CHAPTER 1
INTRODUCTION TO ELECTROCHEMICALLY ACTIVE BIOFILMS

Jerome T. Babauta and Haluk Beyenal

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, USA

1.1 INTRODUCTION

Microbial respiration is based on electron transfer from electron donors to electron acceptors - a series of reactions facilitated by a cascade of energetic substances; these are well-known reactions described in the literature [1-4]. The donors and acceptors of electrons are typically dissolved substances; however, some microorganisms can use solid electron donors and/or solid electron acceptors, such as minerals and metals, in respiration [5]. Electron transfer by microorganisms to and from external electron acceptors or donors is termed extracellular electron transfer. Extracellular electron transfer is typically studied using the model organisms Shewanella oneidensis and Geobacter sulfurreducens. However, the focus on extracellular electron transfer has been extended toward interspecies electron transfer, as well as electron transfer into microbes [6-10]. The exact mechanisms of extracellular electron transfer between microorganisms and solid substances remain a matter of debate in the literature [1, 2, 11-20]. One of the goals of this book is to provide fundamental knowledge needed to study the exact nature of extracellular electron transfer processes. Two points of view are usually presented in this debate: (1) that electrons are transferred by conduction through extracellular materials or elongated appendages called nanowires [12, 14, 17, 18, 21, 22] and (2) that electrons are transferred across a redox gradient by electrochemical reactions using either freely diffusing redox mediators, also known as electron shuttles [11, 15, 16, 19, 20, 23-25], or bound redox mediators at sufficient density within the biofilm to allow for electron hopping across redox sites [26, 27]. This book covers both mechanisms and describes how to perform the measurements for each mechanism. Biofilms with microorganisms capable of electron transfer to and from solid electron acceptors have been used in microbial fuel cells (MFCs) to harvest energy from various environmental processes [28]. The biofilms grown on the electrodes of MFCs are called electrochemically active biofilms (EABs), which admittedly is a misnomer, as all microorganisms are electroactive in the respiration process. EABs are also known under several other names in the literature dedicated to MFCs, such as electricigens, electrochemically active microbes, exoelectrogenic bacteria, and anode-respiring or anodophilic species. However, because the hallmark of EABs is the ability to exchange electrons with solid surfaces such as electrodes, we believe that the term "EABs" refers to the most basic property of these biofilms. Because this book focuses on extracellular electron transfer processes between biofilms and inert electrodes, the term "EABs" is fully appropriate. The link connecting EABs, electrochemistry, and electrochemical techniques is also considerably clearer for a more general audience.

The use of EABs in MFCs is not new. For various reasons, these devices have attracted some attention in the literature recently. In particular, researchers have recognized their potential use as alternative sources of energy. The attention MFCs receive is fully justified, although some expectations of their ability to deliver large amounts of energy combined simultaneously with high power appear overly optimistic. It has been demonstrated that MFCs are successful sources of energy to power electronic devices that consume low levels of power continuously or to power electronic devices requiring higher power intermittently [29-35]. Lately, the concept of power management systems is gaining popularity [33, 36-44]. Much of the interest in using MFCs stems from the idea of harvesting energy from wastewater treatment processes, which at present are wasteful processes in which energy-rich streams are reclaimed without useful energy being obtained [45]. There has been an estimate presented at conferences referring to the amount of energy that could be harvested from all wastewater treated in the United States if the entire chemical oxygen demand were converted to the equivalent number of electrons and used to power external devices. In our opinion, this calculation resembles the computations estimating how much gold could be extracted from seawater - and how rich one could get by doing so - if we were to treat all the oceans in the world. There is some truth in these calculations, of course, but they neglect the costs and the technical problems associated with harvesting energy from wastewater or extracting gold from seawater. MFCs are fascinating devices, and they no doubt will find practical applications through integrated power management systems. In the short term, however, it is difficult to see how they can meaningfully contribute to solving the impending energy crisis. We are afraid that similar claims may be promoted on similar microbial technologies based on MFCs [46, 47]. Just how practical these applications will become remains to be seen.

Collectively, MFCs and the newer biologically catalyzed electrochemical cells have come to be known as bioelectrochemical systems (BESs) [48-52]. As BES research becomes more sophisticated, it appears that BESs can provide new insights into the fundamental mechanisms of electron transfer between microorganisms and solid substances. This application can deliver interesting results sooner than the expectations of harvesting large amounts of energy from wastewater processes can be fulfilled. There is a lot of excitement about using BESs, and understandably, all expectations may not be fulfilled.

The immediate goal of this book is to describe the state-of-the-art research techniques for studying electron transfer processes in EABs. The future of MFCs or any technology based on them will be based on fundamental electrochemical and microbial research on electron transfer processes in biofilms. This directly translates to revealing the processes governing electron transfer between a biofilm and an electrode and not the incremental improvement of power output from MFCs. Thus, we would also like to call attention to something we find promising: the use of BESs as tools of discovery in studying the process of electron transfer in EABs. For example, high-throughput, efficient BESs could be used to select for new EABs [53, 54]. Many researchers could use this technology as a tool for understanding the biochemistry of these unique microorganisms. This use of BESs may be much less glamorous than the promise of delivering power to the national grid, but at the same time, the expectations we set are more realistic than converting all or even a large part of the chemical oxygen demand in wastewater into electron equivalents and using it to power external devices. As BES researchers, we are somewhat concerned that focusing on glamorous, but, in our opinion, currently unrealistic, expectations may do damage to a legitimate and very interesting field of research. We hope that this book can help identify and evaluate the strengths and the limitations of BESs for both generating energy and studying the mechanisms of electron transfer between microorganisms in biofilms and solid substances. Electron transfer between biofilms and solid surfaces was known long ago [55]; however, MFC research and related research tools have critically contributed to new developments and mechanisms of extracellular electron transfer by combining electrochemistry with microbiology. In the following sections of this chapter, we describe EAB research from an electrochemical perspective, focusing on single electrodes rather than on reactor systems. We use the term BES to describe generic electrochemical cells utilizing microbe-based half-cells. The term "biofilm electrode" refers to the microbe-based half-cell where the EAB is grown. Finally, we note that these sections have been extended and re-written from a previously published work to serve as an introduction to this book [56].

1.2 ELECTROCHEMICALLY ACTIVE BIOFILM PREPARATION AND REACTOR CONFIGURATIONS

Although there are many techniques for quantifying EAB extracellular electron transfer mechanisms, the quality and interpretation of the results are highly dependent on the way the study is conducted. Factors that are often selected arbitrarily, such as (1) the biofilm electrode material, (2) how the EABs are grown on the biofilm electrodes, (3) the reactor configuration used to grow the EABs, and (4) the reactor configurations used to study the extracellular electron transfer processes, have a critical impact on the resulting EAB and its ability to participate in extracellular electron transfer processes. Identifying the effects of each factor on EAB performance may serve as a basis for optimizing systems toward maximizing the rate of energy conversion.

1.2.1 Electrode Materials

The electrode material used to construct a biofilm electrode affects the measured current and open circuit potential (OCP) of the EAB grown on it, and therefore, the choice of electrode material is important for the standardization of reported values. Traditionally, cheaper graphite, carbon paper, carbon granule, carbon brush, or carbon felt electrodes are used in MFC practical applications [57, 58]. These carbon materials suffer from high background currents that can mask the electrochemical responses of redox species at low concentrations. In our...