A Fresh Take on the Search for Extraterrestrial Life, Inspired by MBL

Ligia Coelho, an alumna of the MBL Microbial Diversity course, is developing purple bacteria as a biosignature to detect life on other planets. Credit: Cornell Chronicle

In the search for extraterrestrial life, we Earthlings naturally look for signs of water or the green, lush lifeforms that cover our planet. But the Universe is not just our mirror, and our imaginations must stretch way beyond the familiar. Last summer, a student in the MBL Microbial Diversity course was inspired to take up this challenge, leading to her new publication, “Purple is the New Green," in Monthly Notices of the Royal Astronomical Society: Letters.

“During the course, I encountered so much biodiversity that the astronomy community is still missing in their models to search for extraterrestrial life,” says lead author Ligia F. Coelho, a postdoctoral associate working with Lisa Kaltenegger at the Carl Sagan Institute at Cornell University. “Most models still ignore the concept of diverse microbial communities, focusing instead on single, green photosynthetic species. As soon as I got back to Cornell, I immediately started working on this paper, bridging the fields of microbial diversity and astronomy.”

The paper explores how “purple bacteria” – some of which she collected in Woods Hole -- may cover the surface of planets that orbit M stars, the cool, reddish-looking stars that are the most abundant in the Universe. Unlike green bacteria and plants that use the pigment chlorophyll to synthesize their food in the visible light range, purple bacteria contain purple pigments that photosynthesize in the invisible, infrared light range.

purple microbial mat
Purple microbial mat in Little Sippewissett Marsh, Woods Hole, where Coelho sampled some of the microorganisms used in the study. Credit: Ligia Fonseca Coelho
microbial mat cut
Purple microbial mat cut from Little Sippewissett Marsh. Credit: Ligia Fonseca Coelho.

Coelho and her colleagues measured the spectral signatures of a diverse set of purple bacteria and “modeled how exoplanets would look if they were covered in these organisms, and how they would look through the lens of upcoming telescopes” such as the Extremely Large Telescope and the Habitable Worlds Observatory. These next-generation telescopes are specifically engineered to look for life on exoplanets (planets beyond our solar system).

green and purple bacteria
Green bacterial communities on left, purple bacteria on right. Credit: Cornell Chronicle

“We were able to measure part of this diverse set of purple bacteria thanks to the 2023 Microbial Diversity course led by Rachel Whitaker and Scott Dawson, and my amazing colleagues in the course who worked hard on the cultures,” says Coelho. The team sampled from nearby Trunk River and its sediments, as well as from Sippewissett Marsh, and then cultured and enriched purple bacteria from those samples.

“The Microbial Diversity course gave me an effective toolset that I'll use for the rest of my life to keep searching for life in the cosmos,” Coelho says.

The study’s authors include Stephen Zinder of Cornell University, a former co-director of the Microbial Diversity course, who encouraged Coelho to apply.

Here is a story about the research from Cornell University.


Coelho, Ligia Fonseca, et al. (2024) Purple is the new green: biopigments and spectra of Earth-like purple worlds. Monthly Notices of the Royal Astronomical Society, DOI: 10.1093/mnras/stae601