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Discovery in New Mexico caves challenges astrobiology assumptions
Scientists have found cyanobacteria deep inside Carlsbad Caverns harnessing near-infrared light for photosynthesis, suggesting extraterrestrial life could thrive in environments previously considered uninhabitable.
The unexpected green glow
Hazel Barton, a cave biologist at the University of Alabama, made an astonishing observation while exploring the pitch-black depths of Carlsbad Caverns National Park in southern New Mexico. The walls of an alcove, far from any natural light source, were covered in a vibrant green microbial layer.
"The wall was bright green. It was the most iridescent green you'd ever seen, and yet the microbes were living in complete darkness," Barton said.
Hazel Barton, Professor of Geological Sciences, University of Alabama
The Carlsbad Caverns, formed 4 to 11 million years ago by sulfuric acid dissolving limestone, consist of 119 caves beneath the Chihuahuan Desert. While the show cave, Carlsbad Cavern, attracts nearly 350,000 visitors annually, few realize the scientific breakthroughs hidden in its darker recesses.
Photosynthesis without sunlight
Tests revealed the green microbes were cyanobacteria, organisms typically dependent on sunlight for photosynthesis. However, these particular cyanobacteria use chlorophyll d and f, pigments capable of capturing near-infrared light-wavelengths invisible to the human eye but abundant in the cave's reflective limestone environment.
"The limestone rock absorbs almost all visible light, but to near-infrared light, caves are pretty much a hall of mirrors," Barton explained. Measurements showed near-infrared light concentrations 695 times higher in the cave's darkest areas compared to the entrance, with cyanobacteria thriving in these conditions.
Implications for extraterrestrial life
The discovery has significant implications for the search for life beyond Earth. Most stars in our galaxy are red dwarfs (M-type stars), which emit primarily near-infrared light. Previously, scientists assumed photosynthesis-and thus life-could only exist on planets receiving visible light, limiting the habitable zone around such stars.
"The vast majority of stars in our galaxy are these M- and K-type stars," Barton noted. "This means most of the stars in our galaxy are putting out near-infrared light, and yet we barely know anything about how photosynthesis and life could survive under those conditions."
Redefining the habitable zone
Traditionally, astrobiologists defined the habitable zone based on the presence of liquid water and the ability to support photosynthesis using visible light (up to 700 nm). However, the Carlsbad cave cyanobacteria can photosynthesize using light up to 780 nm, expanding the potential habitable zone around red dwarf stars.
Barton and microbial biologist Lars Behrendt, from Uppsala University, are proposing a NASA-funded study to determine the minimum light requirements for these cyanobacteria. Their findings could help prioritize exoplanet targets for telescopes like the James Webb Space Telescope (JWST).
"What our work is trying to do is figure out what is the longest wavelength of light and lowest level of light at which you can photosynthesize. Then what you can do is take the 100 billion potential stars that we can point the James Webb Space Telescope at, and reduce it down to say 50 stars [which may host life]."
Hazel Barton
Broader scientific context
The Carlsbad discovery builds on over a century of research into extremophile microbes. In 1890, microbiologist Sergei Vinogradskii found microbes surviving on inorganic matter through chemosynthesis. In 2018, scientists at Imperial College London discovered cyanobacteria in low-light environments using chlorophyll f to harness near-infrared light.
"We showed that not only do they live down there, but that they photosynthesize in a completely sheltered environment where they've probably been untouched for 49 million years," Behrendt said of the Carlsbad cave microbes.
Future research and potential breakthroughs
The team's upcoming research aims to pinpoint the exact light thresholds for near-infrared photosynthesis. This data could refine the search for habitable exoplanets by identifying which planets receive sufficient near-infrared light to support life.
Oxygen detection in an exoplanet's atmosphere could serve as a strong indicator of life, as Barton explained: "There are very, very few ways that oxygen can be made in an atmosphere without life."
The discovery in Carlsbad Caverns not only challenges our understanding of life on Earth but also broadens the horizons for finding life elsewhere in the universe.
