Lead
In early surveys of remote passages beneath Carlsbad Caverns National Park in southern New Mexico, researchers led by cave biologist Hazel Barton and microbial scientist Lars Behrendt discovered green microbial films that photosynthesise in the absence of visible sunlight. First noticed during a 2018 expedition, tests showed these organisms use chlorophyll variants that harvest near-infrared light, extending photosynthesis into zones previously considered too dark. The find—observed across multiple off‑trail caverns in a 119‑cave network formed 4 to 11 million years ago—suggests photosynthetic life could survive under very different stellar light regimes than Earths surface plants. The result has immediate implications for how astrobiologists prioritise targets when searching for life beyond Earth.
Key takeaways
- Researchers found cyanobacterial biofilms deep in Carlsbad Caverns (a park with 119 known caves), including in passages where human-visible light is absent.
- Cave cyanobacteria contain chlorophyll d and f and can capture near‑infrared wavelengths up to about 780 nm, beyond the conventional 700 nm photosynthetic limit.
- Measurements showed near‑infrared light levels in the deepest alcoves were about 695 times higher than at the cave entrance, due to limestone reflectivity.
- The Big Room in Carlsbad Cavern measures roughly 4,000 ft (1,220 m) by 625 ft (191 m); about 350,000 visitors use the show cave annually, while the discoveries were made off the tourist trail in sheltered alcoves.
- Findings may expand the types of exoplanetary environments considered potentially habitable, especially around abundant M‑type (red dwarf) and K‑type stars that emit proportionally more near‑infrared light.
- Past work relevant to this discovery includes 1890 descriptions of chemosynthetic microbes and later discoveries of infrared‑adapted cyanobacteria such as Acaryochloris marina in 1996.
- Barton and Behrendt have submitted a NASA proposal to quantify the minimum light intensity and longest usable wavelength for photosynthesis in these dark cave ecosystems.
Background
Carlsbad Caverns National Park in southern New Mexico contains a complex subterranean network carved primarily by sulphuric acid dissolution of limestone roughly 4 to 11 million years ago. The park is best known for the Big Room, a massive show cave chamber that is accessible to the public, but the full system includes many off‑trail alcoves and passages where natural light does not penetrate. The caves rest within the Chihuahuan Desert and are part of a landscape shaped by deep karst processes and microbial rock interactions over geological time.
Microbial life has long been found below Earths surface by two different metabolic strategies: photosynthesis, which requires light energy, and chemosynthesis, which extracts energy from chemical reactions with inorganic compounds. Since Sergei Vinogradskii described chemosynthetic microbes in 1890, scientists have documented organisms that thrive without sunlight, including communities around hydrothermal vents and inside deep rock. Discoveries of organisms that switch or extend photosynthetic capability into red and near‑infrared wavelengths have gradually challenged assumptions about the strict light limits for photosynthesis.
Main event
During a 2018 field collaboration, Lars Behrendt reached out to Hazel Barton to sample poorly lit alcoves along the Carlsbad cave network. In one such dark niche, flashlight illumination revealed a strikingly green biofilm on the limestone wall. Laboratory analyses identified the organisms as cyanobacteria, but with a twist: they carried chlorophyll variants known as d and f, which absorb longer wavelengths than the chlorophyll a found in most surface plants.
As the team pushed into ever darker passages where even headlamp illumination was minimal, the green pigment persisted. Instrumental light measurements in the back alcoves showed that near‑infrared intensity increased dramatically relative to the entrance, because limestone transmits and reflects longer wavelengths more effectively than visible light. In multiple side caves across the park the team repeatedly found photosynthesising microbes occupying sheltered surfaces.
Quantitatively, chlorophyll a based photosynthesis is commonly thought to fall off near 700 nm; the Carlsbad organisms demonstrate photochemistry up to about 780 nm when carrying chlorophyll f. The researchers report a strong spatial concentration of these infrared‑harvesting cyanobacteria in the darkest zones, implying ecological partitioning based on accessible photon energy. Behrendt and Barton note the communities may have been isolated for tens of millions of years, which could explain adaptation to the local spectral environment.
Analysis & implications
At a practical level for astrobiology, the ability of cyanobacteria to perform photosynthesis using near‑infrared light widens the spectral range at which life could harness stellar photons. If photosynthesis can operate further into the infrared than previously assumed, planets orbiting cooler stars that emit a larger fraction of their energy in near‑infrared wavelengths may be more favourable for surface or near‑surface primary production than models that limit usable light to 700 nm suggest.
