December 2002

Microbe Power

A new fuel cell taps the energy of seafloor sediments.

By Ken Garber

Leave organic sediments on the seafloor for 80 million years, and they might turn into crude oil. But some microbiologists and geobiologists aren't willing to wait that long to exploit the sediments' latent energy. They're developing a simple, inexpensive fuel cell-just two disk-shaped electrodes and a connecting circuit-that generates electricity when planted in bottom-of-the-ocean muck. "The seafloor constitutes a ready-made battery," says the Naval Research Laboratory's Leonard Tender, who coinvented the device with Clare Reimers of Oregon State University.

The marine sediment along continental coasts is about two percent organic carbon, mostly from dead plankton. Microbes ingest and oxidize the carbon, transferring sheared-off electrons to chemicals in the sediment. The transfer creates a voltage between the ocean bottom and the overlying seawater, a potential difference that generates a current when one electrode placed in the muck is joined together with another above it. Such devices, tested off the New Jersey and Oregon coastlines, have generated steady low-level power for nine months at a stretch. "Every indication is that they would have run forever," says Tender.

An unexpected finding was that much of the power comes from the biological activity of bacteria, called Geobacter, that colonize the electrodes. These bacteria transfer electrons directly to the buried electrode rather than the surrounding sediment, according to recent studies by Derek Lovley, a microbiologist at the University of Massachusetts, Amherst.

The fuel cells can now generate about one watt of electricity-enough to power oceanographic instruments such as temperature sensors. That power level is "remarkable," says Michigan State University microbiologist Greg Zeikus, who is working on similar microbial fuel cells for generating electricity from municipal wastewater. "That's the target we all want to achieve." Powering military surveillance devices is one possible application. Harbor pollution cleanup is another because, Tender says, the fuel cells accelerate the microbial decomposition of organic toxins.

Tender speculates that seawater fuel cells could even contribute clean, abundant energy to the nation's power grid. Placing hundreds of the devices over deposits of frozen methane found in some coastal sea-floor sediments should, in theory, yield large electrical currents. "Right now we're at the one-watt level," says Tender. "But there's no reason why any of this cannot be scaled up to the megawatt region in the right environment."