Wednesday, September 20, 2017

Purple and Green: Earth's Early Primary Colors

The earliest life on Earth might have been just as purple as much 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 would chlorophyll have this dip in the area that has the most energy?" said Shil DasSarma, a microbial geneticist.

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?

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, 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.
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.

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.
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.
These ideas are currently little more than speculation, but they fit with other things scientists know about retinal and early Earth. 
For example, retinal has a simpler structure than chlorophyll, and would have been easier to produce in the 
low-oxygen environment of early Earth.
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.
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....."