A study in Nature Communications has mapped how certain cyanobacteria run oxygen-producing photosynthesis on far-red light, a dim glow near the edge of human vision. The research, led by scientists at Imperial College London, identifies where the unusual pigments sit inside the water-splitting machinery and assigns a specific absorbed wavelength to each one.
The finding tackles a long-standing mystery in photosynthesis. Most plants and cyanobacteria use visible red light near 700 nanometers as their practical limit. Some microbes living in shade can push beyond that boundary and harvest light reaching toward 800 nanometers.
That ability matters because far-red light is common in the places where visible light has already been filtered away. It slips through microbial mats, shaded rock surfaces and other tight habitats where ordinary photosynthesis has little room to operate.
The new work gives researchers a detailed wiring diagram for this hidden survival mode. It shows how small changes in pigment chemistry and protein structure let life use a weaker form of light to split water and release oxygen.
The red edge of photosynthesis
The old boundary is known as the red limit. It describes the longest wavelength that most photosynthetic organisms can use efficiently for the demanding chemistry of photosynthesis.
For familiar plants, algae and many cyanobacteria, that limit sits around the red end of visible light. Their main pigment, chlorophyll a, is especially good at absorbing red and blue light. That simple fact shapes much of the color and energy flow of life on Earth.
Far-red light carries less energy than ordinary red light. Even so, a small group of microbes has evolved a way to use it. The study describes this adaptation as a survival response in places where white light has been depleted.
The paper’s abstract puts the point directly: “Far-red light photoacclimation enables some cyanobacteria to survive in white-light-depleted environments by extending the red limit of photosynthesis.” In practical terms, the microbes adjust their photosynthetic hardware when their habitat gives them mostly far-red light.
This is a subtle feat. Photosynthesis must move electrons with enough force to drive reactions that support life. In oxygenic photosynthesis, that includes pulling electrons from water, which is one of biology’s most demanding energy jobs.
How cyanobacteria switch pigments
The microbes in the study are cyanobacteria, ancient bacteria that helped shape Earth’s atmosphere through oxygen-producing photosynthesis. Under ordinary light, they can use the usual green pigment system. Under far-red conditions, some species build a different version of their photosynthetic machinery.
The key change happens in the pigments. Standard photosynthesis relies heavily on chlorophyll a. Far-red-adapted cyanobacteria add small amounts of red-shifted chlorophylls that absorb longer wavelengths.
One major player is chlorophyll f. This pigment is tuned to catch far-red light more effectively than chlorophyll a. The studied complexes also contain one molecule of chlorophyll d, another red-shifted pigment that helps reshape the system’s energy landscape.
These pigments are sparse, which makes them hard to study. A photosynthetic complex contains many pigment molecules and only a few are swapped for far-red versions. Finding the exact positions of those few molecules takes high-resolution structural work and careful comparison with light-absorption measurements.
The Imperial-led team combined several approaches to build that map. Structural information showed the physical layout. Spectroscopy showed how the complex absorbs light. Evolutionary comparisons helped identify which protein changes matched the far-red adaptation.
A closer look at Photosystem II
At the center of the study is Photosystem II, the protein complex that starts the water-splitting side of oxygenic photosynthesis. It draws electrons from water and helps release the oxygen that animals, plants and many microbes depend on.
Photosystem II is a dense molecular machine. It holds pigments in precise positions so absorbed light energy can move from one site to another. The arrangement must guide energy efficiently while keeping the chemistry safe enough for the organism to survive.
To see the far-red version in detail, the researchers used cryo-electron microscopy. In this method, purified biological machinery is rapidly frozen and imaged with an electron beam. The resulting data can reveal the shapes of proteins and the placement of key molecular parts.
The team compared far-red Photosystem II from two cyanobacteria. One came from Chroococcidiopsis thermalis PCC 7203. The other came from Calothrix sp. NIES-3974. The comparison helped separate shared far-red features from species-specific additions.
That side-by-side design is important. A single structure can show what one organism does. Two related systems can reveal which changes are widely conserved and which ones are optional solutions.
