Volume 6, Number 2, 1997

Not your average stick-in-the-mud

No one was dumb enough to stick these things in the mud before," oceanography professor George W. Luther III says jokingly, when asked why his rugged microelectrode for measuring swamp scum and sea slime wasn't invented sooner. "It's a solid-state device, and we take special steps to protect it from fouling."

Microelectrodes are not new, but most existing sensors rely on membranes that don't perform reliably in field environments. Also, Luther says, they only characterize a single gaseous compound.

Containing a gold wire plated with mercury, the UD sensor is hardy enough to withstand salt marshes, harbors, bays and other swampy marine settings where the delicate balance of nature is constantly changing because of natural chemical events such as the decomposition of organic matter. The sensor also lets researchers measure key components of these environments-including dissolved oxygen, iron, manganese, hydrogen sulfide and iodide-simultaneously. Hydrogen sulfide, for example, can be detected at levels as low as one part per billion.

Thus far, Luther and his colleagues have used the probe primarily to learn more about the complex natural chemistry of wetland, coastal and ocean ecosystems. But, the findings could ultimately help researchers develop more effective strategies for preventing or mitigating damage to the environment. After all, Luther notes, "If we want to understand how pollution resulting from human activities might impact fresh water and marine environments, we first need to know exactly what's happening in these systems, on a day-to-day basis. The microelectrode probe is an extremely useful new tool for gathering that information."

Luther and graduate student Paul Brendel developed the original glass-encased device by inserting a tiny gold wire into the center of a very thin-walled glass tube that's just 200 microns in diameter and about 4 centimeters long. Then, they removed undesirable chemical species by applying electrical voltages to the surface of the electrode.

Now, Luther has teamed up with marine studies professor Stephen C. Dexter and graduate student Kunming Xu, who tweaked the technology to analyze thin organic "biofilms" on metals in seawater. Corrosive ocean biofilms can quickly damage metal parts on boats, docks and off-shore platforms, Dexter says. Excess metal in seawater also may endanger marine wildlife under some conditions, he adds.

To make sure his invention had the 'right stuff,' Luther field-tested the original microelectrode in Hawaii's Kaneohe Bay. "It's very much like a Delaware salt marsh," he says, "in that it cycles iron very rapidly." In the future, he might subject the probe to more extreme tests: in Hawaiian volcanoes and hydrothermal vents, which are loaded with gases such as hydrogen sulfide and methane, as well as iron and other metals. If the probe can withstand Hawaii's low-oxygen hydrothermal vents, he argues, "it could probably go just about anywhere."

Even, he hopes, on a deep-sea lander, where it would travel to the ocean floor. Someday, it might be possible to measure chemical species on board a ship, using the UD microelectrode by remote-control, Luther says.

Already, marine studies Associate Prof. Craig Cary is using the microelectrode probe to measure the chemical components of samples retrieved from deep-sea thermal vents. When oceanic plates move in opposite directions, Cary explains, the resulting volcanic activity can super-heat water deep in the earth's crust, ejecting it through the ocean floor at temperatures as hot as 680 degrees Fahrenheit. While thermal vents produce dramatic towers of black smoke, or "chimneys," he says, they also generate "diffuse-flow sites," where bacterial communities thrive on a diet of hydrogen sulfide.

Cary, who has made nine trips to the ocean floor in a submersible vehicle, brings samples of these organisms back to the laboratory, then measures the chemistry of their dynamic habitat. The resulting data provide crucial information about the organisms' geochemical impacts on diffuse-flow sites.

Dexter, meanwhile, is developing more earth-bound uses for the microelectrode. By shrinking the sensor's tip to a mere 25 microns in diameter, Dexter and Xu refined the probe to study ocean biofilms. Whenever metals corrode in seawater, Dexter notes, they interact with microscopic organisms and metabolic by-products-commonly known as "slime." These interactions trigger an elaborate series of reactions as microorganisms consume oxygen to produce various other chemical species. In this way, microbes may speed the corrosion of metal surfaces.

Researchers won't be able to prevent biofilms from forming on boats, docks and the ocean's surface until they know more about the chemical reactions taking place inside these slimy films, he says. That's easier said than done, because biofilms are "extremely heterogeneous, meaning that their chemistry varies from point to point within the surface," he adds.

Fortunately, UD researchers are now using the miniaturized microelectrode to simultaneously measure concentrations of dissolved oxygen, manganese and iron, as well as pH levels, in seawater biofilms grown on platinum and stainless steel surfaces. The sensor's tiny tip can generate accurate measurements at 1.5-micron intervals within the biofilm, Dexter says.

"Dissolved manganese species were found in the presence and absence of oxygen, whereas iron species were only detected in anaerobic (oxygen-free) conditions within the biofilms," Luther reports. "So, we're finally beginning to get a little bit better handle on what's happening, chemically, within these fascinating systems."

-Ginger Pinholster and Tracey Bryant