Purple Earth hypothesis

The Purple Earth hypothesis is an astrobiological hypothesis that photosynthetic life forms of early Earth were based on the simpler molecule retinal rather than the more complex chlorophyll, making Earth appear purple rather than green.[1][2] An example of retinal-based organisms that exist today are the photosynthetic microbes collectively called Haloarchaea.[3] That time would date somewhere between 2.4 and 3.5 billion years ago, prior to the Great Oxygenation Event.[4] Many Haloarchaea contain the retinal protein, bacteriorhodopsin, in their purple membrane which carries out light-driven proton pumping, generating a proton-motive gradient across the cell membrane and driving ATP synthesis. The haloarchaeal purple membrane constitutes one of the simplest known bioenergetic systems for harvesting light energy.

Purple culture (left) and isolated purple and red membrane components (right) of Haloarchaea

Retinal-containing purple membrane exhibits a single light absorption peak centered in the green-yellow energy-rich region of the visible spectrum, but allows transmission of red and blue light, resulting in a deep purple color.[5] Chlorophyll pigments, in contrast, absorb red and blue light, but little or no green light, which results in the characteristic green color of plants, cyanobacteria, and photosynthetic membranes. Microorganisms with purple and green pigments frequently co-exist in stratified communities where they may utilize complementary regions of the solar spectrum.

The simplicity of haloarchaeal retinal pigments in comparison to the more complex chlorophyll-based photosynthetic membrane, 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 photosynthesis.

Co-existence of purple and green pigment containing microorganisms in many environments suggests their co-evolution. Astrobiologists have suggested that retinal pigments may serve as remote biosignatures in exoplanet research.[6]

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.