It’s 3:30 a.m. on April 14, 2018, and biophysicists Tjaart Krüger and Michal Gwizdala sit on top of a desert mountain in the southwest African country of Namibia, examining small rocks. They look for fine films of bacteria that cling to life under small quartz pebbles.

Unlike insects and other creatures that live in the gravel plains of the desert, these so-called hypoliths can’t escape the relentless sun or search for food – they have to find ways to photosynthesize. But in the Namib Desert, where the sun scorches the earth and rain occurs once or twice a year, if at all, photosynthesis can be dangerous. Exposure to excessive UV radiation and a lack of water can lead to DNA damage and the build-up of reactive oxygen molecules. This is what makes the existence of the bacteria so intriguing for the two biophysicists.

In an article published earlier this year in Environmental microbiology, Krüger and Gwizdala, of the University of Pretoria in South Africa, and their colleagues have shown that these photosynthetic cyanobacteria have a new strategy for coping with the harsh environment. Using a mix of physics, biochemistry, and metagenomics, the team found that bacteria shut down their photosynthetic machinery for most of the year before taking action once water was available. They are saved from certain death by their quartz rock houses, which allow enough light for photosynthesis but reduce its intensity.

“The Namib Desert is one of the most interesting for the study of photosynthetic organisms that live in low water levels in deserts,” says Chris McKay, senior scientist at NASA’s Ames Research Center. “This article addresses the key question of how light passes through stones and how organisms use it.” Insight into how these microorganisms survive these harsh conditions and how they photosynthesize could prove important in understanding how life can respond to extreme conditions on planets such as Mars.

Once the sun rises in Namib, the fall temperature will exceed 40 ° C. But before dawn there is a cool breeze and only the light of the stars. In the dark, Krüger and Gwizdala, who sampled and collected rocks from three different sites in the desert, use fluorometry to measure the bacteria’s photosynthetic abilities, lifting stones and touching a probe to the green film of microorganisms. below.

In addition to absorbing light, photosynthetic organisms fluoresce it via their chlorophyll green pigments. Measuring light absorption and emission can provide information about an organism’s ability to photosynthesize and metabolize the microbial community, Gwizdala explains. The probe contains an LED that illuminates bacteria and can also detect and analyze emissions between 680 and 900 nanometers.

The sun crept over the horizon as Gwizdala and Krüger finished surveying the rocks atop the mountain. It takes 20 minutes to cross the silent early morning landscape towards the White Water Tower which rises above the gravel plains and the sea of ​​red dunes. This is the distinctive profile of the Gobabeb Namib Research Institute, a scientific outpost in the middle of the Namib. Krüger and Gwizdala place the rocks in a dark humid chamber and hydrate the dried bacteria. After the bacteria are fully hydrated, the researchers lay their stones on the desert sand with the bacteria facing the ground, exposing the rocks to full sunlight before re-measuring the bacteria’s fluorescence.

After removing the thin green film for genetic and pigment analysis, Krüger and Gwizdala clean the rocks to study their optical properties. Using a spectrometer, they measure the intensity and wavelength of light passing through stones. They say only about 3% of the photons that hit the surface get to the bacteria below. Quartz rock also lengthens the average wavelength of light by disproportionately absorbing shorter wavelengths – stones do not transmit any of the energy-rich blue and UV lights that are so dangerous to life.

Because of its stone niche, the bacteria do not need the coping tricks other plants, algae and bacteria have developed in such harsh environments, the researchers determined. A 2019 study found that some organisms had adapted special mechanisms to extend the spectrum and perform photosynthesis using far red light, which is less efficient for photosynthesis but gives them an advantage in relatively shaded or filtered light conditions. . Gwizdala and Krüger expected the Namib bacteria to do something similar, given the extreme environment and the fact that the light filtered by the stone is relatively red, but it seems the hypoliths are opting for efficiency. . In the rare moments when water is available, they photosynthesize as efficiently as possible and build up safe energy for the lean months to come.

With the rock providing protection, bacteria don’t have to worry about solar radiation damaging their photosynthetic machinery; instead, they can save energy when it’s dry, and then kick in when a rainy event arrives. When they have no water, bacteria completely shut down their photosynthetic machinery. In fact, when colleagues Krüger and Gwizdala at the Center for Microbial Ecology and Genomics at the University of Pretoria performed a metagenomic analysis of DNA under rocks, they did not find the microbial genes typically associated with high light stress.

“We went to Namib with the preconceived idea that hypolithons live in a constantly stressful environment and have a range of photoprotective mechanisms constantly activated,” says Krüger. But their research has shown that this is not the case. The main threat from extremophiles is lack of water, not solar radiation.

In addition to exposing the secrets of life that thrive in Earth’s extremes, these habitats give scientists the opportunity to study environments similar to those on other planets, both present and past, and how life can inhabit these places.

“From NASA’s point of view, this [research] is quite interesting for the implications for life on Mars in the past, as the climate on this planet has become drier over time, ”says McKay. “These photosynthetic desert communities could be examples of the last stages of biological surface production on Mars.”

As for Krüger and Gwizdala, they hope their research will show that fluorometry and spectroscopy should be part of the “standard toolbox” for experiments in microbial ecology, and that such methods should be replicated in other extremely deserts. dry world, as in the Atacama Desert in Chile and those of Antarctica.