Purple Earth hypothesis
The Purple Earth hypothesis is an astrobiological hypothesis that life forms of early Earth were retinal-based rather than chlorophyll-based, 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] 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.
Retinal-containing purple membrane exhibits a single light absorption peak centered in the green-yellow energy-rich region of the solar spectrum, allowing transmission of red and green light, and resulting in deep purple color.[4] 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.[5]
See also
- Bacteriorhodopsin – A proton pump used by Haloarchaea to harvest light energy
- Red edge – region of rapid change in reflectance of vegetation in the near infrared range of the electromagnetic spectrum
References
- ^ DasSarma, Shiladitya; Schwieterman, Edward W. (11 October 2018). "Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures". International Journal of Astrobiology: 1–10. arXiv:1810.05150. Bibcode:2018arXiv181005150D. doi:10.1017/S1473550418000423. ISSN 1473-5504.
- ^ Sparks, William B.; DasSarma, S.; Reid, I. N. (December 2006). "Evolutionary Competition Between Primitive Photosynthetic Systems: Existence of an early purple Earth?". American Astronomical Society Meeting Abstracts. 38: 901. Bibcode:2006AAS...209.0605S.
- ^ DasSarma, Shiladitya (2007). "Extreme Microbes". American Scientist. 95 (3): 224. doi:10.1511/2007.65.224. ISSN 0003-0996.
- ^ Stoeckenius, Walther (1976). "The Purple Membrane of Salt-loving Bacteria". Scientific American. 234 (6): 38–47. Bibcode:1976SciAm.234f..38S. doi:10.1038/scientificamerican0676-38. ISSN 0036-8733. JSTOR 24950370. PMID 935845.
- ^ Schwieterman, Edward W.; Kiang, Nancy Y.; Parenteau, Mary N.; Harman, Chester E.; DasSarma, Shiladitya; Fisher, Theresa M.; Arney, Giada N.; Hartnett, Hilairy E.; Reinhard, Christopher T. (1 June 2018). "Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life". Astrobiology. 18 (6): 663–708. arXiv:1705.05791. Bibcode:2018AsBio..18..663S. doi:10.1089/ast.2017.1729. ISSN 1531-1074. PMC 6016574. PMID 29727196.
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
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