Root deposits comprise water-soluble exudates: secretions,
lysates, gases, and mucilage. Within exudates, carbohydrates,
carboxylic acids, and amino acids are highly degradable to microorganisms[
18] and, therefore, the most responsible for electron
donation. Bacilio-Jimnez et al. tested the composition of
carbohydrates, carboxylic acids, and amino acids in rice root
exudates and found that glucose comprises approximately
90% (mol/mol) of the total amount of components.[19] Glucose
is one of the most suitable substrates for MFCs, and glucosefed
MFCs generate the highest power density.[20] However, coulombic
efficiency in glucose-fed MFCs is the lowest (15%) compared
to that in acetate-fed (72 %), butyrate-fed (43 %), and
propionate-fed MFCs (36 %). A possible reason is that glucose
can be anaerobically degraded by non-electricity-generating
bacteria, and thus a large proportion of electrons are not transferred
to the anode.