The Electric Bacteria That Could Save Wastewater Treatment

By Peter Chawaga

A research team from the University of California, Santa Barbara has been working on a unique bacterium that may one day naturally produce the energy needed to treat wastewater.

The bacterium, Shewanella oneidensis, has the rare ability to live without oxygen, breathing instead through the reduction of heavy metals like iron, lead, and uranium. What’s truly incredible is that as it breathes this way, the bacterium also produces electricity.

“Shewanella has naturally evolved to live in environments that contain no oxygen,” said Zach Rengert, a member of the research team, Bazan Research Group. “It has a special set of membrane proteins that allow it to respire on solid-state substrates, such as iron-containing minerals in a process that is essentially the opposite of rust formation.”

This makes the bacterium particularly appealing for wastewater treatment, as it can remove heavy metal contaminants and simultaneously produce energy in an anaerobic, or oxygen-free, environment.

“One of the costs of wastewater treatment is pumping oxygen into tanks to allow aerobic bacteria to consume organic contaminants,” said Rengert. “By using anaerobic bacteria, you can produce electricity and negate the need for oxygenating the water.”

The Bazan Research Group first became interested in the bacterium as part of its work using electricity-conducting compounds known as conjugated oligoelectrolytes (COEs) to improve organic contaminant removal.

“Our original motivation for using Shewanella originated from its popularity as an electrogenic organism in microbial fuel cells,” said Rengert. “We had been studying the use of COEs to modify the properties of model lipid bilayers and to increase organic matter removal in wastewater containing a complex consortium of microorganisms. Shewanella represented a model organism in which we could study the effects of COEs in a more controllable fashion.”

The group created a synthetic molecule known as DSFO+ that could seamlessly incorporate itself into Shewanella bacteria and, hopefully, increase its capacity to generate electricity.

“DSFO+ will spontaneously incorporate into the membrane of a cell,” Rengert said. “This is due to its molecular structure having charged groups on the periphery and ‘greasy’ constituents in the center, thus mimicking the nature of a lipid bilayer.”

Upon studying the effects of the molecule on the bacterium, the researchers found that they had successfully modified its conductive capacity.

“It improved Shewanella’s current generation by catalyzing current production from the inside of the cell to the electrode of the microbial electrochemical cell,” said Rengert. “It approximately doubled current generation.”

What all of this means is an enhanced capacity for the bacterium to be used in anaerobic digestion and to save on energy costs at wastewater treatment plants.

“It takes energy to treat wastewater and this [electrical generation] can offset the energy needed, thus reducing costs,” Rengert said.

While the study focused on Shewanella, Bazan Research Groups sees potential in applying COEs more broadly to other sources of power generation.

“Wastewater treatment plants have naturally-occurring bacteria from the wastewater,” said Rengert. “Shewanella represents a model organism to study the effects of our COEs, but we envision this COE to be applicable to broad classes of organisms, such as those found in a wastewater treatment facility. Further work needs to be performed to ascertain the applicability to other microbes and its toxicity to human and other organisms, as well as its efficacy on large scales.”

The team hopes to modify the COEs and make them more effective by altering their reduction-oxidation (redox) reaction, a critical component of electron transfer. This will hopefully mean more efficient, organic, cost-saving wastewater treatment coming to a plant near you.

“[We envision] designing new redox-active COEs that have a better ability to transport charge in a way similar to the native redox-active proteins and which have a lower toxicity,” Rengert said. “Also, additional mechanistic studies can elucidate key design principles for the creation of better COEs.”