The thick biofilm is a combination of cells loaded with cytochromes, metal-based proteins, and pili, hairlike protein filaments discovered and patented by Gemma Reguera, MSU’s associate professor of microbiology.
She said that microbes have a taste for waste in their natural state. Her bioelectrodes also have a big appetite for waste and are ready to be scaled up and used to cleanup industrial sites while producing electricity as a byproduct.
The biofilms are like an electrical grid. Each cell is a power plant that generates electrical discharges, which are then delivered to the underlying electrode using a network of cytochromes and pili. The cytochromes are the transformers and towers supplying electricity to the city. The pili represent the sparse-but-mighty powerlines that connect the towers, even those far away from the power plant, to the grid.
Cytochromes and pili work together for shorter ranges. As more cells stack on the electrode, the efficiency of the cytochrome as electron carrier diminishes, and the pili do all of the work – discharging electrons 1,000 times faster than normal.
“The pili do all of the work after the first 10 layers, and allow the cells to continue to grow on the electrode, sometimes beyond 200 cell layers, while generating electricity,” said Ms. Reguera.
“This is the first study to show how electrons can travel such long distances across thick biofilms; the pili are truly like powerlines, at the nanoscale,” added Ms. Reguera.
The cytochromes lose their transfer speed as the distance increases. One cannot continue to grow the biofilm on the electrode without the wires, she added.
The methodical approach to dissect the contribution and interactions between the cytochromes and the pili was the key to this discovery. The researchers used a genetic approach to eliminate key electron carriers in the biofilms, cytochromes and conductive pili, and studied the effect of the mutations in the growth of the biofilm and ability to generate electricity. The researchers also generated a mutant that produced pili with reduced conductivity.
“We used the mutants to grow biofilms of precise thickness and capacity to produce electricity. This information allows us to reconstruct the paths – cytochromes or pili – used by the cells to discharge electrons across the biofilm and to the underlying electrode,” Ms. Reguera said.
A key finding was how the biofilm is mechanistically stratified as it grows in thickness on the electrode without compromising electricity generation.
“We went from constructing the cell equivalent of a 10-story building to a 15- and a 20-story building and demonstrated the coordinated action of cytochromes and pili in the bottom floors and the need to discharge electrons via the wires in the upper floors to grid. We know that we can build 200-story buildings, which really opens up opportunities for which these biofilms can be used,” added Ms. Reguera.
The next phase of this research will look at potential spinoff company options to bring the bioelectrodes to market.