Microbial communities and electrochemical performance of titanium-based anodic electrodes in a microbial fuel cell

Abstract

Four types of titanium (Ti)-based electrodes were tested in the same microbial fuel cell (MFC) anodic compartment. Their electrochemical performances and the dominant microbial communities of the electrode biofilms were compared. The electrodes were identical in shape, macroscopic surface area, and core material but differed in either surface coating (Pt- or Ta-coated metal composites) or surface texture (smooth or rough). The MFC was inoculated with electrochemically active, neutrophilic microorganisms that had been enriched in the anodic compartments of acetate-fed MFCs over a period of 4 years. The original inoculum consisted of bioreactor sludge samples amended with Geobacter sulfurreducens strain PCA. Overall, the Pt- and Ta-coated Ti bioanodes (electrode-biofilm association) showed higher current production than the uncoated Ti bioanodes. Analyses of extracted DNA of the anodic liquid and the Pt- and Ta-coated Ti electrode biofilms indicated differences in the dominant bacterial communities. Biofilm formation on the uncoated electrodes was poor and insufficient for further analyses. Bioanode samples from the Pt- and Ta-coated Ti electrodes incubated with Fe(III) and acetate showed several Fe(III)-reducing bacteria, of which selected species were dominant, on the surface of the electrodes. In contrast, nitrate-enriched samples showed less diversity, and the enriched strains were not dominant on the electrode surface. Isolated Fe(III)-reducing strains were phylogenetically related, but not all identical, to Geobacter sulfurreducens strain PCA. Other bacterial species were also detected in the system, such as a Propionicimonas-related species that was dominant in the anodic liquid and Pseudomonas-, Clostridium-, Desulfovibrio-, Azospira-, and Aeromonas-related species.

FIG. 1.

(a) Cell voltage and anode potential for each electrode (R = 1,000 Ω) during the first 8 days of the microbial fuel cell operation. (b) Power output for each electrode as determined by cyclic voltammetry on an Fe(III)-reducing cathode. (c) Current monitored over time for 25-mV sequential anode potential steps, ranging from −450 mV to −350 mV, for each electrode.

FIG. 2.

Dominant bacterial populations present in the MFC analyzed by DGGE, where A indicates the anolyte, B indicates the Ta-Ti electrode surface, and C indicates the Pt-Ti electrode surface. (a) Major acetate-oxidizing populations present in the anolyte and on the surface of each electrode. (b) Comparison of dominant acetate-oxidizing bacterial populations in the anolyte (A), on the surface of the two electrodes (B and C), and in selected Fe(III)-reducing enrichments (D and H), subsequent transfers (E and I), and NO₃⁻-reducing enrichments (J). The following bacterial isolates are represented: Lac319 (F), T33 (I-7), N968 (K), and reference strain Geobacter sulfurreducens PCA (G). Arrowheads at bands A-5, E-1, E-6, H-2, H-7, and J-9 indicate excised bands. Samples were run on a single DGGE gel. The digital image was cropped and minimally edited for brightness and contrast; desired sample lanes were chosen for presentation.

FIG. 3.

Phylogenetic relationships, based on 16S rRNA gene sequences of isolates and excised DGGE bands (shown in bold) and close relatives, with bootstrap values of >70% shown. Nitrate reducers N968, N959, and band J-9 and Fe(III) reducers T32, T33, C328, C314, Lac319, and bands E-6, H-7, H-2, and E-1. The cladogram was created using the neighbor-joining method of the MEGA4 application (54). The bootstrap consensus tree inferred from 1,000 replicates was constructed to represent the evolutionary distances of the taxa analyzed.