Halobacteria, Retinal, and Chlorophyl: A Purple Earth

Early Earth Was Purple, Study Suggests
By Ker Than: 10 April 2007 10:41 am ET

The earliest life on Earth might have been just as purple as it is green today. 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?"

DasSarma thinks it is because chlorophyll appeared after another light-sensitive molecule called retinal was already present on early Earth. Retinal, 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 thus tinting some of the first biological hotspots on the planet a distinctive purple color. 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,

"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 in Maryland

The researchers speculate that chlorophyll- and retinal-based organisms coexisted for a time, but after a while the balance tipped in favor of chlorophyll because it is more efficient than retinal.

Retinal has a simpler structure than chlorophyll, and would have been easier to produce in the low-oxygen environment of early Earth, DasSarma said. 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.