By coordinating their metabolic activities, it is possible for bacterial cells to produce electricity.

Despite their tiny size, these specialized bacterial cells interact with one another in intricate ways.

The relationships of two important bacterial forms show an ability to produce electricity by coordinating their metabolic activities, according to research by Jonathan Badalamenti, César Torres and Rosa Krajmalnik-Brown at Arizona State University’s Biodesign Institute.

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A light-sensitive green sulfur bacterium Chlorobium can act in tandem with Geobacter, an anode respiring bacterium and the result is a light-responsive form of electricity generation.

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“Geobacter is not light responsive in its own right because it’s not a photosynthetic organism,” said Badalamenti, lead author of the two new papers on the subject. In contrast, photosynthetic Chlorobium is unable to carry out the anode form of respiration necessary for electricity production. “But when you put these two organisms together, you get both a light response and the ability to generate current.”

The electrons Geobacter acquires from its photosynthetic partner Chlorobium can end up measured and collected in the form of electricity, using a device known as a microbial fuel cell (MFC) — a kind of biological battery.

Microbial fuel cells may one day generate clean electricity from various streams of organic waste, simply by exploiting the electron-transfer abilities of various microorganisms.

The team conducted its research at the Swette Center for Environmental Biotechnology, which is under the direction of Regents’ Professor Bruce Rittmann. The goal of the Center is to exploit microorganisms for the benefit of society. These efforts typically involve the use of bacteria to clean up environmental pollutants or to provide clean energy. In the case of MFC research, bacteria can assist in both of these activities, generating useable electricity from energy-rich waste.

In the new studies, the researchers explore the possibility of enhancing electricity production in MFCs by examining the function of light-responsive Chlorobium, a photosynthetic green sulfur bacterium. The resulting experimental configuration, in which light responsive bacteria play a role in energy generation, is a microbial photoelectrochemical cell (MPC).

To explore the behavior of photosynthetic bacteria in a MPC, the team first used a means of selectively enriching phototrophs such as Clorobium in a mixed culture, by poising the device’s anode at a particular electrical potential that was favorable for phototrophic growth, yet unfavorably low for the growth of non-photosynthetic anode respiring bacteria.

The researchers then noted an intriguing result: Electricity production measured at the anode linked to phases when the MPC was in total darkness and dropped during periods when the bacterial culture ended up exposed to light.

The group detected the presence of Chlorobium in the enrichment cultures using pyrosequencing and reasoned the observed negative light responsiveness was either due to photosynthetic Chlorobium directly transferring electrons to the anode during dark phases or instead, transferring these electrons to a non-photosynthetic anode respiring bacterium like Geobacter, through an intermediary reaction.

Phototrophic organisms like Chlorobium do not have the reputation of carrying out direct anode respiration. “The follow up scientific question was to discern if we had discovered a novel phototrophic anode respiring bacteria or if the phototroph was giving something to the anode respiring bacteria Geobacter and that was the response we were reporting,” Krajmalnik-Brown said.

In subsequent experiments, pure cultures of either Chlorobium or anode-respiring Geobacter ended up examined as well as co-cultures combining the two. In the case of Chlorobium alone, light responsive electricity generation was not there. Similarly, pure Geobacter cultures failed to produce electrical current when deprived of an electron donor like acetate in the medium.

Only when the photosynthetic Chlorobium combined with anode respiring Geobacter in co-culture experiments did electricity generation occur and it did so in a negative light-responsive manner — increasing in periods of darkness and falling off during light phases.

The experimental results of the co-culture study suggest the following scenario: Chlorobium bacteria gather energy from light in order to fix carbon dioxide and fuel their metabolism. During dark phases however, they sustain themselves by switching from photosynthesis to dark fermentation, using energy they have stored. Acetate produces as a metabolic byproduct of this dark phase fermentation.

During periods of darkness, anode respiring Geobacter gains electrons from the acetate produced through Chlorobium metabolism, transferring them to the MPC anode, thereby producing the observed rise in electrical current.

“In this second study, we deliberately removed any sources of electrons that were present in the growth medium,” Badalamenti said. When the two bacterial communities interacted, it was clear Chlorobium was helping to provide food for the Geobacter, in a light-responsive manner.

The authors said one of the attractive advantages of their study is electricity generation measured at the anode can be a highly accurate surrogate for the complexities of bacterial metabolism taking place in the MPC culture.

“Unlike having to measure metabolites or cell growth either microscopically or through chemical intermediates, we are able to construct a co-culture system in which one of the readouts is electricity,” Badalamenti said. “We can then monitor metabolism in the system in real time.”

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