Photosynthetic water oxidation driven by near infra-red light

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Photosynthesis is the process that converts solar energy into the chemical energy that powers life. The light is used to split water, removing some of its electrons and using them to pull down carbon dioxide from the atmosphere to make the building blocks and fuel for life. When water is split in this way, protons (hydrogen ions) and oxygen are released. The oxygen accumulates in the atmosphere, reacting with UV to form the protective ozone layer. The oxygen also provides a reactive environment that allows respiration to occur. Both of these roles of oxygen were crucial for the development of multicellular organisms: life as we know it.The most important photosynthetic enzyme is photosystem II (PSII), the water splitting enzyme. It is the enzyme that changed the planet. Water is very unreactive and splitting it is hard to do. An enzyme capable of splitting water seems to have evolved only once and all O2-producing photosynthesizers, from the most ancient cyanobacterium to the oak tree, use the same enzyme.Such difficult chemistry requires a lot of energy and this comes from sunlight. The amount of energy in light depends on its colour and PSII uses red light absorbed by a pigment called chlorophyll a. Until recently it was thought that all PSIIs have chlorophyll a at the heart of the process. There have been decades of discussion about why red light (680nm) is the lowest energy needed to perform water oxidation: this is known as the red-limit.The red limit was questioned when it was found that a marine bacterium, which was shaded by a green sea-squirt (!), had chlorophyll d performing the photochemistry at around 710nm. An even longer wavelength pigment, chlorophyll f, was discovered recently. This time it was not just a quirky one-off in a weird ecological niche, chlorophyll f was found to be present in a wide range of common cyanobacteria. However the chlorophyll f is only made when they grow in near-darkness, shaded from visible light but exposed to far-red/near-IR light, e.g. deep in bacterial mats in hot springs, or in some rocks. The role of chlorophyll f is generally considered to be only for gathering light but not for the photochemical part of photosynthesis. We have now found that the chlorophyll f does seem to perform photochemistry in PSII. This surprising result represents a major extension of the red limit.These strange far-red PSIIs perform normal PSII chemistry and yet they are quite different from normal PSII in energy terms. In the present project we intend to study this new world of long-wavelength photosynthesis, to follow up our surprising discovery, to understand how it works, to assess what changes have occurred that allow PSII to function with less energy, and to see if the move to lower energy gives better energy efficiency. Since it seems unlikely that there is such a thing as a free lunch, we shall also test if the improved energy efficiency comes with penalties in terms of it resilience to variations in light intensity, for example. This project will involve studying PSII in living cells, membranes and in the isolated enzyme using a range of biochemical and biophysical methods.This demonstration of oxygenic photosynthesis working well beyond the established red limit, takes us into a realm of the subject that is largely unstudied; and yet longer wavelength photosynthesis is already a high profile engineering target aimed at making crops and bioenergy more efficient. Normal photosynthesis is inefficient and much effort goes into thinking up ways of improving it. Engineering longer wavelength photosynthesis seemed a far-off pipedream but now it turns out that nature has already done the engineering. Our aim here is to determine if moving to far-red photosynthesis will provide a useful technological target with an improved energy budget and to test if it comes with a loss of resilience that could restrict the use of engineered long-wavelength photosynthesis to specific growth conditions.