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Research Feature: Benthic Microbial Fuel Cells
In December, Mark Nielsen, an Earth’s Subsurface Biosphere IGERT student, defended his Ph.D. on benthic microbial fuel cells. These devices use sea floor microbes to generate electricity and could one day be used to power oceanographic instruments. In this Web interview, Mark describes his research, particularly experiments he carried out at deep ocean seeps in Monterey Canyon, California. Mark also described this work in a November 2008 article in Energy and Environmental Science.
What are benthic microbial fuel cells and what are some of their potential uses?
A benthic microbial fuel cell converts chemical energy into electrical current. Microorganisms play two key roles in the system: 1) they establish the electrochemical redox potential, and 2) they facilitate the transfer of electrons from donors to the circuit. The objective is to generate a steady supply of power that could be used to operate remote oceanographic instruments – things like water quality sensors or acoustic receivers that track the movements of tagged animals. These devices are usually powered by batteries. If you could power them with fuel cells instead, then you’d have a continuous power supply and the life of the instrument wouldn’t be limited by battery life.
My academic advisor, Clare Reimers, and others developed early prototypes of benthic microbial fuel cells. My project has focused on improving the technology and scaling it up to provide useful amounts of power.
Can you explain the fundamentals of how benthic microbial fuel cells work?
When particulate organic matter settles to the bottom of the ocean, it provides a carbon source for microbes living in sea floor sediments. As they consume the organic matter, the microbes use up oxygen and create anoxic conditions in the sea floor sediment. There is a voltage difference between the anoxic environment of the sediment and the overlying oxygen-rich sea water – it is that voltage difference that the fuel cell exploits. Microbes mediate the transfer of electrons from the environment to the electrical circuit in two ways. Some microbes can metabolize organic carbon and transfer the electrons directly to a solid surface (the electrode in this case). Other microbes produce reduced metabolites, such as sulfide, that can react with the electrode to deliver electrons to the circuit.
In your paper in Energy and Environmental Science, you describe experiments you carried out at cold seeps – places where chemical-rich fluids emerge from sea floor sediments. Why did you choose this setting?
In earlier studies, we had found that we could increase the amount of power generated by our fuel cells if we constantly flushed water past the anode. We could create this flushing by pumping, but that sort of defeated the purpose of the fuel cell because then we had to expend power to run the pump. So we decided to experiment with settings in the ocean where natural forces produced flow or advection, and cold seeps are one of those settings.
What were the major findings of your Energy and Environmental Science paper?
First, this paper shows that a new design of the fuel cell, called a chambered benthic microbial fuel cell, can produce substantially more power than the solid anode fuel cells. The solid anode seemed to plug the seep, so after a while, power production dropped off. The chambered design produced more sustained power – up to 600 times the amount produced by the solid anode cell. The chambers were made out of half-cylinders of sewer pipe with a carbon-fiber brush anode inside. We positioned them over a seep, and water would flow through the chamber and out through one-way valves. We produced up to 60 mw of power with this design – we were excited about this because that’s enough power to run some low-power oceanographic instruments.
A second thing we describe in this paper is an analysis of the organisms that make the fuel cell work. We extracted DNA from the microbes that were attached to the anode fibers in the chambers to identify the type of microbes that were present in the fuel cells. This was an opportunistic experiment and we don’t have a lot of replicates – but our limited number of samples suggests that the microbial community changes depending on the amount of circulation or advection within the chamber. We had the most diverse microbial community in our chamber with the most advection.
What were some of the logistics involved in setting up this experiment?
As with many oceanographic studies, we were limited by the remoteness of the site. Our site in the Monterey Canyon was about 1000 meters under water and only reachable by submersible. It’s not like a lab experiment where you can have a lot of replicates and run a lot of trials! Due to a short lead time, I had one chance to design and build the fuel cells and there was no way to know if they were working until we retrieved the equipment at the end of the experiment. I finished building the fuel cells a couple of days before we had to leave for California and then Clare and I drove about 13 hours down to Monterey for the cruise. We sailed on the R/V Pt Lobos from the Monterey Bay Aquarium Research Institute (MBARI) and I was amazed at the technological capabilities of their remotely operated submersible.
What are your plans now that you are finishing your Ph.D.?
I am going to do a postdoc at Harvard University with Peter Girguis. He was one of our collaborators on this paper and is a microbiologist. The focus of my dissertation was very applied – I spent a lot of time figuring out how to make the fuel cells work. With Peter I will focus more on basic research relating to the process of extracellular electron transfer. Some of the questions we will be studying include “What are the functions of the various organisms within benthic microbial fuel cells? And what biological processes are involved in transferring electrons? I will also be looking more at the sediment geochemistry.
For more information about Mark and his work with Clare Reimers and benthic microbial fuel cells see also:
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