Purple Earth hypothesis
The Purple Earth hypothesis is an astrobiological hypothesis that life forms of early Earth were retinal-based rather than chlorophyll-based thus making Earth appear purple rather than green. An example of a retinal-based organism today is the photosynthetic microbes called halobacteria.
The difficulty with this hypothesis is that retinal-based photosynthesis requires either arginine or oxygen in the environment. As a complex amino acid, arginine is not likely to have been available in the levels needed. Oxygen only became available in earth's environment after the development of chlorophyll-based photosynthesis, and given that retinal itself also requires oxygen to synthesize, the possibility that retinal could have preceded chlorophyl as the most common photosynthetic molecule is unlikely.
- LiveScience.com: Early Earth Was Purple, Study Suggests Accessed May 9, 2007
- Sparks, William B.; DasSarma, S.; Reid, I. N. "Evolutionary Competition Between Primitive Photosynthetic Systems: Existence of an early purple Earth?". Bulletin of the American Astronomical Society, Vol. 38, p.901. Bibcode:2006AAS...209.0605S.
- Lozier, R.H.; Bogomolni, R.A.; Stoeckenius, W. (September 1975). "Bacteriorhodopsin: a light-driven proton pump in Halobacterium Halobium". Biophysical Journal. 15 (9): 955–962. Bibcode:1975BpJ....15..955L. doi:10.1016/S0006-3495(75)85875-9.
- Colorful Worlds: Plants on Other Planets Might Not Be Green
-  2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #06.05; Bulletin of the American Astronomical Society, Vol. 38, p. 901
-  BR Green, E Gantt - J Phycol, 2000 - interscience.wiley.com
-  Softpedia article
-  Fox News article
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