# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 3 Research Progress in Energy Harvesting from Microorganisms
### Impact of Other Micro and Macro Organisms in MFCs
#### 3.3.3 Photo-Reactor Aided MFCs  
Biocathodes for MFCs emerged as an efficient solution to the limiting factor that cathodes require presence of expensive ORR catalysts and continuous supply electron acceptors [69]. [PERSON] et al. [176] proposed one of the first photo-reactor coupled MFCs for paddy fields where the air cathodes got abundant oxygen supply from the green rice plants and the system works as an ecological solar cell. In line with the development of plant based MFCs, emerged algae [71,72] and microalgae [177,178] supported cathodes that provide an oxygen supply as well as acting as ORR catalysts. There are reports of air biocathodes [179] and even floating marine biocathodes [68,159]. Chronological improvement of plant based MFCs paved the way for a newer sub-group called photo-reactor coupled MFCs. Photo-reactor coupled MFCs are very promising since they focus on cohabitation of multiple organisms, especially phototrophic ones, for efficient bio-energy production. Such photosynthetic organisms act as in situ oxygenators and facilitate cathodic reactions [180]. In addition to oxygenation, such microorganisms can also act as a regular source of substrate [79]. [PERSON] et al. [181] showed that algae biomass, when used with activated sludge, can be used as an effective MPC substrate component. In line with this, [PERSON] et al. [72] reported an algae-based MFC-biocathode configuration where cathode aeration was achieved by the oxygen generated from the photosynthetic activity of the cathode itself. [PERSON] et al. [182] reported a similar work on a cost-effective, MFC configuration where photosynthetic activity of cyanobacteria was utilized to improve cathode oxygenation. A similar work was reported by [PERSON] et al. [178] where a Photosynthetic Alga MFC was developed for bio-electricity generation using _Chlorella vulgaris_ in the cathode. [PERSON] et al. [183] reported their study on bio-cathodes with aerobic microorganisms as catalysts. [PERSON] et al. [180] summarized current photo-reactor coupled MFC configurations and limitations in their scale-up operations. [PERSON] et al. [78] wrote a mini-review on the recent studies on microalgae processes, MFC process and their coupled systems. [PERSON] et al. [184] concentrated on summarizing wide applications of photo-reactor coupled MFCs. These classes of MFC can be further classified into plant and algae/microalgae based MFCs. [PERSON] et al. [185] first summarized the use of plant-based biocathodes to be included with MFCs. These plant-based MFCs generate bio-electricity using both living plants and bacteria [184] and offer immense possibilities for powering wetlands on a large scale [186]. The reason is that wetland plants possess the unique ability to release oxygen as well as organic matter into the rhizosphere. This principle was successfully utilized to implement a biocathode, buried in the rice rhizosphere that successfully acquired electrons from root excreted oxygen [187]. Similar plant-based MFCs have also been reported by [184,188,189]. This method is extensively used in combined constructed wetland-MFC systems [137,186,190,191]. They are also found to be more efficient in energy generation than photovoltaic cells and are also capable of removing various pollutants as well as biosensing [184]. In addition to the finding that phototrophic organisms grown on cathodes can provide additional oxygen supply [180], the finding that bacteria can also be used as bio-catalysts to accept electrons from the cathode electrode emphasized the development of bio-cathodes [192]. The closed loop of bio-mass with oxygen utilization and generation with microalgae have been intensively studied [78]. Works of [PERSON] et al. [193] and [PERSON] et al. [71] also prove this observation. Microalgae-based MFCs are capable of removing CO2 from air and nitrogen and phosphorus contaminants from waters [78; 180]. [PERSON] et al. [194] utilized this principle for simultaneous biodegradation of textile dye waste and generation of bioelectricity and obtained promising results.