The surfaces of the world’s oceans are vast, but the depths are far vaster. And for scientists who suspect that what goes on down there is crucial to understanding climate change, getting data has proved challenging. Only so much of what goes on below can be inferred from the surface data provided by satellite images or floating ocean sensors.
Ocean processes take place in extremes of time and scale. Things happen in many dimensions at once. While the amount of carbon from atmospheric carbon dioxide transferred into the ocean can be measured, where that carbon goes between the time it sinks into the ocean’s depths, when it becomes part of the ocean’s ecosystem, and when it returns to the surface remains a mystery. The process takes decades, involves both chemical and biological interactions, and is subject to ocean currents that move over enormous distances.
Scientists from national and international research consortiums have now begun scattering a new generation of sensors over the world’s seabeds. These are not your everyday seabed sensors, designed to collect basic data on salinity, temperature, and depth. The new sensors, known by demanding acronyms such as palace, solo, biomaper, and apex, come equipped with arrays of electronics, including miniaturized mass spectrometers able to analyze dissolved gases, Doppler radar that can read currents, DNA analyzers, biosensors, and high-resolution optics.
Inventing a cool sensor is one thing. Keeping it running is another. While land-based sensors can readily relay their data, connect to electricity, and be calibrated, serviced, and replaced -- and while some ocean-surface sensors can run on solar power -- scientists have had to devise some creative methods of keeping seabed and deep-sea sensors, sometimes lying miles below the surface, in operation.
Which is why Clare Reimers, a professor of chemical oceanography at Oregon State University, became intrigued by the prospect of a battery powered by bacteria. She and her colleagues at the College of Oceanic & Atmospheric Sciences depend upon the data collected by scores of electronic sensors set out on the seabed. Replacing their batteries requires Reimers to board a research vessel and spend at least a day at sea. A microbial fuel cell (MFC) that generated enough electricity to run their sensors indefinitely would save a great deal of time and money.
The seabed is full of bacteria, and bacteria release electrons when they break down molecules of organic or inorganic material (in the same way human cells break down molecules of sugar). So why not find a way to harness those charges? While the electrical potential created by even billions of bacteria might be small, Reimers hoped it would be enough to power her seabed sensors. She embedded a graphite plate, a 19-inch disk, into the seabed to attract the electrons freed by the bacteria’s metabolism. Once the electrons gave the plate a negative charge, she connected it to a cathode in the waters above to allow a current to flow.
It worked. A first generation of microbial fuel cells set out on the seabed produced power for more than a year without interruption; during that time Reimers might have had to change the regular batteries in her sensors twice. She and her group are now refining their early models with the goals of increasing the power, now only a few milliwatts, and decreasing manufacturing costs -- critical advances if MFCs are ever to replace batteries on a large scale.
Other nifty sensor-related technologies are also being deployed, and they too need efficient power sources. Scientists have developed sensors that sink, gather data, and then rise back to the surface. (Those deployed beneath Arctic sea ice have been programmed so that if they hit ice when they surface, they’ll sink and repeat the process until they find open water.) Instruments have been attached to roving undersea robotic probes that gather data as they trace a path along the seabed. These autonomous underwater vehicles (or AUVs, as they’re known) could be programmed to find and dock at deep-sea filling stations, where they can transmit their data, refuel, and move on. Refueling would of course be unnecessary if these AUVs could be outfitted with internal microbial fuel cells, enclosed chambers in which bacteria, fed on plankton, for instance, produce electricity. In theory, the AUV could collect the plankton it needs as it roams the sea.
While bacteria power may not work for sensors that transmit large amounts of data at high resolution, Reimers says the technology will have a clear advantage in basic sensors designed for the long haul -- for years and decades of use. Considering that changing the batteries in a deep-sea sensor costs $10,000 to $17,000, the potential savings would be substantial. This year Reimers will deploy her first MFC-powered sensors. Designed to track tagged marine animals, they will replace sensors requiring biannual battery changes.
The principle of microbial fuel cells may soon find applications on land, too. Wastewater treatment plants already make use of bacteria to metabolize organic waste. If harnessed in a series of fuel cells, the bacteria might produce enough electricity to provide all or part of a plant’s needs. For bacteria, producing energy is as easy as breathing.
-- Bruce Stutz
