Reveal the secrets of desert extremophiles
It’s 3:30 a.m. on April 14, 2018, and biophysicists Tjaart Krüger and Michal Gwizdala are sitting on top of a desert mountain in southwestern Namibia, examining small rocks. They look for thin films of bacteria clinging to life on the underside of small quartz pebbles.
Unlike the insects and other creatures that live on the gravel plains of the desert, these so-called hypoliths cannot escape the relentless sun or search for food – they must find ways to photosynthesize. But in the Namib Desert, where the sun scorches the earth and it rains once or twice a year, if at all, photosynthesis can be dangerous. Exposure to excessive UV radiation and lack of water can lead to DNA damage and the buildup of reactive oxygen molecules. This is what makes the existence of bacteria so intriguing to both biophysicists.
In an article published earlier this year in Environmental microbiology, Krüger and Gwizdala, from the University of Pretoria in South Africa, and their colleagues showed that these photosynthetic cyanobacteria have a novel strategy to cope with the harsh environment. Using a mix of physics, biochemistry and metagenomics, the team found that the bacteria shut down their photosynthetic machinery for most of the year before springing into 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 studying photosynthetic organisms that live on low water levels in deserts,” says Chris McKay, senior scientist at NASA Ames Research Center. “This paper addresses the key question of how light travels through stones and how organisms use it.” Insight into how these microorganisms survive these harsh conditions and how they carry out photosynthesis could prove important in understanding how life can respond to extreme conditions on planets such as Mars.
Once the Sun rises in the Namib, the autumn temperature will reach over 40°C. But before dawn there is a cool breeze and only starlight. In the dark, Krüger and Gwizdala, who sampled and collected rocks from three different sites in the desert, use fluorimetry to measure the photosynthetic abilities of the bacteria, by lifting the stones and touching a probe to the green film of micro -organisms below.
In addition to absorbing light, photosynthetic organisms emit it by fluorescence via their green chlorophyll pigments. Measuring light absorption and emission can provide information about an organism’s ability to photosynthesize and the metabolism of the microbial community, says Gwizdala. The probe contains an LED that illuminates the bacteria and can also detect and analyze emissions between 680 and 900 nanometers.
The sun crept over the horizon as Gwizdala and Krüger finished pacing the rocks at the top of the mountain. It takes 20 minutes to cross the quiet early morning landscape towards the white water tower that rises above the gravel plains and sea of red dunes. It is the distinctive outline of the Gobabeb Namib Research Institute, a scientific outpost in the middle of the Namib. Krüger and Gwizdala place the rocks in a dark moist chamber and hydrate the dried bacteria. Once the bacteria have been fully hydrated, the researchers rest their rocks on the desert sand with the bacteria facing the ground, exposing the rocks to full sunlight before measuring the fluorescence of the bacteria again.
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 the light passing through the stones. They say that only about 3% of the photons that hit the surface reach the bacteria below. Quartz rock also lengthens the average wavelength of light by disproportionately absorbing shorter wavelengths – the stones do not transmit any of the energy-rich blue and UV light that is so dangerous to life.
Because of its stone niche, the bacterium doesn’t need the adaptive tricks that other plants, algae and bacteria have developed in such harsh environments, the researchers determined. A 2019 study found that some organisms have adapted special mechanisms to broaden the spectrum and photosynthesis by 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 bacterium to do something similar, given the extreme environment and the fact that the light filtered by the stone is relatively red, but it seems that the hypoliths opt for efficiency. In the rare times when water is available, they perform photosynthesis as efficiently as possible and accumulate safe energy for the lean months ahead.
Thanks to the rock that protects them, the bacteria do not have to worry about solar radiation damaging their photosynthetic machinery; instead, they can save energy when it’s dry and then spring into action when a rainy event arrives. When they have no water, bacteria completely shut down their photosynthetic machinery. In fact, when Krüger and Gwizdala’s colleagues from the Center for Microbial Ecology and Genomics at the University of Pretoria performed a metagenomic analysis of DNA under the rocks, they did not find the microbial genes commonly associated with high light stress.
“We went to the Namib with the preconception that hypoliths live in a permanently stressful environment and have a range of photoprotective mechanisms that are permanently activated,” says Krüger. But their research showed that is not the case. The main threat from extremophiles is lack of water, not solar radiation.
As well as exposing the secrets of the life that thrives 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 desert photosynthetic communities could be examples of the last stages of surface biological production on Mars.”
As for Krüger and Gwizdala, they hope that their research will show that fluorometry and spectroscopy should be part of the “standard toolkit” for experiments in microbial ecology, and that such methods should be replicated in other extremely deserts. dry in the world, such as in the Atacama Desert in Chile and those in Antarctica.