Hiking though the redwood forest along California’s Lost Coast.
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The evolution of intelligent life around red dwarf stars is likely to be an uphill slog, due to the demands needed to jumpstart oxygenic photosynthesis. Given that red M-dwarf stars are by far the most prevalent of any in the cosmos, this could represent the ultimate limiting factor on the prevalence of intelligent life in our galaxy and beyond.
Or so says a leading U.K. theoretical biophysicist who specializes in the study of photosynthesis, both here on Earth and its possibility to evolve on extrasolar planets throughout the galaxy.
Oxygenic photosynthesis on a planet circling an M-dwarf would be a lot harder as there is less visible light, Chris Duffy, a theoretical biophysicist at the U.K.’s Queen Mary University in London, told me via email. Full sun would feel like strongly shaded sub-canopy environments here, he says. This might mean you are limited to single cell oxygenic organisms like cyanobacteria, says Duffy.
Photosynthesis allows organisms — everything from the most basic cyanobacteria to the tallest California redwoods trees — to harvest energy from the Sun so that they can grow biomass. Here on Earth, most photosynthesis is optimized in the visible part of the spectrum, which ranges from 400 to 700 nanometers in wavelength.
On Earth, oxygenic photosynthesis uses a fairly complex system in which four visible photons are used to create one molecule of O2. Oxygenic photosynthesis takes electrons and hydrogen from water (H2O) and gives them to molecular carbon dioxide (CO2) to create glucose (C6H12O6).
The Rise Of Oxygen
On Earth, oxygenic photosynthesis led to an oxygen-rich atmosphere which then enabled aerobic respiration, says Duffy. On Earth multicellular life evolved after oxygenation of the atmosphere by early cyanobacteria, though it is unclear if this was a necessary precondition, he says.
But an oxygen-rich atmosphere gives a planetary atmosphere a tremendous advantage in terms of access to energy needed to drive aerobic respiration. Aerobic respiration on which we all depend is simply how our cells convert oxygen and sugars like glucose into energy that we can use. This energy is mostly in the form of the energy-carrying molecule ATP (adenosine triphosphate) which, in turn, powers cellular functions.
Aerobic respiration can then supply the intense energy demands of multicellular and eventually intelligent life, says Duffy.
Rapid Rise Of Earth Life
Single cell autotrophs — organisms that create their own food from inorganic sources — seem to have evolved on Earth as soon as the surface became solid.
Light Harvesting Antennae Are Key.
All photosynthetic organisms have antennae — large assemblies of pigment-binding proteins that absorb light over a large area and wavelength range, says Duffy. These antennae funnel that energy to specialized protein complexes called reaction centers where initial photosynthetic reactions take place, he says.
Photon Absorbing Pigments
The primary reaction of photosynthesis is the removal of an electron from a chlorophyll molecule by absorption of a photon, Duffy says. Rather than have every chlorophyll carry out this reaction, evolution devised a structure where a majority of chlorophylls (and other pigments) simply absorb photons and pass the energy on until it arrives at one of the small subsets of chlorophylls that carry out the reaction, he says.
Ironically, although photosynthetic organisms use visible light, they are just as susceptible to photodamage from ultraviolet light as any other organism. That’s because UV light can cause cellular damage to an organism’s DNA and proteins.
It’s a problem mainly on the surface since water is fairly opaque to UV radiation, says Duffy. So, plants (and all surface life) possess UV defenses, he says. These are typically yellow, orange and red pigments like carotenoids as well as flavonoid pigments which both absorb very blue and UV light and dissipate it very quickly and harmlessly as heat, says Duffy.
Proxima B Planet, Orbiting Proxima Centauri, a Red Dwarf Star. (Photo by: Education Images/Universal Images Group via Getty Images)
Universal Images Group via Getty Images
The presence of oxygen in an exoplanet atmosphere could be detectable as absorption lines in light passing through its atmosphere, making it a priority bio-signature, Duffy and co-authors write in a 2025 paper appearing in the journal PLOS Computational Biology. On Earth, the authors note, the global coverage of chlorophyll generates the vegetation red edge in its reflectance spectrum (as first observed by NASA’s Galileo probe in 1993.
The ‘red edge’ was a term coined by Carl Sagan when measuring our planet’s surface reflectance spectrum during NASA’s Galileo spacecraft’s Earth fly-by, says Duffy. This is a very clear remote signal of oxygenic photosynthesis on Earth so there is a huge drive to detect similar red edges in starlight reflected by exo-world surfaces, he says.
The Bottom Line?
We often don’t appreciate just how profoundly oxygenic photosynthesis reshaped our planet and made it habitable for the kind of majestic old growth forest bounty that we see globally. It’s just another reminder of how unique our planet appears to be.
That’s in part, because the odds appear to be long for oxygenic photosynthesis arising on a planet circling a puny red dwarf.
Oxygenic photosynthesis might be just out of reach on a planet closely orbiting an M-dwarf, says Duffy.