By Nala Rogers

Two sensors adorn the shell of a live pocketbook mussel. One measures the mussel’s heartbeat, while the other tracks when the shell opens.
Credit: Lee Hauser

In tanks at the University of Iowa, mussels equipped with heart rate monitors are purifying water with their excrement. Like human heart monitors, the gadgets glued to the mussels’ shells provide information about activity and metabolism. But in the mussels’ case, this information is helping researchers understand how mussels cleanse the water of agricultural runoff.

“We’ll have very dirty river water, and we’ll put a mussel into a little bucket and the next day it will be just clear as day,” said Daniel Vial, an electrical engineering student at the University of Iowa in Iowa City who helped design the mussel heart rate monitor.

The new device uses the same principle as human heart rate sensors that clip to people’s fingers, according to Vial. An infrared light penetrates the mussel’s shell and reflects off its internal organs, tracking the ebb and flow of blood. The mussels are also equipped with “gape sensors” that detect when their shells are open.

To learn how mussels purify water, the researchers are recording how their gapes and heart rates change as they digest meals of algae. The team presented their research during a poster session Tuesday morning at the American Geophysical Union’s fall meeting in San Francisco.

Mussels could help solve problems caused by nitrogen-containing fertilizers, according to the researchers. When nitrogen washes from fields into water sources such as the Mississippi River, it can spur large growths of microscopic algae known as phytoplankton. After the phytoplankton dies, its decay can strip the water of oxygen. This process has led to algal blooms that poison other organisms with toxic byproducts, as well as oceanic “dead zones” where the oxygen is too depleted for most creatures to survive.

Mussels eat phytoplankton, including the nitrogen locked in algal tissues. They excrete nitrogen in the form of ammonium, and bacteria then convert some of the ammonium into harmless gas. But in order to design effective programs to combat pollution with mussels, engineers need to know how much nitrogen mussels will scrub from the water under different conditions, according to Craig Just, an assistant professor of environmental engineering at the University of Iowa and one of the team’s leaders.

“I hope that out of this research, we’ll learn when and how much mussels eat, poop, pee, and puke with respect to nitrogen,” said Just. “Then we can quantify their impact on that system.”

Today’s mussel populations in the upper Mississippi River basin are just a tenth of their historic levels, according to Just. Before the invention of plastic, people harvested mussels to make shell buttons. Now, sediment runoff from agriculture smothers mussels in mud. Just plans to use his findings to advocate for mussel habitat restoration.

The heart rate and gape monitors could also turn mussels into “biological sensors” for water quality, according to Vial. Currently, the monitors transmit information through wires, but eventually, Vial says, they will send wireless signals through the water to a nearby base station that can launch data over the internet. This setup will allow the team to build networks of monitored mussels in natural habitats. Signals from the mussels could provide an early warning of contamination with toxic chemicals, including chemicals that water treatment plants don’t routinely test for.

Nala Rogers is a science communication graduate student at UC Santa Cruz