Desert potholes may provide clues to the evolution of life on Earth.
We’re all familiar with potholes – those nasty ruts in the road that produce an uncomfortable bump when we drive over them. But there is another kind of pothole that can be found in the desert, far away from human traffic. Instead of being a nuisance, these potholes provide shelter for life in a hostile environment. These potholes also may provide clues to the evolution of life on Earth and the possibility for life on other worlds.
In the high desert of the Colorado Plateau, wind and rain carve out hollows in the desert sandstone, creating potholes over time. These potholes range from several centimeters to tens of meters in depth, and when they fill with water they become home to a variety of organisms.
However, the water in desert potholes doesn’t last long. Rainfall in the desert is intermittent at best. In the subzero temperatures of winter, pothole water freezes, and in summer, when temperatures reach as high as 140 degrees F, the water evaporates.
For aquatic creatures, losing water is like losing the ozone layer. Water acts as protective insulation, and when it is gone organisms are exposed to intense heat, UV radiation, drying and freezing. Organisms that can survive such environmental extremes are of much interest to astrobiologists: these resilient Earth creatures may indicate the survival strategies used by life on other, less hospitable worlds.
Margie Chan, a geologist at the University of Utah, is working with a team of scientists to study the origin of desert potholes and the organisms that live in them. Her team members include Katrina Moser, a paleolimnogist at the University of Utah, Gordon Southam, a geomicrobiologist at the University of Western Ontario, and Tim Graham, a biologist with the U.S. Geological Survey. Chan says there are three adaptive strategies for living in the precarious environment of desert potholes: escape, desiccation resistance, and anhydrobiosis.
Escapers are organisms that move away when the water dries up. Insects, toads, and salamanders make up the majority of the escapers.
Desiccation resistors are able to seal moisture inside their bodies and become dormant. This “Tupperware” strategy is practiced by water snails and some species of mites.
Anhydrobiosis is an ability to survive an almost complete loss of body water. The cells of this type of cryptobiotic organism replace the water with sugar, which acts as a sort of antifreeze. The metabolism shuts down while the organism waits out the harsh conditions. Once water is available, the organism emerges from its dormant state with relatively little physical damage. Anhydrobiotic pothole organisms include bacteria, algae, microscopic animals such as tardigrades, rotifers, nematodes, and the eggs of crustaceans like fairy shrimp, clam shrimp and tadpole shrimp.
“Moser also recently discovered over a dozen different species of diatoms in sediment from the potholes,” says Chan. “We hope to do more studies documenting the types and the relationship to environmental conditions.” [Diatoms are unicellular algae, and they can act as indicators of environmental change.]
Conditions in the desert follow a general cycle, but it is not a dependable one. One year the water may last for several weeks to months, another year, just a few days. While most pothole organisms have short life cycles, the briefest life span is ten days. If the water evaporates after two days, the organisms don’t stand much of a chance. But Graham says it’s hard to pin down a life span for some pothole organisms because of the mechanisms they’ve developed to deal with such short-term water.
“Some species like gnat larvae and mites can be active for a day or two, then may be dormant for 2 months, then active the next time it rains for a few days,” says Graham. “We don’t know their total life span: it could be 3 months, it could be a year.”
During the wet period, pothole organisms emerge from their dormant state in order to reproduce. However, there’s always a risk that the water won’t last long enough for the next generation. Since it is difficult to detect in advance how long the water will last, many organisms lay several kinds of eggs that have different cues for hatching. By hedging their bets, they are able to ensure that at least some of their progeny will survive. Organisms that lay a wide range of eggs are more likely to survive throughout the course of evolution.
“If one individual produces only eggs that hatch the very first time they get wet, and another produces some eggs like that, others that don't hatch until they dry out once, and others that must dry three times and be frozen once before they hatch, the later organism would be more likely to have representatives in the next generation, especially in arid, unpredictable environments,” says Chan.
Some of the species that inhabit desert potholes may represent some of the Earth’s earliest animals. Graham primarily studies pothole invertebrates, especially insects and branchiopods (the crustaceans tadpole shrimp, fairy shrimp, and clam shrimp).
“The crustaceans of potholes have an ancient heritage,” says Graham. “It is thought that branchiopods were among the first crustaceans, first appearing in the Cambrian. Large branchiopods today are relatively diverse, yet many of their traits have not changed appreciably for thousands, perhaps millions of years.”
