Microbial Fuel Cells for Wastewater Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 414

18.5 Exoelectrogens
In nature, there are many microorganisms possessing the ability to transfer
electrons derived from the metabolism of organic matters to the anode. Micro-
organisms capable of extracellular electron transfer are generally called ''exo-
electrogens''. These microorganisms attain their required energy by oxidizing
organic matter with the release of protons and electrons that are used in MFC to
produce electricity. Marine sediment, soil, wastewater, fresh water sediment and
activated sludge are rich sources for these microorganisms (Niessen et al. 2006 ;
Zhang et al. 2006 ). In the beginning, it was considered that only a few types of
bacteria were capable of producing electricity and most of them were gram neg-
ative Proteobacteria such as Shewanella putrefaciens (Park and Zeikus 2002 ),
Geobacter sulfurreducens (Bond and Lovley 2003 ), etc. However, now gram
positive bacteria also have been discovered to produce electricity, including
Clostridium butyricum within the Firmicutes (Park et al. 2001 ). The capability to
produce electricity generally depends on the nature of bacterial species and their
ability to utilize different substrates. Power generation also depends on the optimal
growth condition of bacteria, e.g., pH and temperature.
Up to now, most of isolated exoelectrogens are bacteria (Table 18.1 ) and were
isolated from different MFCs using large varieties of substrates. Scientists are
trying to discover new exoelectrogenic bacteria, which will have the capacity to
achieve high power density. The pure strain Geobacter sulfurreducens operated in
a two-chamber MFC with PEM and graphite electrode produced an electric current
density of 65 mA/m 2 using acetate as the substrate (Bond and Lovley 2003 ). Other
pure strains such as Comamonas denitrificans DX-4 and Citrobacter sp. SX-1
produced the highest power and current density of 35 mW/m 2 and 205 mA/m 2
using acetate and citrate as electron donors in MFC respectively (Xing et al. 2010 ;
Xu and Liu 2011 ). One scientific report showed that power output in a MFC
inoculated with a pure culture (Geobacter metallireducens) or a mixed culture
(wastewater inoculums) was similar, with 40 ± 1mW/m 2 for Geobacter metal-
lireducens and 38 ± 1mW/m 2 for the wastewater inocula (Min et al. 2005 ).
However, Rhodopseudomonas palustris DX-1, isolated from an air cathode MFC,
produced electricity at higher power densities (2720 ± 60 mW/m 2 ) than mixed
culture in the same device using complex substrates including volatile acids, yeast
extract and thiosulfate (Xing et al. 2008 ). In addition, some bacteria require
exogenous redox compound to increase maximum power production. For example,
Shewanella putrefaciens generated the maximum power density of 10.2 mW/m 2
when operated in the absence of exogenous electron acceptors in a single cham-
bered MFC, but current production by Shewanella putrefaciens was enhanced
10-folds when an electron mediator, i.e., Mn 4+ or neutral red was incorporated into
the graphite anode (Park and Zeikus 2002 ).