Some types of bacteria living in seafloor mud can generate enough electricity to power small electronic devices, field tests have shown. The months-long trials may herald a new generation of fuel cells that can supply reliable low-voltage power to future networks of ocean-bottom sensors in long-lived instruments for which changing batteries would be impractical, if not impossible.
Many microbes live in the sediments that accumulate at the bottom of the ocean, says Leonard M. Tender, a chemist at the Naval Research Laboratory in Washington, D.C. As these bacteria consume the ooze’s organic carbon, which comes primarily from the decayed remains of marine organisms, they produce waste products that include negatively charged ions. As a result, there’s a voltage difference of up to 0.8 volts between the top few centimeters of seafloor sediment and the seawater immediately above it.
In other words, it’s a natural fuel cell. Placing one electrode of a fuel cell in the microbe-infested mud and the other in the overlying water provides a low-resistance path for the electrons to follow and produce usable current.
First, using large saltwater aquaria, Tender and his colleagues demonstrated that the bacteria could generate enough current to operate devices such as pocket calculators. Then, last year, the researchers conducted long-term field experiments in a New Jersey salt marsh and an Oregon estuary. The electrodes in their prototype fuel cells were graphite disks about the size of a manhole cover. Small holes drilled in the disks increased the surface area that the microbes could colonize.
They also enabled the devices to sink into the mud more easily, says Tender.
Within a few days of being placed in the sediments, prototype fuel cells were producing several milliwatts of power. The chemically inert graphite electrodes didn’t corrode and weren’t consumed by the electricity-generating reactions. The researchers report their results in an upcoming Nature Biotechnology.
Tender and his colleagues are now investigating various coatings for the electrodes that might increase the efficiency of electron transfer and thus boost the electrical power the fuel cells can generate.
Although the microbe-powered fuel cells tap into a source of free energy, they may not generate enough electricity to power some ocean-bottom instruments, says F. Beecher Wooding, a mechanical engineer at Woods Hole (Mass.) Oceanographic Institution. For example, they would be too weak to run the seafloor seismometers that he deploys. Those devices, which record data 200 times each second on small, rotating disk drives, need about 600 milliwatts of power.
Instruments that acoustically transmit information to scientists at the surface–a technique Wooding describes as “power-hungry but data-slow”–could use such fuel cells only if the electrodes measured several square meters.
Nevertheless, says Wooding, the proposed microbial fuel cell could work for devices that sample data infrequently and record them in digital memory. Such instruments include those that measure temperature, salinity, and ocean current.