Red dwarf (M‑type) and K‑type stars dominate stellar populations in the Milky Way. Many rocky exoplanets discovered to date orbit these smaller, cooler stars because their planets are easier to detect, but habitability assessments often emphasise liquid water and atmospheric retention while treating photosynthetic viability conservatively. The Carlsbad cave findings argue for revisiting those assessments: if organisms can adapt to near‑infrared regimes, the effective habitable zone for photosynthesis could extend inwards or outwards depending on stellar spectra and planetary surface properties.
There are caveats. Near‑infrared photons carry less energy per photon than visible light, which can limit reaction efficiency and affect the structure of autotrophic food webs. Substrate availability, nutrient supply, and stable ecological niches are also necessary conditions; spectral availability alone does not guarantee complex biospheres. Still, the cave data provide real‑world constraints that can be translated into observational strategies—helping decide which stars and planets to prioritise with instruments such as the James Webb Space Telescope.
Comparison & data
| Parameter | Conventional limit | Carlsbad cave finding |
|---|---|---|
| Photosynthetic wavelength cutoff | ~700 nm (chlorophyll a) | up to ~780 nm (chlorophyll f/d) |
| Relative near‑IR intensity (deep alcove vs entrance) | baseline | ~695× greater in deepest measured zones |
| Cave network age | — | formed ~4–11 million years ago |
| Big Room dimensions | — | ~4,000 ft (1,220 m) long × 625 ft (191 m) wide |
The table highlights how the caves host light conditions very different from open terrestrial habitats and provides quantitative anchors researchers can use when modelling photosynthesis under alternative stellar spectra. Translating these measurements to exoplanet contexts requires accounting for planetary albedo, atmospheric filtering and the spectral output of the host star.
Reactions & quotes
Field leaders emphasise both the surprise of the finding and its broader implications for searching for life.
Discovering green, photosynthesising films meters into a dark alcove shifted our idea of where light‑driven life can exist.
Hazel Barton, University of Alabama (cave biologist)
Context: Barton led cave sampling and biochemical analyses that identified chlorophyll d and f in the cave cyanobacteria. She framed the discovery as evidence that photosynthesis can operate under spectral regimes previously thought unusable.
We found that these microbes photosynthesise in sheltered zones that may have been isolated for tens of millions of years.
Lars Behrendt, Uppsala University (microbial biologist)
Context: Behrendt coordinated cave access and measurements in 2018 and emphasises the apparent antiquity and isolation of the communities, which may have facilitated spectral adaptation.
If we can quantify the lowest light levels and longest useful wavelengths, we can narrow exoplanet targets for telescopes like JWST.
Research team statement (Barton & Behrendt proposal summary)
Context: The team has proposed targeted measurements to NASA to establish empirical limits for photosynthesis under very low near‑infrared fluxes, linking cave observations to telescope prioritisation.
Unconfirmed
- The estimate that these cave communities have been completely untouched for 49 million years is a plausible interpretation based on isolation, but direct geochronological evidence tying microbial mats to that exact timescale is limited.
- The precise lower bound of light intensity and longest usable wavelength for sustained photosynthetic growth in situ remains to be established; the NASA proposal aims to quantify these limits.
- Scaling the cave measurements directly to exoplanet surfaces requires additional modelling of planetary atmospheres and surface reflectance, so any extrapolation to specific exoplanets should be treated as provisional.
Bottom line
The discovery of near‑infrared photosynthesising cyanobacteria in Carlsbad Caverns challenges a long‑standing assumption about spectral limits for photosynthesis and provides an empirical basis for expanding the range of stellar environments considered potentially habitable. By demonstrating that specialised pigments can exploit wavelengths beyond 700 nm in natural, sheltered settings, the work connects terrestrial microbial ecology to exoplanet target selection.
Next steps include rigorous measurement campaigns to determine minimal photon flux and maximal usable wavelength for photosynthesis in those caves, plus laboratory experiments that replicate cave spectral regimes. If those efforts establish robust thresholds, astronomers can use stellar spectra measured by facilities such as JWST to prioritise a narrower set of stars and planets for follow‑up atmospheric characterisation, improving the efficiency of the search for extraterrestrial life.