Science Twitter is a great place – for education and entertainment. This ‘cyanobacteria meme’ was tweeted by Jaida Elcock, also known as @soFISHtication, on June 15, 2020, and quickly racked up more than 1000 retweets and 5000 likes. As a physicist with a somewhat sketchy memory of first-year biology, I felt like I needed a bit of a refresher on the whole photosynthesis thing. Next thing I know, I’d fallen down a fascinating rabbit hole, learning a ton about some very cool research into generating electricity from biophotovoltaic cells. But, first, we need to back up a bit.
I freely admit that I fall into the naïve category of “everyone” here in thinking that trees and greenery are the suppliers of our oxygen. Or, I should say, I thought that, until this tweet led me to discover that photosynthesizing trees and land-based plants actually only provide about half of our oxygen. The other half comes from phytoplankton, an eclectic mix of (mostly) single-celled creatures that live in fresh and salt water (see illustration at the top). Cyanobacteria are just one form; collectively these organisms can be thanked for not only producing the air we breathe, but also for forming the foundation of the aquatic food chain.
Now that we are all in on @soFISHtication’s joke, you may be wondering how physics enters this story. Don’t get me wrong, I’m always happy to learn something new no matter what the subject, but my physicist tendencies draw me toward physical systems and applications. So, the papers that grabbed my eye when digging into this topic were all about the possibility of harnessing the photosynthesizing power of these teeny little organisms to generate electricity, in a nascent field called biophotovoltaics. When cyanobacteria use sunlight to form sugar and oxygen, there are steps along the way that generate electrons. The idea is to intercept some of these electrons and send them into a connected electrical circuit, thereby generating a current out of sunlight, carbon dioxide, and water. It really doesn’t get much greener than that in terms of electrical generation – microorganisms doing what microorganisms do, but providing us with a little bit of electricity as a by-product!
And when I say a little bit, I do mean a little bit. The devices that have been built so far have peak currents that are measured in microamps, which means that biophotovoltaics are not going to single-celledly solve the world’s energy problems. But, as with any young field, there are rapid developments occurring to improve both the overall efficiency as well as the total output of these systems. One paper published last month in Nano Letters by a team from South Korea coupled cyanobacteria to a nanoscopic structure consisting of gold (Au) particles and zinc-oxide (ZnO) rods. Their setup was designed to expand the range of light frequencies that could be converted to electricity, as well as to amplify the signal from the organisms.
Unlike conventional solar-powered devices, biophotovoltaics (BPVs) generate current both when the light is shining and when it isn’t, since electrons are generated during darkness from a chemical reaction involving the sugars formed by photosynthesis. And, although it almost sounds like science fiction, a British team reported in Nature Communications in 2017 that they had successfully produced a BPV using an inexpensive commercial inkjet printer to print a ‘bio-ink’ of cyanobacterial cells onto a carbon nanotube conducting surface. The device provided enough current to generate flashes in a connected LED light, suggesting that their paper-based printed BPV could work as an environmentally-friendly power supply for biosensors that only require intermittent bursts of energy. Printing BPVs is certainly a tantalizing hint that this technology could scale, with these authors suggesting the possibility of “bioenergy wallpaper” in our future.
As I said at the beginning, this particular rabbit hole has been a fascinating exploration into a world I knew nothing about. These studies really did blow my mind: fabricating devices that extract electrons from individual living cells is truly a technological marvel. I’m looking forward to following this work in the future and learning much more about the microscopic but mighty cyanobacteria. Thanks Twitter!
Electricity generation from digitally printed cyanobacteria (2017) Marin Sawa, Andrea Fantuzzi, Paolo Bombelli, Christopher J. Howe, Klaus Hellgardt & Peter J. Nixon Nature Communications volume 8, Article number: 1327 (2017) https://www.nature.com/articles/s41467-017-01084-4
Biophotovoltaics: Green Power Generation From Sunlight and Water (2019) J. Tschörtner, B. Lai, and J. O. Krömer Frontiers in Microbiology 10:866 doi: 10.3389/fmicb.2019.00866
A Broadband Multiplex Living Solar Cell (2020) M. J. Kim, S. Lee, C.-K. Moon, J.-J. Kim, J. R. Young, Y. S. Song Nano Letters doi: 10.1021/acs.nanolett.0c00894