea aquatic_ and unplanted CWMFC systems. It was evident from their study that the plants' presence can boost the system's electricity production by 142% [48]. They attained a maximum power density (P\({}_{\text{D}}\)) of 12.42 mW\(\cdot\)m\({}^{-2}\) generated from the system incorporated with _I. aquatic_, which was 142 percent greater than the 5.13 mW\(\cdot\)m\({}^{-2}\) generated from the system without aquatic macrophytes [48]. It was also observed by [PERSON] et al. [55] that _Ipomoea aquatic_ in CWMFC improved power generation from 0.191 to 0.302 W\(\cdot\)m\({}^{-3}\) by improving the cathode potential from 0.522 V to 0.603 V as a result of the oxygen availability by the plant root [35; 54]. This result agrees with earlier experimentation from [PERSON] et al. [44], where, to ascertain the contribution of _Hydrilla verticillata_ in a CWMFC with regards to D6 fixation, they observed that the DO of the planted system was 9 times higher than the unplanted. Further studies by [PERSON] et al. [40] also assessed the DO concentration of four CWMFCs: unplanted CWMFC, and three CWMFCs planted with _Canna indica_, _Acorus calamus_, and _Ipomoea aquatic_.