For a long stretch of Earth鈥檚 history, our planet might have looked green from a distance, instead of the pale blue dot we know today.
Earth鈥檚 green period, which lasted from around 3 billion years to 600 million years ago, probably shaped the evolution of the cyanobacteria that filled the atmosphere with breathable quantities of oxygen, says at Nagoya University in Japan.
Like plants, cyanobacteria capture energy from sunlight via photosynthesis. In plants, the main pigment used in this process is chlorophyll, which absorbs blue and red light and reflects green light. But cyanobacteria also have pigments called phycobilins, which absorb red and green light, as part of their light-harvesting systems called phycobilisomes.
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Matsuo and his colleagues wanted to understand why cyanobacteria use these additional pigments and what this tells us about the environment in which the first photosynthesising organisms evolved.
During the Archaean Eon, from 4 billion to 2.5 billion years ago, the ocean was rich in iron hydroxide, says Matsuo. 鈥淚ron hydroxide absorbs the blue light, while water absorbs the red light, meaning that the leftover light would have created a green light window,鈥� he says.
Matsuo and his colleagues carried out simulations to estimate the concentrations of iron hydroxide and other chemicals in the ancient ocean and determine the spectrum of light that was available to photosynthetic organisms. They found that this light window closely matched the spectrum that would be absorbed by phycobilin pigments.
They also carried out experiments in which cyanobacteria were grown in different light environments. Under green light replicating the conditions of the Archaean, cyanobacteria with a green-specialised phycobilin pigment called phycoerythrobilin grew much more quickly than cyanobacteria without this pigment, suggesting it would have been favoured by natural selection. Genetic analysis suggested that phycoerythrobilin was present in the common ancestor of today鈥檚 cyanobacteria.
The team also ran field tests around Satsunan-Iwo Island in southern Japan, where thermal vents result in iron-rich waters with a similar green-dominated spectrum at a depth of 5.5 metres. They found that green-light harnessing cyanobacteria species were more prevalent at this depth than at the surface.
Today, Earth appears to be blue when seen from space, primarily because of a phenomenon called Rayleigh scattering. This describes how light waves get scattered by particles in the atmosphere that are smaller than the wavelength of light.
鈥淚f we assume an atmosphere similar to today鈥檚, the green hue reflected by the ocean would have mixed with the blue from Rayleigh scattering, likely creating a more bluish-green colour rather than the blue we see today,鈥� says Matsuo.
鈥淎nother important consideration is that the oceans may have covered a larger portion of Earth鈥檚 surface compared to the present day, making the ocean鈥檚 colour an even more dominant factor.鈥�
The oxygen produced by cyanobacteria reacted with dissolved iron in the oceans to form iron oxides, which sank to the bottom of the ocean. These deposits can be seen as thin layers within ancient rocks that were once under the sea. From about 600 million years ago, these layers are no longer seen, suggesting that by this time, the iron in the ocean was fully oxidised and the green period was over.
Many aquatic environments are still dominated by green light: for example, in coastal waters at shallow depths, organic matter from the land such as dead plants and animal waste absorbs blue light, leaving predominantly green light for cyanobacteria to harness in their photosynthesis.
Matsuo says most efforts at trying to determine whether there is life on other planets have focused on looking for oxygen in the atmosphere produced by photosynthesis.
鈥淗owever, because a green ocean could result from photosynthetic oxidation, it may serve as an indicator of life on exoplanets,鈥� he says.
at the University of Sydney, Australia, isn鈥檛 convinced that green light explains why cyanobacteria use phycobilisomes to absorb light energy.
鈥淐yanobacteria themselves are distributed widely, not only in oceans, rivers and other water bodies, but also in soil and other environments,鈥� she says. 鈥淢any soil cyanobacteria have these green-tinged phycobilisomes, and most of them play protective roles against oxidative stresses or light damage.鈥�
It is also very difficult to determine how Earth would have looked from space in the ancient past, says Chen.
Nature Ecology & Evolution
