Quirks & Quarks

Purple reign: life on Earth might once have been dominated by purple microorganisms

Purple microorganisms may once have dominated life on Earth, and might be found on extrasolar planets too

Simpler haloarchaea might have preceded green photosynthesizers

Researchers suggest Earth might once have been dominated by purple microorganisms, and that alien life might be purple too (NASA)

American researchers are suggesting that life on Earth might have been purple before it was green. This means that as we seek out life in other extrasolar planets, we should keep an eye out for purple life forms as well.

Modern life on Earth is dominated on land and in the sea by green organisms — photosynthetic plankton in the oceans and green plants on land. Their green colour comes from chlorophyll, the pigment that absorbs sunlight and allows them to harvest the power of the Sun for energy. 

But photosynthesis is a complex process and chlorophyll is a complicated molecule, which is why NASA astrobioloigst Ed Schwieterman thinks a simpler, and purpler, form of life might have evolved first. 

"There are these organisms that exist in isolated environments today, called haloarchaea, and they have a purple membrane that uses a unique pigment," Schwieterman told Quirks & Quarks. This pigment is made in a relatively simple biological process, which he thinks might have evolved before chlorophyll-based life.

These salt ponds in San Francisco bay have been colonized by purple-red haloarchaea (Grombo, cc-by-sa-2.5)

Haloarchaea are visible today in extreme conditions, like super-saline salt ponds, in ocean waters, and even on rocks in Antarctica. "They can impart a bright purple or red colour when they exist in these blooms," said Schwieterman.

Their purple pigment is used to capture energy from the sun, and is based on a molecule called retinal. Retinal is about half the size of chlorophyll and chemically much simpler.

The process where organisms use retinal to absorb sunlight is also simpler, though less efficient, than chlorophyll-based photosynthesis.

Purple-pigmented haloarchea absorb sunlight for energy using a process called phototrophy. Phototrophy works for grabbing energy from sunlight, but it doesn't harvest carbon from the atmosphere the way photosynthesis does.

This puts phototrophic organisms at a disadvantage compared to photosynthesizers because they can't use carbon from the air to build their tissues or store energy in sugars.

So in the modern world, under most circumstances, the photosynthesizers outcompete phototrophs. 

The oceans of ancient Earth might have been dominated by purple haloarchaea before green photosynthetic cyanobacteria evolved (NOAA)

But Schwieterman thinks that because phototrophy using purple pigment is a simpler process, it might have evolved before photosynthesis. "We think photosynthesis evolved up to about three and a half billion years ago," he said, "and so our retinal phototrophs, we're placing them before that." 

They might have dominated the planet for some time before evolution found a better way to harness sunlight. 

"We think as photosynthesis took over, the purple organisms retreated to more isolated ecological niches such as we find today," he explained. 

This, incidentally, was an essential development for animal life on Earth.

Photosynthesis produced the oxygen in the Earth's atmosphere which made the evolution of animals, including modern humans possible. Animals could not have evolved on a planet that stayed purple.

Schwieterman admits we have no hard evidence for this — there is no fossil or geochemical evidence that preserves traces of Earth as a purple planet.

He argues, though, that the possibility that this happened on an ancient Earth means we might at least consider it as we look for life on other planets.

"We don't want to miss something because our imagination was too limited, or we thought about Earth as it exists today rather than Earth as it might have existed in the past."

Future space telescopes are now being designed that will look at planets for signs of life — including the characteristic light signature of green photosynthetic plants. Schwieterman is hoping his research helps convince astronomers to also keep an eye out for the colour purple, in case the planets host a more primitive form of life.


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