Branchiopods originally lived in all waters, before the existence of insects and fish. Once these predators appeared, the branchiopods were quickly eliminated from most aquatic environments. In the face of this evolutionary pressure, branchiopods diverged into two groups: small forms that mostly swim in the water column, (primarily ‘water fleas,’ which were quick enough to avoid fish) and large branchiopods (tadpole shrimp, fairy shrimp, and clam shrimp) that were relegated to hypersaline waters and temporary bodies of water - environments where fish could not survive, and where insects could not build up large populations. Thus, the story of branchiopod evolution shows how interactions between species can have profound effects, directing the long-term trajectory of evolutionary paths.
Graham says that organisms capable of anhydrobiosis would have needed to have water available at least part of the time during the course of evolution. But no one knows how long anhydrobiotic creatures can survive without water. The potholes usually get at least a little bit of water each season. There are some indications, however, that water can be withheld for centuries without killing the organisms.
“I've had sediment with eggs in it for at least 5 to 7 years and can still hatch critters out,” says Graham. “Eggs of tadpole shrimp or fairy shrimp have been kept for 50 years in the lab, and were still viable. There is a record from England of tadpole shrimp occurring in a particular pool only twice in 150 years because the pond hadn't dried out in that time, and tadpole shrimp eggs must dry before they will hatch. A study of eggs of a different crustacean found that eggs estimated to be over 350 years old were still viable.”
By surviving without water, anhydrobiotic animals become resistant to numerous environmental extremes. Eggs of brine shrimp, for instance, have been carried into space and placed outside the space vehicle. After being exposed to the full radiation of the sun and the stresses of the vacuum of space, the eggs were brought back to Earth and successfully hatched.
“The eggs can be incredibly resistant to many environmental stresses, not just desiccation,” says Graham. “Understanding the mechanisms of anhydrobiosis may help us understand how life might survive periods without free water elsewhere in the solar system.”
The anhydrobiotic creatures in desert potholes make an interesting case study for possible life on Mars. Mars once had (and may still have) extensive liquid water resources. Scientists believe that most of the water on Mars is now locked up in the form of ice underneath the planet’s surface.
“The desert potholes make an analogy to Mars mostly because of how life adaptations might occur in extreme environments,” says Chan. “If we can use remote sensing types of techniques or mechanical life-detection devices, potholes are good analogies of meter-scale biomarkers that might be detectable.”
Chan also says the potholes might teach us something about life on Jupiter’s moon Europa. Although Europa is quite different from Mars – it has a frozen ice layer that may overlie a salty ocean – Europa’s icy surface is essentially “dry.” If Europa has liquid water beneath the frozen outer layer, the moon could harbor life. Such life probably would undergo similar wet and dry cycles as the anhydrobiotic organisms in desert potholes.
Chan and Southam have been studying a dried film of material that helps the potholes retain water longer than the surrounding sandstone. This film is composed of dried organic matter, and forms when the water is gone. Within two weeks of rehydration, the surface of the black film becomes green from algal growth, and bacterial colonies increase. Besides preserving pothole water, the biofilm also may enlarge the potholes by dissolving the cement between the sandstone grains.
“Both bacteria and algae are present, but we can't yet say how they work together,” says Chan. “The algae presumably serve as the primary producers supporting the ecosystem, while the heterotrophs (bacteria) are likely doing most of the mineral weathering, which helps create space for the algae.”
Chan says her group is only just beginning their research on desert potholes. She hopes to collaborate with astrobiologists who are doing parallel studies examining life in extreme environments. Chan also plans to participate in the Mars Society field project in Hanksville, Utah, where her team will study organisms that live inside rocks.
“In particular, we would like to try to understand how life develops, sustains, and evolves in this extreme setting,” says Chan.
Graham, meanwhile, is interested in how environmental factors determine which species will be found in a particular pothole. He hopes to see whether pothole communities create their own food through photosynthesis, or instead rely on organic matter blown in from the surrounding area. Graham also wants to learn more about a species of mite he discovered in potholes a few years ago.
“There are only two species in the genus it belongs to: this undescribed species, known only from the Colorado Plateau, and a species found in ephemeral pools in South Africa,” says Graham. “I would like to explore the intervening areas to see if there are other forms, to try to explain this intriguing biogeography. I am also interested in the ecology of the species on the Colorado Plateau because it lives in very small pools that would seem to have little productivity, yet there can be hundreds of these little mites in a single pool.”