Microbiologist Robert Burnap is decoding the principles of the most fundamental process of life on planet Earth: photosynthesis. The Einstein Visiting Fellow looks into the evolution of the molecular machines that drive photosynthesis – and uses their mechanisms as a blueprint for artificial catalysts to produce future solar fuels.
When I think about photosynthesis, I see a green tree teeming with life all over its branches. Three billion years ago, when photosynthesis evolved from within the roots of this tree, its growth suddenly exploded and gave rise to the diversity and abundance of life on this planet. Before that, there was no oxygen on Earth and only a primitive form of photosynthesis. This type of anoxygenic photosynthesis still exists today, for example in bacteria that carry out photosynthesis using light energy without producing oxygen. These types of organisms predated the “modern” oxygenic photosynthetic mechanism. Biomass only really exploded when oxygen production became possible through water splitting.
It is difficult to think of a more important process than photosynthesis. Everything emanates from it from the oxygen we breathe and the food we eat to the fossil fuels that are buried in the Earth. Every carbon atom in our bodies has passed through the photosynthetic mechanism at some point. We are essentially manifestations of a chain of consumption that follows this fundamental process.
We do not completely understand yet how the molecular machine that drives photosynthesis evolved, and how its catalyst first assembled. But it turns out that the assembly of the extant catalyst occurs by a process that – like photosynthesis itself – is driven by light. The project I am working on with Holger Dau at the Freie Universität Berlin focuses on this hypothesis. We have demonstrated that anoxygenic photosynthesis uses metals that still comprise the “modern” photosynthetic catalyst today, although in a more simplified way. The ancient molecular mechanism used manganese from the environment as a resource and over time reconfigured itself to use water instead. Nature learned how to use manganese to join two processes to produce high-energy carbon compounds: the harvesting of light energy and the splitting of water molecules. This enabled the emergence of biomass.
Understanding the design principles that have evolved in catalysts in the natural system can provide us with blueprints for artificial devices. We are now combining molecular genetics with biophysical instrumentation and computer models to develop them. Essentially, we reprogram the genetic code of the molecular machines and observe how their functioning changes. Then we model the molecular mechanisms to understand how it all fits together. We hope that this will enable us to emulate the way plants function as highly efficient solar machines to produce future solar fuels and reduce our reliance on fossil fuels.
The Dau Lab is an absolute pioneer in the development of artificial water splitting and is one of the reasons I wanted to come to Berlin. We wanted to make hydrogen by mimicking the water splitting reaction in an artificial catalyst. But to pull hydrogens away from oxygen molecules in water is really difficult chemically because water is a very stable compound. The whole process requires a huge amount of energy. But if we can solve this engineering problem, artificial photosynthesis will help minimize our impact on the environment - and ensure that the tree of life continues to flourish.