Substrate Degradation Kinetics, Microbial Diversity and the Current Efficiency of Microbial Fuel Cells supplied with Marine Plankton

College of Oceanic and Atmospheric Sciences, Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365; Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331; Harvard University, Biological Labs, 16 Divinity Avenue, Cambridge, Massachusetts 02138

^* To whom correspondence should be addressed. Email: creimers{at}coas.oregonstate.edu.

	Abstract

The decomposition of marine plankton in two-chamber, seawater-filled, microbial fuel cells (MFCs) has been investigated and related to resulting chemical changes, electrode potentials, current efficiencies and microbial diversity. Six experiments were run at varying discharge potentials, and a seventh served as an open-circuit control. The plankton consisted of a mixture of freshly captured phytoplankton and zooplankton (0.21 – 1 mm) added at an initial batch concentration of 27.5 mmol L^-1 particulate organic carbon (POC). After 56.7 days, between 19.6 and 22.2% of the initial OC remained, sulfate reduction coupled to OC oxidation accounted for the majority of the OC that degraded, and current efficiencies (of the active MFCs) were between 11.3 and 15.5%. In the open-circuit control cell, anaerobic plankton decomposition (as quantified by the decrease in total organic carbon, TOC) could be modeled by three terms, two first-order reaction rate expressions (0.79 d^-1 and 0.037 d^-1, @ 15°C) and one constant, no-reaction term (representing 10.6% of the initial OC). However, in each active MFC, decomposition rates increased during the third week, lagging just behind periods of peak electricity generation. We interpret these decomposition rate changes to have been due primarily to the metabolic activity of sulfur reducing microorganisms at the anode, a finding consistent with the electrochemical oxidization of sulfide to elemental sulfur, and the elimination of inhibitory effects of dissolved sulfide. Representative phylotypes, found to be associated with anodes, allied with -, - and -proteobacteria as well as the Flavobacterium-Cytophaga-Bacteroides and Fusobacteria. Based upon these results, we posit that higher current efficiencies can be achieved by optimizing plankton-fed MFCs for direct electron transfer from organic matter to electrodes, including microbial pre-colonization of high surface area electrodes, and pulsed flow-through additions of biomass.