In a stellar piece of science reporting, Jasmin Fox-Skelly informs us about a finding which challenges some of the foundational principles of modern biology and astronomy which we have been taught in high school. The setting for this reportage is dramatic by itself: “Beneath the deep rocky canyons of the Chihuahuan Desert in southern New Mexico, lies a network of 119 caves. The caves, part of the Carlsbad Caverns National Park, formed four to 11 million years ago due to sulphuric acid dissolving the limestone rocks.”
The two protagonists of this discovery are Lars Behrendt, a microbial biologist at Uppsala University, and biologist Hazel Barton, professor of geological sciences at the University of Alabama.
Deep inside the Carlsbad Caverns, far away from where natural light can reach, Barton & Behrendt made a startling discovery in 2018: “For more than 20 years, Barton has been studying microscopic life found deep underground. Yet what happened next was a surprise, even to her.
Behrendt shone a torch on the wall. Although the alcove was pitch black, the light revealed a blanket of green microbes clinging to the wall. Later tests revealed they were cyanobacteria; single celled organisms related to bacteria. Unlike most bacteria, though, cyanobacteria (also known as blue-green algae) use light from the Sun to make food.
“We started going deeper and deeper into the cave,” says Barton. “Eventually we were a point where we couldn’t see without using flashlights. We had to use a headlight to be able to see our hand in front of our face, and yet you could still see green pigment on the wall.”
Plants are green due to a chemical called chlorophyll, which absorbs light energy. In photosynthesis, this energy is used to convert carbon dioxide and water into glucose and oxygen. The process is much the same in cyanobacteria. Yet here, in the cave, there was no sunlight.”
So, what was going on? How were these bacteria thriving in a place with no sunlight? Further research revealed that some forms of bacteria don’t need sunlight thrive: “It turns out that the cyanobacteria in the cave have a special version of chlorophyll that can capture near-infrared light. This light has a longer wavelength than visible light, and appears just before infrared on the electromagnetic spectrum. It is undetectable to the human eye.
While plants and cyanobacteria use chlorophyll a for photosynthesis, the cyanobacteria in the Carlsbad caves use chlorophyll d and f, which are able to generate energy from near-infrared light.
Although visible light can only travel a few hundred feet into the caves, near-infrared can journey a lot further due to the reflective nature of the limestone rocks. “The limestone rock that the cave is made of will absorb almost all visible light, but to near-infrared light, caves are pretty much a hall of mirrors,” says Barton.
In fact, when the researchers measured the light in the back of the cave where it was darkest, they found the levels of near-infrared light were 695 times more concentrated than at the entrance. At the same time, while chlorophyll d and f containing cyanobacteria were present in all parts of the cave, they were particularly concentrated in the darkest and deepest places.
The researchers also hiked out to other caves in the Carlsbad Caverns National Park and tested other off-the-beaten-track caves and caverns. In each case they found photosynthesising microbes deep down underground.
“We showed that not only do they live down there, but that they photosynthesise in a completely sheltered environment where they’ve probably been untouched for 49 million years,” says Behrendt.”
Behrendt & Barton’s finding has profound implications for astronomers. Why? Because so far our search extra-terrestrial life has focused on Earth-like planets. Behrendt & Barton’s finding opens up the prospect that life could exist in an altogether different type of planet:
“As the existence of liquid water is essential for life on Earth, this measure, known as the star’s “Goldilocks zone”, is what astrobiologists have focused on when looking for extraterrestrial life. So far, they have found dozens of candidates. However not all of these planets could sustain life, and directing telescopes like the James Webb Space Telescope (JWST) takes time and considerable resources.
Another important factor which governs whether life can exist is whether photosynthesis can take place. On Earth, photosynthesis forms the base of most food chains and provides the oxygen we breathe. For this reason, it makes sense to limit the search to planets that can support photosynthesis. This could dramatically reduce the zone around a star where life could exist.
In the past, astrobiologists set the limit for photosynthesis at a wavelength of 700nm in the light spectrum, which is the equivalent wavelength of the colour red. This is the point at which the efficiency of photosynthesis using chlorophyll a declines. However, the cyanobacteria discovered in the Carlsbad cave systems can harvest light up to wavelengths of 780nm using chlorophyll f.
“The vast majority of stars in our galaxy are these M- and K-type stars,” says Barton. “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 the conditions of light that would be produced by a star like that.”
Barton plans to change this. Together with Behrendt, she has put in a proposal to Nasa to find the limits of where photosynthetic life can survive. The work would involve going deep down into the darkest caves to measure exactly how much light is needed for cyanobacteria to survive. That information can then be used to narrow down the search for habitable worlds. For example, with the JWST, scientists can measure the amount and type of light exoplanets receive.
“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 photosynthesise,” says Barton.
“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].””
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