(or, a love letter to cyanobacteria)
Cyanobacteria are little green critters that have been happily thriving in almost every habitat on this planet for the last 2.7 billion years. They were around long before creatures like us could even survive. In fact, cyanobacteria might very well be the reason you’re around to read this. Before about 2 billion years ago, the Earth’s atmosphere consisted almost entirely of nitrogen and carbon dioxide – not a shred of oxygen to be seen. Definitely not the best habitat for organisms that need oxygen to live and breathe!
Cyanobacteria changed all that. I mentioned they’re green? That’s because they contain a lot of chlorophyll, a pigment that absorbs sunlight (apart from in the green – otherwise they’d be black) and helps harvest its energy to turn carbon dioxide and water into chemical energy that the bacterium can use. This is a process called photosynthesis. You’ve probably heard of it – plants are quite famous for doing it nowadays, although our friends the cyanobacteria got there first.
Importantly to the story at hand, though, one of the products of photosynthesis is oxygen. It’s thought that eventually all these photosynthesising cyanobacteria produced so much oxygen that they irreversibly changed the makeup of the Earth’s atmosphere and accidentally killed off a whole lot of species of early life for whom oxygen was poisonous. This paved the way for multicellular, oxygen-breathing life forms like the ones we know and love to evolve. You win some you lose some, I guess.
In summary, cyanobacteria have been around for ages, caused the death of thousands of species and the evolution of thousands more, and turn carbon dioxide and water into oxygen and sugars using the power of our sun. So what?
Well, for one, photosynthesis is an absolutely fascinating process, and cyanobacteria are excellent model organisms to study it.
Nature has a habit of producing systems that are beautifully suited to their purpose. What if we could harness that, use it for our own ends? Photosynthesis takes sunlight and water, two things that are sustainable and abundant, and produces chemical energy in a form that can be used by the cyanobacterium. There’s a lot of complex biological catalysis and machinery that carry out this process, but the single important takeaway is that it’s all electrons. The sunlight collected by those green pigments excites an electron, which is passed between pigments and quinones and iron clusters and proteins until it’s finally used to reduce carbon dioxide. It’s this electron that I’m interested in.
If you put cyanobacterial cells onto an electrode and shine light at them (and your circuit is all connected up happily) then a current will flow! As well as being the world’s oldest photosynthesisers, these cells also excrete electrons when they do so, meaning we can use them as a means of generating electricity from sunlight. Adding a ‘mediator molecule’ (for example 2,6-dichloro-1,4-benzoquinone, aka DCBQ) will increase this current significantly. Essentially, DCBQ is stealing electrons from photosynthesis, which is, quite frankly, very cool.
In my work, I’m using a combination of electrochemistry and fancy uberfast spectroscopy to investigate how this process works and whether it can bypass the protective mechanisms cyanobacteria have in place to dissipate energy when they’re overwhelmed with too much light. The hope is that we can one day understand these biological processes well enough to hack into them, to design molecules that will efficiently divert excited electrons from the cell’s biological machinery so we can use them for our own ends – whether that be in the form of biological solar cells, bio-hybrid batteries or some other exciting new technology.
If we truly understand nature’s beautiful ways of doing things, we might be able to harness them for a cleaner, greener future.
NanoDTC PhD Student, c2021