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Early life-forms on Earth may have been able to generate metabolic energy from sunlight using a purple-pigmented molecule called retinal that possibly predates the evolution of chlorophyll and photosynthesis. If retinal has evolved on other worlds, it could create a distinctive biosignature as it absorbs green light in the same way that vegetation on Earth absorbs red and blue light.
Earth's atmosphere has not always contained significant amounts of oxygen. For the first two billion years of our planet’s history, the atmosphere was rich in carbon dioxide and methane, but around 2.4 billion years ago something changed: the Great Oxygenation Event that saw the abundance of free oxygen in our atmosphere dramatically rise. The cause of this is thought to be cyanobacteria, which are able to perform photosynthesis — the transformation of sunlight and carbon dioxide into metabolic energy to produce sugars that fuel life's processes, and oxygen as a 'waste' product — using a green pigment called chlorophyll.
Metabolisms - World - Oceans - Processes - Earth
"Retinal-based phototrophic metabolisms are still prevalent throughout the world, especially in the oceans, and represent one of the most important bioenergetic processes on Earth," DasSarma tells Astrobiology Magazine.
Chlorophyll absorbs light peaking at wavelengths of 465nm and 665nm. This is why leaves appear green, because they reflect green light rather than absorb it. However, the Sun’s spectrum peaks at ~550nm, which includes yellow and green light.
Number - Proteins - Contain - Molecule - Protein
A number of proteins that absorb sunlight contain a molecule of retinal, including one protein called bacteriorhodopsin that absorbs light peaking at 568nm, close to the wavelength at which the Sun’s light peaks, and most notably in the range that chlorophyll does not absorb in. "This is exactly what got us thinking that the two pigments — retinal and chlorophyll — may have co-evolved," says DasSarma, who argues that because retinal is the simpler molecule, it would have...
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