## 4 The Role of Macrophyte in Bioelectricity Generation
The DO concentration of the unplanted was remarkably lower (1.95 mg/L) than that of the planted. However, among the planted, the effluent of CWMFC planted with _Canna indica_ demonstrated the highest DO concentration (3.31 mg/L), which indirectly led to higher bioelectricity production than the unplanted. Similarly, [PERSON] et al. [42] also assessed the influence of plant radial oxygen loss (ROL) in CWMFC in the removal of nitrobenzene from an aqueous solution. They compared three macrophyte species: _Scripus validus_, _Typha orientalis_, and _Iris pseudacorus_, with unplanted CWMFC. Generally, their results demonstrated that the planted CWMFC were more efficient in power production and nitrobenzene removal than unplanted. However, relatively, _Scripus validus_ attained the highest power density 19.5 mW/m\({}^{2}\) due to its high DO production of 2.57 \(\pm\) 0.17 mg/L and root biomass of 16.42 \(\pm\) 0.18 g\(\cdot\)m\({}^{-2}\). In CWMFC, because of root oxygen exudation, aquatic plants play a crucial role in achieving a high oxidation-reduction potential between the two regions. In comparing the contribution of macrophytes in the creation of redox conditions necessary for power production, Doherty  
Figure 3: Oxidation and reduction reaction in their respective chambers. et al. [33] in their experiment, also reported that the presence of macrophytes had a more significant impact on redox conditions by creating a redox gradient that was 20 percent greater than that of the unplanted. Likewise, in a study conducted by [PERSON] et al. [56], the macrophytes' overall influence on redox conditions within the wetlands was analyzed by contrasting the planted and unplanted wetlands wrought under a continuous flow regime. Their results proved that plants increased the redox gradient with a subsequent energy production of 16 mW/m\({}^{2}\) higher than the unplanted. Similarly, [PERSON] et al. [54], in their study to assess the role of macrophytes in an up-flow CWMFC by comparing planted with unplanted systems, established that DO at the cathodic region for planted systems was higher with a high reduction potential, compared to unplanted, making the planted system perform better than the control (unplanted) in their electrical output. The main transport mechanisms of oxygen in macrophyte CWMFC is by the air pressure gradient from the atmosphere and mainly via diffusion from the aerenchyma cells of roots during plant photosynthesis [39; 40; 57]. The oxygen secreted from the root aerenchyma cells creates a micro-oxidizing environment in the rhizosphere; hence, the redox potential of the system is greatly influenced [58; 59]  
However, aside from plant root oxygenation, the secretion of exudates also plays a crucial role in bioelectricity production. Root exudates secreted from wetland plants are organic matter, and electroactive bacteria that form naturally within the anode region oxidize the exudates, producing electrons for the production of bioelectricity. Hence, macrophyte species with such propensity will have a great influence on the system's electrical output [59]. [PERSON] et al. [60] investigated the effect of four different native species--_Aglaoema commutatum_, _Epiremum aureum_, _Dranaceae braumii_, and _Philodendor cordatum_--as the only endogenous substrate from their root exudate and the effect on bioelectricity production. _Philodendor cordatum_ attained the highest power density of 12.5 mW\(\cdot\)m\({}^{-2}\) with a 20.6 mg/L maximum production of root exudates. Therefore, at an optimal condition for root exudate release, the CWMFC performance in terms of bioelectricity was observed to be higher. Different species based on their substrate type, transport, and exudate release will have a different effect as demonstrated by [60]. [PERSON] et al. [61] also investigated the effect of root exudates and oxygen release by four wetland plants, _Canna indica_, _Cyperus alterniferlius_, _Acorus calamus_, and _A. donax_, with unplanted CWMFC on power production.