The far-red-only protein piece
One of the clearest findings came from Chroococcidiopsis thermalis. Its far-red Photosystem II contained a protein piece called PsbH2′, which appears only in the far-red form of the complex.
The structure showed PsbH2′ forming part of a binding site for chlorophyll f. That places the protein directly beside one of the unusual pigments that helps the system absorb longer wavelengths.
This matters because pigments do their work inside protein pockets. The surrounding protein changes a pigment’s behavior by shaping its local electrical environment and holding it at the right angle. A small structural shift can alter which wavelengths a pigment absorbs.
In this case, the far-red-only subunit appears to help create a custom pocket for a far-red pigment. The finding links a specific protein adaptation to a specific pigment position. That connection gives researchers a clearer explanation for how the altered machinery is assembled.
The discovery also shows that far-red photosynthesis involves more than adding new pigments. The protein scaffold itself changes so those pigments can sit in useful positions. The machinery is rebuilt in a targeted way.
Two microbes, two survival setups
The two cyanobacteria used different versions of the far-red system. Chroococcidiopsis thermalis had four chlorophyll f sites assigned in its Photosystem II complex. Calothrix sp. NIES-3974 had two of the same sites.
Calothrix also lacked the psbH2′ gene and the PsbH2′ subunit seen in Chroococcidiopsis thermalis. That difference gave the researchers a natural comparison between a more elaborate far-red setup and a simpler one.
The shared sites point to core features of far-red Photosystem II. If both organisms keep particular chlorophyll f positions, those sites likely play important roles in the energy flow of the complex.
The extra sites in Chroococcidiopsis thermalis suggest additional refinements. Some species may tune far-red photosynthesis more extensively, depending on their evolutionary history and the light environments they occupy.
This kind of comparison is valuable because far-red photosynthesis appears in organisms that live under varied conditions. Desert crusts, shaded rocks, hot springs and dense microbial communities can all create unusual light niches. Different cyanobacteria may solve the same energy problem with slightly different hardware.
Why every pigment’s color matters
The hardest part of the study was assigning each red-shifted pigment to its specific absorbed wavelength. In a photosynthetic complex, pigments sit close together and their signals can overlap. That makes it difficult to tell which molecule is responsible for which part of the absorption spectrum.
The team used structure, spectroscopy and phylogenetic analysis together. The structure placed the pigments. Spectroscopy measured how the far-red system absorbed light. Evolutionary patterns helped identify which protein changes were linked to the unusual pigment sites.
The study reports that the researchers assigned specific wavelengths to all red-shifted chlorophylls in the far-red Photosystem II complexes. That is a major step because light energy does a choreographed handoff through the machinery.
Each pigment’s color helps determine the path of that energy. A redder pigment can act like a lower-energy stepping stone. The exact order and position of those steps influence how efficiently the system delivers energy to the reaction center.
With the new map, scientists can begin modeling how excitation energy moves through far-red Photosystem II. That makes the machinery less like a black box and more like a circuit with labeled components.
A blueprint for future crops
The far-red system has attracted interest beyond microbiology. Sunlight reaching Earth includes a sizable far-red portion that most crop plants use poorly. If researchers could safely extend the usable light range of crops or algae, photosynthesis might become more efficient in dense canopies or engineered growth systems.
The new study offers a more detailed blueprint for that goal. It shows which pigments are placed where, which protein changes support them and which features are shared between two cyanobacterial species.
Even with that map, crop engineering remains a difficult challenge. Photosynthesis is tightly balanced. Changing pigment placement or wavelength use could affect energy transfer, water splitting, photoprotection and growth. The microbial system gives clues, while plants bring additional layers of complexity.
Still, the work sharpens the questions engineers can ask. Instead of broadly trying to make plants absorb farther-red light, researchers can focus on specific pigment sites and protein environments that nature already uses.
The wider scientific payoff is just as striking. These cyanobacteria show how life can bend photosynthesis toward the edge of visible light. By mapping that machinery in detail, the Imperial-led team has revealed how a few pigments and protein changes help microbes turn faint far-red glow into the chemistry of life.
