Chlorophyll-f-containing Photosystem I

In 2018 we discovered a new type of photosynthesis that does photochemistry using chlorophyll-f, which absorbs infra-red light. Photosynthesis uses sunlight to provide the energy for life on the planet and put the oxygen into the atmosphere. Since its appearance, the oxygen generated by photosynthesis formed the ozone layer that screens out the deadly UV and allowed respiration to occur, leading to the evolution of complex life.

Photosynthesis also pulled down most of the CO2 from the atmosphere and converted it into living matter, resulting in conditions on the planet appropriate for the current inhabitants.

Given the importance of photosynthesis, the discovery of a new kind did cause a stir. The new process is found in some bacteria that do normal, visible-light photosynthesis. However, when these bugs find themselves in darkness, shaded by other photosynthetic organisms that use the visible light but not the infra-red, they are able to switch-on a special suite of genes to make new photosynthetic enzymes that works with infra-red light.

We showed that chlorophyll-f does the key light-driven chemical reactions at the heart of this type of photosynthesis.

This discovery was a surprise as the standard type of photosynthesis shows little or no fundamental variation across all of the wide range of photosynthetic species, from cyanobacteria to oak trees. It had been assumed that the energy of visible light absorbed by chlorophyll-a was only just sufficient to do the demanding chemistry. The discovery that lower energy, longer wavelength light could be used to do exactly the same process, was thus highly unexpected.

Photosynthesis is inefficient in energy terms and this makes agriculture inefficient too. This is why we put in enormous quantities of energy, in the form of fertilizers, pesticides, and processes, to get the yields we need. Unsurprisingly, for years scientists have been researching ways of improving photosynthesis.

Recently there have been remarkable advances in which increased crop yields were obtained by modifying the regulation processes that optimize light use and protect plants under changing light conditions. A major intrinsic inefficiency in crops is that leaves in the lower canopy are shaded from the light by those in the upper canopy. The new infra-red photosynthesis could in principle be introduced into crops to function when needed in the shaded leaves.

This could give a marked increase in photosynthetic yields. Extending the spectrum of photosynthesis to longer wavelengths has been talked about for years but it seemed a rather unrealistic task. The finding that evolution has already done it, makes the whole idea more feasible.

The current project involves studies that are needed to learn what evolution has done to get the system to work with less energy. By comparing the new system with the standard one, we have already advanced rapidly in this area, but the present project is the first to deal specifically with the infra-red-driven Photosystem I, the enzyme that provides the energy boost needed to fix CO2 into living matter.

To do this we will identify exactly which of the enzyme's chlorophylls are chlorophyll-f and how they are tweaked by the protein to make them do the job. We will combine molecular biology, biochemistry and biophysics to sort out the structural and mechanistic details required to understand how it works. In this way we can provide the knowledge needed to determine the feasibility of crop improvement and the best ways to implement it.

The new Photosystem I also provides an opportunity to disentangle the individual chlorophyll contributions. Unlike the conventional chlorophyll-a systems, where all (95) pigments are the same color, these new systems have a small number of distinct chlorophylls-f in key positions. This undreamt-of decluttering of the color spectrum should allow old mysteries to be resolved. We hope to do that too in this project.