Purple is the New Green: Purple Bacteria Could Dominate Wide Range of Exoplanetary Environments

With more than 5,500 detected exoplanets, the search for life is entering a new era. Using life on Earth as a guide, astrobiologists from Cornell University and the University of Minnesota looked beyond green landscapes to expand their ability to detect signs of surface life on other worlds. In new research, they characterized the reflectance spectra of a collection of purple sulfur and purple non-sulfur bacteria from a variety of environments.

To expand our baseline for finding life in the cosmos, Coelho et al. have measured the reflectance of purple bacteria that thrive across a range of anoxic and oxic environments. Image credit: Sci.News.

From house plants and gardens to fields and forests, green is the color we most associate with surface life on Earth, where conditions favored the evolution of organisms that perform oxygen-producing photosynthesis using the green pigment chlorophyll a.

But an Earth-like planet orbiting another star might look very different, potentially covered by bacteria that receive little or no visible light or oxygen, as in some environments on Earth, and instead use invisible infrared radiation to power photosynthesis.

Instead of green, many such bacteria on Earth contain purple pigments, and purple worlds on which they are dominant would produce a distinctive ‘light fingerprint’ detectable by next-generation ground- and space-based telescopes.

“Purple bacteria can thrive under a wide range of conditions, making it one of the primary contenders for life that could dominate a variety of worlds,” said Dr. Lígia Fonseca Coelho, a postdoctoral researcher with the Carl Sagan Institute at Cornell University.

“We need to create a database for signs of life to make sure our telescopes don’t miss life if it happens not to look exactly like what we encounter around us every day,” added Dr. Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University.

For the study, the authors collected and grew samples of more than 20 purple sulfur and purple non-sulfur bacteria that may be found in a variety of environments, from shallow waters, coasts and marshes to deep-sea hydrothermal vents.

What are collectively referred to as purple bacteria actually have a range of colors including yellow, orange, brown and red due to pigments related to those that make tomatoes red and carrots orange.

They thrive on low-energy red or infrared light using simpler photosynthesis systems utilizing forms of chlorophyll that absorb infrared and don’t make oxygen.

They are likely to have been prevalent on early Earth before the advent of plant-type photosynthesis and could be particularly well-suited to planets that circle cooler red dwarf stars — the most common type in our Galaxy.

“They already thrive here in certain niches,” Dr. Coelho said.

“Just imagine if they were not competing with green plants, algae and bacteria: a red sun could give them the most favorable conditions for photosynthesis.”

After measuring the purple bacteria’s biopigments and light fingerprints, the researchers created models of Earth-like planets with varying conditions and cloud cover.

“Across a range of simulated environments, both wet and dry purple bacteria produced intensely colored biosignatures,” Dr. Coelho said.

“If purple bacteria are thriving on the surface of a frozen Earth, an ocean world, a snowball Earth or a modern Earth orbiting a cooler star, we now have the tools to search for them.”

The team’s work appears in the Monthly Notices of the Royal Astronomical Society.

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Lígia Fonseca Coelho et al. 2024. Purple is the new green: biopigments and spectra of Earth-like purple worlds. MNRAS 530 (2): 1363-1368; doi: 10.1093/mnras/stae601

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