Light Wavelength Analysis Nets Purple Earth

This visualization shows global chlorophyll concentrations. Data taken with the MODIS instrument on NASA’s AQUA spacecraft, between July 1, 2002, and Dec. 31, 2004. Image Credit: NASA.

Light Wavelength Analysis Nets Purple Earth

Researchers here on the Oblate Spheroid, struggle to explain why microbial evidence throughout the Earth is varied yet so stratified.

The answer may come from the fact that the evolution of microbe life on the earth to date has come down to how these organisms were able to convert certain wavelengths of light.

Simpler structures on early Earth were able to absorb and utilize light from the green spectrum of visible light. If this is true, it suggests that the Earth would reflect light from the red and blue spectrums thus yielding a purple Earth.

Further, if this theory holds up, wavelength analysis may give astronomers greater understanding of the evolution of potential life through biomarkers on distant planetary objects.

Lakes in Australia appear purple because they are filled with halobacteria, a microbe that uses retinal to capture the Sun's rays. Image Credit: Cheetham Salt Limited

Excerpts from Life Science -

Early Earth Was Purple, Study Suggests
By Ker Than - LiveScience Staff Writer - originally posted: 10 April 2007

The earliest life on Earth might have been just as purple as it is green today, a scientist claims.

Ancient microbes might have used a molecule other than chlorophyll to harness the Sun’s rays, one that gave the organisms a violet hue.

Chlorophyll, the main photosynthetic pigment of plants, absorbs mainly blue and red wavelengths from the Sun and reflects green ones, and it is this reflected light that gives plants their leafy color. This fact puzzles some biologists because the sun transmits most of its energy in the green part of the visible spectrum.

“Why would chlorophyll have this dip in the area that has the most energy?” said Shil DasSarma, a microbial geneticist at the University of Maryland.

After all, evolution has tweaked the human eye to be most sensitive to green light (which is why images from night-vision goggles are tinted green). So why is photosynthesis not fine-tuned the same way?

Possible answer

DasSarma thinks it is because chlorophyll appeared after another light-sensitive molecule called retinal was already present on early Earth. Retinal, today found in the plum-colored membrane of a photosynthetic microbe called halobacteria, absorbs green light and reflects back red and violet light, the combination of which appears purple.

Primitive microbes that used retinal to harness the sun’s energy might have dominated early Earth, DasSarma said, thus tinting some of the first biological hotspots on the planet a distinctive purple color.

Being latecomers, microbes that used chlorophyll could not compete directly with those utilizing retinal, but they survived by evolving the ability to absorb the very wavelengths retinal did not use, DasSarma said.

“Chlorophyll was forced to make use of the blue and red light, since all the green light was absorbed by the purple membrane-containing organisms,” said William Sparks, an astronomer at the Space Telescope Science Institute (STScI) in Maryland, who helped DasSarma develop his idea.

The retinal pigment in halobacteria absorbs green light and reflects red and blue light. Chlorophyll absorbs red and blue light and reflects green. Some scientists think this mirror relationship suggests chlorophyll evolved to exploit parts of the spectrum unused by retinal. Image Credit: American Scientist

Chlorophyll more efficient

The researchers speculate that chlorophyll- and retinal-based organisms coexisted for a time. “You can imagine a situation where photosynthesis is going on just beneath a layer of purple membrane-containing organisms,” DasSarma told LiveScience.

But after a while, the researchers say, the balance tipped in favor of chlorophyll because it is more efficient than retinal.

“Chlorophyll may not sample the peak of the solar spectrum, but it makes better use of the light that it does absorb,” Sparks explained.
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Also, the process for making retinal is very similar to that of a fatty acid, which many scientists think was one of the key-ingredients for the development of cells.

“Fatty acids were likely needed to form the membranes in the earliest cells,” DasSarma said.

Lastly, halobacteria, a microbe alive today that uses retinal, is not a bacterium at all. It belongs to a group of organisms called archaea, whose lineage stretches back to a time before Earth had an oxygen atmosphere.

Taken together, these different lines of evidence suggest retinal formed earlier than chlorophyll, DasSarma said.
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“I’m a little cautious about looking at who’s using which wavelengths of light and making conclusions about how things were like 3 or 4 billion years ago,” said Des Marais, who was not involved in the research.
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Implications for astrobiology

If future research validates the purple Earth hypothesis, it would have implications for scientists searching for life on distant worlds, the researchers say.

“We should make sure we don’t lock into ideas that are entirely centered on what we see on Earth,” said DasSarma’s colleague, Neil Reid, also of the STScI.

For example, one biomarker of special interest in astrobiology is the “red edge” produced by plants on Earth. Terrestrial vegetation absorbs most, but not all, of the red light in the visible spectrum. Many scientists have proposed using the small portion of reflected red light as an indicator of life on other planets.
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