An artistic rendering with a lot of artistic license. PaulPaladin/Shutterstock

Aliyah Kovner 19 Oct 2018, 21:26

Before the evolution of chlorophyll-based algae turned large swathes of our planet green, young Earth might have sported patches of a lovely purple hue.

This theory, put forward by molecular biology professor Shiladitya DasSarma and astrobiologist Edward Schwieterman, is based on observations that nearly all forms of life, back to most ancient single-celled organisms, produce a pigment that can drive a light-to-energy reaction using photons in the yellow to green spectrum – making it appear purple when light in the blue and red wavelengths are bounced back.  

Outlining the “Purple Earth” argument in the International Journal of Astrobiology, the authors explain that the chromoprotein retinal is excellent at absorbing light in the 490-600 nm range. When bound inside a cell membrane, retinal can use this Sun-harvested energy to build ATP, the cellular fuel molecule. Now, this version of the phototrophic process is not very efficient compared to photosynthesis – and it does not result in the creation of free oxygen or sugars – but retinal is a much simpler molecule than chlorophyll, and is therefore easier for cells to make.

“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 told Astrobiology Magazine.

(a) An Australian salt pond with a bloom of purple microorganisms. (b) A gradient of the pigments found in Halobacterium sp., an Archaea with retinal-based metabolism. 

Attempts to estimate when early life gained the ability to make the pigment – using a genetic mutation analysis technique called the “molecular clock” – have been hazy thus far, but the evidence suggests it was very soon after life emerged some 4 billion years ago. Similarly, researchers are unsure about the precise timeframe of chlorophyll’s emergence, but we do know that it happened at some point before 2.3 billion years ago – when, due to a combination of poorly understood factors, existing single-celled photosynthetic algae suddenly took over the ecosystem, causing what is called the Great Oxygenation Event.

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