Unsolved:Purple Earth hypothesis

Short description: Astrobiological hypothesis regarding early photosynethetic organisms

File:Purplemembrane.tif The Purple Earth hypothesis is an astrobiological hypothesis that the earliest photosynthetic life forms of Early Earth were based on the simpler molecule retinal rather than the more complex porphyrin-based chlorophyll, making the surface biosphere appear purplish rather than greenish like nowadays. The time would date somewhere between 2.4 and 3.5 billion years ago, prior to the Great Oxygenation Event and Huronian glaciation.

An example of retinal-based organisms that exist today are the photosynthetic microbes collectively called Haloarchaea. Many Haloarchaea contain the retinal derivative protein bacteriorhodopsin in their cell membrane, which carries out photon-driven proton pumping, generating a proton-motive gradient across the membrane and driving ATP synthesis. The process is a form of anoxygenic photosynthesis, and the haloarchaeal membrane protein pump constitutes one of the simplest known bioenergetic systems for harvesting light energy.

Retinal-containing membrane exhibits a single light absorption peak centered in the energy-rich green-yellow region of the visible spectrum, but transmit and reflects red and blue light, resulting in a deep purple color. Chlorophyll pigments, in contrast, absorb red and blue light, but little or no green light, which results in the characteristic green color of plants, green algae, cyanobacteria and other organisms with chlorophyllic organelles. The simplicity of retinal pigments in comparison to the more complex chlorophyll, their association with isoprenoid lipids in the cell membrane, as well as the discovery of archaeal membrane components in ancient sediments on the Early Earth are consistent with an early appearance of life forms with purple membrane prior to green photosynthetic organisms.

Microorganisms with purple and green photopigments frequently co-exist in stratified colonies known as microbial mats, where they may utilize complementary regions of the solar spectrum. Co-existence of purple and green pigment-containing microorganisms in many environments suggests their co-evolution. It is possible that the Early Earth's biosphere was dominated by retinal-powered archaeal colonies that absorbed all the green light, leaving the eubacteria that lived in their shadows with no accessibilities except to evolve utilizing the residual red and blue light spectrum. However, when porphyrin-based cyanobacteria started to photosynthesize, dioxygen was released as a byproduct and started to accumulate, first in the ocean and then in the atmosphere. When large enough quantities of oxygen had been produced, the reducing capabilities of chemical compounds on the Earth's surface were depleted, and the once-reducing atmosphere eventually became a permanently oxidizing one with abundant free oxygen molecules. This coincided with a global glaciation (which might also have been partly caused by the depletion of the atmospheric methane — a powerful greenhouse gas — due to the Great Oxygenation) and devastated the anaerobic archaeal biota, leaving the niches open for eubacteria (both the aerobic proteobacteria and the photosynthetic cyanobacteria) to exploit and prosper. However, the porphyrin-based nature of chlorophyll had created an evolutionary trap, dictating that chlorophyllic organisms cannot re-adapt to absorb the energy-rich and now-available green light, and therefore ended up reflecting and presenting a greenish color. The subsequent success of chlorophyllic organisms (particularly early plants) in terrestrial colonization created an overall green biosphere all over Earth.

Astrobiologists have suggested that retinal pigments may serve as remote biosignatures in exoplanet research. The Purple Earth hypothesis has great implications for the search for extraterrestrial life. Historically, planets reflecting light in the green-yellow range were sought out as possible hosts to photosynthetic organisms, due to the implied presence of chlorophyll. The hypothesis suggests that search methods should be expanded to planets reflecting blue and red light, since evolution of retinal-based photosynthesis is also probable, if not more likely than the evolution of chlorophyllic systems.

See also

• Bacteriorhodopsin – A proton pump used by Haloarchaea to harvest light energy.
• Microbial rhodopsin
• Earth:Red edge

External links

• Colorful Worlds: Plants on Other Planets Might Not Be Green
• PBS Eons: When the Earth was purple
• CNN Colorscope-When life on Earth began, it was purple