R E S E A R C H L E T T E R Growth with high planktonic biomass in Shewanella oneidensis fuel cells Martin Lanthier1, Kelvin B. Gregory2 & Derek R. Lovley3 1Eastern Cereal and Oilseed Research Center, Agriculture and Agri-Food Canada, Ottawa, ON, Canada; 2Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA, USA; and 3Department of Microbiology, University of Massachusetts, Amherst, MA, USA OnlineOpen: This article is available free online at www.blackwell-synergy.com Correspondence: Derek R. Lovley, Department of Microbiology, University of Massachusetts, Amherst, MA 01003, USA. Tel.: 11 413 545 9651; fax: 11 413 545 1578; e-mail: dlovley@microbio.umass.edu Received 5 July 2007; accepted 3 September 2007. First published online 8 November 2007. DOI:10.1111/j.1574-6968.2007.00964.x Editor: Skorn Mongkolsuk Keywords Shewanella ; microbial fuel cell; bioﬁlm; anaerobic respiration. Abstract Shewanella oneidensis MR-1 grew for over 50 days in microbial fuel cells, incompletely oxidizing lactate to acetate with high recovery of the electrons derived from this reaction as electricity. Electricity was produced with lactate or hydrogen and current was comparable to that of electricigens which completely oxidize organic substrates. However, unlike fuel cells with previously described electricigens, in which cells are primarily attached to the anode, at least as many of the S. oneidensis cells were planktonic as were attached to the anode. These results demonstrate that S. oneidensis may conserve energy for growth with an electrode serving as an electron acceptor and suggest that multiple strategies for electron transfer to fuel cell anodes exist. Introduction There is a compelling need to identify pure cultures that can serve as appropriate models for electron transfer to the anodes of microbial fuel cells because little is known about the physiology of this process. Shewanella species were the ﬁrst organisms proposed to transfer electrons to the surface of electrodes via electron-transfer proteins (Kim et al., 1999a, b, 1999). This, coupled with the relative ease in growing these organisms, has led to their common use as model organisms for the study of microbial fuel cells (Kim et al., 2002; Park & Zeikus, 2002; Gorby et al., 2006; Ringeisen et al., 2006; Bifﬁnger et al., 2007; Cho & Ellington, 2007). However, little is known about the physiology of Shewa- nella species growing with anodes serving as the sole electron acceptor. Most studies (Kim et al., 2002; Park & Zeikus, 2002; Cho & Ellington, 2007) have evaluated elec- tricity production in short-term studies with cell suspen- sions. Cell suspension studies do not give appropriate information on the long-term functioning of microbial fuel cells that is necessary for applications (Shukla et al., 2004; FEMS Microbiol Lett 278 (2008) 29–35 Lovley, 2006). As previously discussed (Lovley, 2006), in the one previous study designed to evaluate Shewanella growth with an electrode serving as the electron acceptor (Kim et al., 1999a, b, 1999), growth was not directly coupled to electri- city production and a low percentage (o 10%) of the electrons available in the electron donor (lactate) was recovered as current. The possibility that the cultures might have been contaminated (Kim et al., 2002) further compli- cates analysis of the data. In a more recent study, which was designed to optimize power output of a growing culture of Shewanella oneidensis, electron recovery as current remained o10% (Ringeisen et al., 2006). In order to learn more about the long-term sustain- ability of Shewanella-based microbial fuel cells, the growth of the most commonly studied strain of the Shewanella genus, S. oneidensis MR-1, was investigated. The results demonstrate that mechanisms for growth and electron transfer with electrodes serving as the electron acceptor in S. oneidensis are different than the patterns observed in electricigens, microorganisms that completely oxidize organic fuels with direct electron transfer to the anode surface. c 2007 The Authors Journal compilation c 2007 Federation of European Microbiological Societies Published by Blackwell Publishing Ltd.