Chlorophyll-f-containing Photosystem I

Lead Research Organisation: Imperial College London
Department Name: Life Sciences


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.

Technical Summary

We wish to understand how Photosystem 1 (PS1) is able to function using the recently-discovered pigment, chlorophyll-f. This pigment provides less energy than the chlorophyll-a and so could represent a more efficient system.
We wish to understand how evolution has dealt with performing the very reducing PS1 photochemistry while in the presence of oxygen, but with less energy. The structure and energetics must have been tuned by evolution to optimize function under these conditions. By elucidating the nature of these changes in detail, we should obtain the knowledge required for future bioengineering of long-wavelength photosynthesis into other species and ultimately into crops.
We shall determine the locations, arrangement and functions of the 7-8 chl-f molecules present in chl-f PS1 using a combination of site-directed mutagenesis, spectroscopy and structural approaches. We shall determine what other modifications (tuning of absorbed wavelength, redox potentials, distances, etc) have taken place in PS1 to allow the system to work using less energy.
To do this we shall develop molecular biology methods i) to improve the purity and quantity of isolated PS1; and ii) to allow site-directed modification of chl-f PS1. We shall use the improved preparations to solve the structure of chl-f PS1 and compare it to standard PS1. The structural studies will localize the positions of the pigments including the chl-f molecules and monitor the structure of the modified proteins. We shall use the modified PS1 to attribute chlorophylls-f to spectroscopic features.
Our primary aim is to test our current model. 1) We shall test our proposal that chl-f is the "A-1" chlorophyll, and 2) whether both of the symmetrical A-1 chlorophylls are chl-f. 3) We shall locate the remaining 5-6 chl-f molecules among the antenna and establish their absorption maxima. This will serve as a basis for structure-function experiments which are relevant to chl-a PS1.

Planned Impact

The research falls under the remit of two BBSRC strategic priorities: "Bioenergy: generating new replacement fuels for a greener, sustainable future" and "Sustainably enhancing agricultural production". Central to both is photosynthesis research and in particular research aimed at improving the energy efficiency of photosynthesis, as both priorities rely on increases in crop yields.
Photosystem 1 (PS1) is one of the two light-powered enzymes that drive oxygenic photosynthesis, the energy input for life on the planet. The specific focus of this project is the newly discovered long wavelength PS1, (Nurnberg et al 2018 Science). Our goal is to understand how it works, from the electronic level to the ecological level. This work should provide the knowledge base from which long-wavelength photosynthesis may be engineered into crops. Theoretically, long-wavelength photosynthesis could provide a very significant increase in photosynthetic efficiency and thus in crop yields. Our research should determine 1) its feasibility, 2) how to do the engineering, and, 3) how best to implement it, e.g. bioreactors, lower leaf canopies of crop plants.
We shall also the use the unprecedented physical properties of the chlorophyll-f PS1 to understand the structure, function and mechanism of normal PS1.
Academia and education. The proposed research will bring new insights to understanding energy conversion in oxygenic photosynthesis, the energy input into the biosphere. It will test our current understanding normal PS1, as the far-red version does the same job chemically but with less energy. This will have a major impact in the academic field and for non-specialists interested in energy in biology. This is text book stuff and will impact the education sector. The possibility of long wavelength crops interests the academic sector and brings a new world of interesting bioenergetics problems. It impacts the search for life in other worlds, which considered red light as the minimal wavelength needed for oxygenic photosynthesis. Our recent work extended this "red limit".
Our 2018 paper on far-red photosystems was voted an F1000 prime article and no 2 in the top ten advances of 2018, the top research paper, by the Chinese Academy of Sciences and the Chinese Academy of Engineering: a strong endorsement of the potential impact of the subject.
The field of photosynthesis will continue to be boosted by our discoveries. There has been a flurry of activity in the subject in recent meetings. Our collaborators in Germany and Australia have obtained research grants based on our work.
Biotechnology and agriculture: long-wavelength photosynthesis could give a major boost to crop yields in standard agriculture, vertical farming and bioreactor productivity (see above). This project provides the scientific foundation on which such a program can be based.
Environmental, ecological, agricultural policy: Our work in this field has a bearing on the feasibility of existing and future agro/energy policy in government, research councils, and groups interested in ecology and sustainability.
Press and public: Our work on far red photosynthesis captured the attention of the press: our 2018 paper in Science had an Altimetrics score of 937, with exceptional interest level compared to articles of the same age and source. It was covered by 90 news outlets including national and international radio, press and on-line magazines and was the most read Imperial news items in 2018.