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Many bacteria can convert chemical energy to electrical energy: they oxidize diverse organic substrates, transfer electrons to anodic electrodes and thus generate electricity in microbial fuel cells (MFCs). In the marine environment, microbial fuel cells termed either sediment or benthic microbial fuel cells, have been developed to generate power via anodic bacteria in the ocean sediment. Power is dependent upon enriched anodic bacteria that transport their electrons onto the anode. The marine deployed MFC systems can provide renewable, harvested power to trickle charge batteries or other storage devices. Through power management systems these storage devices can power traditional electronic loads of interest. The systems have the promise to allow for long term deployment of in-water sensor and communications systems, providing decreased maintenance and increased operational capabilities. In this study, two sediment microbial fuel cells were deployed in the San Diego Bay over a 60 day time period. The fuel cells deployed in the field for the purpose of sampling bacteria on and adjacent to graphite sheet anodes buried in marine sediment. The anodes were connected electrically via a potentiostat to a carbon fiber brush cathode, which floated freely in the water column. Succession and spatial response of anodic bacterial population structures were monitored. The anodes were buried in the marine sediment containing an organic carbon content of ∼ 1.4% TOC. Sediment cores (1 cm × 5 cm) were extracted on each side of two parallel anode electrodes, in the space between the electrodes (∼2.5 cm away from anode), and 15 cm away from the anode. Sediment cores were individually homogenized and 0.5 g per sample of the sediment was used to determine most probable number (MPN) of iron- reducing bacteria; another 0.5 g per sample of sediment was used for molecular biology analysis of DGGE (denaturing gradient gel electrophoresis) and cloning (data analysis in process; to be presented at conference). Results demonstrated that power increased logarithmically over a two week period; similar to a bacterial growth curve chart. After 15 days, numbers of iron- reducing bacteria were higher by two orders of magnitude next to the anode versus 15 cm away from the anode. When the cathode became anoxic, current production decreased accordingly; demonstrating that anodic bacteria were dependent upon the microbial fuel cell potential and responsible for the power produced. DGGE analysis of the bacteria in the iron- reducing medium demonstrated unique results by Day 60. Relative to the control, observed responses were populations of bacteria that over time became more similar to each other next to the anode, and also, in the space between the two anodes (5 cm)-but were very different 15 cm away from the anode. This result insinuates that bacterial groups not only respond to anodic electrochemistry, but, are using cell to cell contact to transfer electrons or, may be transferring electrons to a quite further distance (cm) via electrically conductive appendages (Reguera, 2005; Gorby, 2006; Nielsen, et al, 2010). This is the first bacterial study investigating the potentially cm scale electron transport of sediment microbial fuel cells in the field environment. This data will be presented at conference. |
