Microscopic organisms are emerging as the sustainability workhorses of the 21st century, having graduated from their traditional composting chores into biofuel production, microbial fuel cells and even self-replicating growable ink. They could do so much more in terms of bio-electronics, but the cell membrane forms a “greasy barrier” that blocks the flow of electrons between the organism and its electronic cousins. That barrier appears likely to fall, though, as researchers at Lawrence Berkeley National Laboratory (LBL) have begun to rev up the electron transfer pathway by using the familiar bacteria E. coli as a testbed.
Discovering the Pathway to Bio-Electronic Systems
Previous research has established that a relatively weak form of electron transfer actually does occur naturally in a species of bacteria called Shewanella oneidensis MR-1, according to LBL writer Lita Stephenson.
That pathway consists of a complex of proteins called MtrCAB, which enables electrons to cross the cell membrane and get to metal oxides or minerals. By this mechanism, S. oneidensis can literally “breathe” on metals, even when oxygen is not available.
In order to develop a more robust system, researchers began engineering E. coli to express large quantities of MtrCAB proteins, and they were able to get it to reduce nanocrystals of iron oxide (reduction refers to the taking on of electrons or releasing oxygen).
However, the research soon hit a wall. The pathway was operational but the engineered E. coli worked even less efficiently than S. oneidensis, with reduced growth and slower electron transfer.
E. Coli Gets a Balanced Workout
The new research has leaped that hurdle to demonstrated that strains of engineered E. coli can be coaxed into generating a measurable current of electricity.
The problem was, as the team discovered, that too much of a good thing really is too much of a good thing. The excess MtrCAB proteins were basically making E. coli sick, by interfering with cell health. In order to find the right balance, the team took a more holistic look at the pathway. They screened hundreds of variants to arrive at a more effective strain of E. coli, as described by Stephenson:
“Interestingly, the strain that produced the greatest current at the anode was not the strain designed to maximize the number of electron conduits expressed in the membrane. Instead, the strains with optimized cellular health produced the maximum observed current, despite having only a moderate level of the electron transport proteins.”
The end result, as published in the journal ACS Synthetic Biology, is that any cell has the potential to be transformed into a living wire that generates electrical energy and interacts directly with manufactured electronics, with the added bonuses of being able to self-repair and self-replicate.
Microbes to the Rescue
We’ve noted before that more than one hundred years ago, science fiction icon H.G. Wells foresaw a future in which the humble bacteria saved the human race from destruction by technologically superior aliens.
As it turns out, he only got the microorganism part right. The aliens is us, in the form of unsustainable energy extraction, production and consumption.
As for the role of microorganisms in our salvation, the newly published E. coli research is just one part of a much broader slate of cutting edge, energy related research programs under the new Molecular Foundry facility at LBL, which after all is a Department of Energy laboratory.
The Naval Research Laboratory is another hotbed of microbial energy systems research, in the form of a fuel cell that uses microbes to harvest energy from sea floor sediment.
Researchers are also looking into strains of “extreme bacteria” that thrive in extremely hot or cold environments, as well as “super-bugs” like Geobacter, which can generate electricity from mud or wastewater.
Image: Courtesy of Lawrence Berkeley National Laboratory
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