Researchers solve colour puzzle of far-red photosynthesis

Researchers have identified the wavelengths absorbed by five specialised chlorophylls that allow some cyanobacteria to carry out oxygen-producing photosynthesis using lower energy, barely visible, far-red light.

A team from the Department of Life Sciences has mapped the energy landscape of far-red Photosystem II, providing new insight on how energy harvested from sunlight is managed and converted into chemical energy.

The study, published in Nature Communications, assigns the specific wavelengths of all five far-red chlorophylls in the photosystem – a long-term goal that the researchers expected would take years to achieve.

"From its sequences I realised Calothrix might provide some vital clues, but I did not expect it to work so neatly, let alone allow us to piece together the full jigsaw puzzle. It was quite a buzz to see it all fall into place." Dr Marco Leong Lead author

Far-red photosynthesis occurs in cyanobacteria that live in shaded environments, where visible light is strongly attenuated and far-red light dominates.

These cyanobacteria replace some of their chlorophyll-a with chemically red-shifted versions, chlorophylls d and f, allowing them to use lower-energy far-red photons for photosynthesis.

This low-energy version of oxygenic photosynthesis provides a unique opportunity not only to understand how photosynthesis can operate with lower-energy photons but also solve unanswered questions of conventional photosynthesis, the process that provides nearly all the energy for life on Earth.

Solving a colour puzzle

A key question has been identifying where the long-wavelength chlorophylls are located, and which wavelengths of light each one absorbs.

The original plan was to genetically modify the proteins around each chlorophyll and look for changes in colour, assigning the pigments one by one. This approach is challenging and time-consuming.

Instead, the team took a broader view. By comparing genomes, the researchers noticed that some rare far-red-capable species appeared to lack amino acid residues needed to bind chlorophyll-f molecules. This suggested that comparing these “incomplete” far-red photosystems with a more standard far-red system could provide clues to solving the colour/location problem.

The team identified Calothrix sp. as a species that could provide this comparison. The organism grows in tough, gloopy clumps, making it particularly challenging to work with in the laboratory.

Nevertheless, the researchers successfully grew the strain, isolated its far-red Photosystem II, solved its structure, and performed comparative absorption spectroscopy.

Using high-resolution cryo-electron microscopy, spectroscopy and chromatography, they found that far-red Photosystem II from Calothrix sp. lacked two of the long wavelength chlorophyll molecules compared with their usual strain for far red photosynthesis studies, Chroococcidiopsis thermalis, which contains five.

This provided a crucial comparison: by matching the missing pigments with the missing absorption peaks, the researchers could link individual chlorophylls to the wavelengths of light they absorb.

Combining cryo-electron microscopy with phylogenetics and low-temperature spectroscopy, the team was able to assign the specific wavelengths of all five far-red chlorophylls in C. thermalis.

Lead author Dr Marco Leong said: ‘From its sequences I realised Calothrix might provide some vital clues, but I did not expect it to work so neatly, let alone allow us to piece together the full jigsaw puzzle. It was quite a buzz to see it all fall into place.’

The researchers describe the process as a kind of ‘scientific Sudoku’, combining different pieces of evidence until the full picture emerged.

A new protein and pigment

The improved structure of far-red Photosystem II from C. thermalis also revealed a previously uncharacterised far-red-exclusive protein subunit, PsbH2′, which forms part of a chlorophyll f binding site.

Professor Bill Rutherford said: ‘Getting the correct locations of the far-red pigments is important in itself but labelling each of them with their specific colour is the big step forward the theory guys have been waiting for.’

The researchers say this is a particularly powerful opportunity because, in conventional chlorophyll-a-based photosynthesis, the assignment of individual pigments to their specific wavelengths has remained a largely intractable problem.

Dr Andrea Fantuzzi said: ‘This is because in conventional chlorophyll-a based photosynthesis all the chlorophyll-a are the same colour ...it’s this recently discovered far-red system that provides this unique opportunity.’

Understanding photosynthesis at the far-red limit

After decades of studying chlorophyll-a-based oxygenic photosynthesis, far-red photochemistry provides researchers with an alternative system for investigating how photosynthesis can operate with less energy and still do the same energy demanding chemistry: splitting water and reducing carbon dioxide.

By revealing both the location and absorption wavelength of each far-red chlorophyll, the researchers now have a detailed map of how the pigments are arranged to capture and transfer energy. The next step is to use this map to understand how energy moves through the photosystem and ultimately drives photosynthesis.