It's easy to assume that few creatures live in the dry and scorching deserts of Moab, Utah. Except for a few desiccated shrubs, the landscape appears to be composed of nothing but dirt, its hard-baked terrain unfurling all the way to the horizon. To the untrained eye, the ground looks crème brûléed—like the aftermath of a wildfire, the ground scorched by the relentless sun.
Scientists conduct a biocrust survey on the Colorado Plateau in Utah, USA. Photograph: Erika Geiger, USGS. www.usgs.gov/media/images/biocrust-survey-utah.
But come rain, the desert landscape is transformed. In a matter of minutes, the tawny patina easily mistaken for bare earth awakens with living colors of green, pink, and orange. Life, of an especially hardy variety, was here all along.
Some down-to-earth desert inhabitants thrive where few others do. In the top few centimeters of soil, covering the landscape in a charred-looking mantle, are biocrusts, short for biological soil crusts. They're a community of unique, interacting microorganisms that collectively make up the primary producers of drylands. They photosynthesize, bind soil, and open the door for thirstier species such as moss, lichen, and even vascular plants to enter the fray. Biocrusts are aptly called the Earth's “living skin.”
Biocrusts occupy some of Earth's most inhospitable environments, but their footprint is not just niche. They populate about a quarter of Earth's dryland eocsystems and some 12% of the planet's surface. Collectively, biocrusts occupy twice as much land area as tropical rainforests. If you've ever visited a desert, chances are you've beheld biocrusts, whether you knew it or not.
They're not sparse; rather, they're invisible. “Biocrust blindness is real,” says Sasha C. Reed, an ecologist at the US Geological Survey. Although biocrusts cover vast areas, their rugged features, forming a miniature of Earth's own terrain, are best appreciated up close, because they constitute just a thin layer of the topsoil. “They are so small that you have to get down to see them, she says.”
A close-up of biocrust filaments in the drylands in Utah, USA. Photograph: Erika Geiger, USGS. www.usgs.gov/media/images/biocrust-filaments.
Reed works in biocrust country in Moab, where the biocrusts are, in her words, “world class” — gorgeous enough to make knowing witnesses weep. This is no hyperbole; Reed recalls attendees of one international biocrusts meeting here shedding tears over the magnificence of Moab's teeming spread, dappled in a chiaroscuro of scorched-Earth shades and textured like a shag carpet.
Reed is also part of a small but growing cadre of scientists working to bring attention to biocrusts as yet another biosphere that's deserving of marvel and appreciation. Just like towering rainforests and showy coral reefs, biocrusts’ ability to pull carbon from the atmosphere and terraform landscapes make them a potentially powerful ally against climate change. Only recently have scientists begun to realize that these puny organisms have planetary power, but their contributions have long been neglected in global models for explaining Earth's cycles. To rectify that, Reed and others around the world are studying biocrusts and revealing their secrets. These efforts include examining what makes a biocrust, how they colonize and exploit the soil, the importance of biocrusts to the planet, and, finally, how humans can give them the respect they're due — by saving them from ourselves.
What are biocrusts?
Biocrust members vary from region to region but share a similar structure. Each of its member micro- and macro- organism fulfills a particular niche. The majority are cyanobacteria, the photosynthesizing backbone of the group. Their filamentous bodies allow for soil aggregation, and they secrete sugary molecules that glue loose particles together into a coherent armor. They're the founding members, setting this literal stage for other microorganisms to join the party. The subsequent arrival of nitrogen and phosphorus-fixing bacteria then allows the community to pull other essential nutrients from the atmosphere. A sufficiently productive community can recruit highly specialized members, such as bacteria that secrete suscreen pigments that protect the entire biocrust community from ultraviolet radiation. On occasion, biocrusts may also attract latecomer macroorganisms such as moss and lichen to ramp up food production.
A broken off piece of biocrusts showcases their ability to cement soil particles together to form the Earth's living skin. Photograph: David Elliott. www.flickr.com/photos/drelliott0net/19908964104.
Late-successional-stage biocrusts with lichen. Photograph: Erika Geiger, USGS. www.usgs.gov/media/images/biocrusts-lichen.
These are the successional stages of every biocrust when they first work bare soil. It is a slow process. In one experiment, researchers measured how long it took biocrusts to recolonize a churned up patch of land in the desert. Left to their own devices, biocrusts took two years to develop. Other researchers have observed that the foray of lichen and moss into biocrusts can take decades.
Microclimate and water availability determine the eventual constituents of biocrusts, but underpinning all that communal diversity are two defining features that biocrusts must be able to fulfill. A crust must be able to photosynthesize, and it must be able to tolerate prolonged desiccation. For the latter, cyanobacteria have a trick up their sleeve: the same sugary secretion that binds soil particles can protect fragile DNA from ultraviolet damage. The cyanobacteria coat their DNA molecules so much that they “become candy,” says Ana Giraldo-Silva.
Biocrusts damaged by vehicle tread. Photograph: Erika Geiger, USGS. www.usgs.gov/media/images/biocrust-damage-vehicle.
Giraldo-Silva is a microbial ecologist at the Public University of Navarre in Spain. Her interest in biocrusts, though, is global; she has soiled her hands in the southern corners of her home country, brushed her fingertips on the undersides of crust layers in Arizona's Sonoran Desert, and roamed all over Colombia to compare the differences between pockets of biocrust communities. On the Colombian coast, she even discovered a salt-tolerant variety living meters away from the Caribbean Sea.
Recently, she has been focused on biocrusts in the Chihuahuan Desert at the US–Mexico border. Like Moab's biocrusts, they boast a lumpy texture. From a microbe's vantage point, biocrusts appear as a series of towering peaks and sunken valleys. Within this irregular topography, the opposite sides of each microbial mountain may harbor entirely different species, thanks to rain shadow effects that play out on the millimeter scale.
Biocrust ecology
Giraldo-Silva suspects that this rough surface serves to trap nutrient-rich mineral dust. An otherwise flat surface wouldn't allow particles to linger. Dust is known to travel the world; for example, particulates from the Sahara have landed on Colorado's Rocky Mountains. Could biocrusts be taking advantage of this windfall to satisfy their nutritional needs?
The ecologist is currently in pursuit of the answer. She has erected dust traps in the Chihuahuan Desert to see whether the same substances found in the air are present among biocrusts. She's particularly interested in the presence and role of phosphorus in this nutrient-poor region. Dust trapping, she suspects, could be one way biocrusts are fertilizing the soil and profoundly shaping the productivity of drylands.
Elsewhere, biocrusts are giving the land a more extreme makeover. In Chile's Atacama desert, where rain is extremely rare, a quirky biocrust variety called grit crust is quietly eking out an existence. It makes its home in the loose, pebbly terrain, but instead of cementing the gravel together, the microbes coat each particle, sinking their hyphae into the nooks and crannies. Over time, the microbes split the grit into finer crumbs, forming, over eons, what we call soil.
Patrick Jung discovered the grit crust of Chile's Pan de Azúcar National Park in 2016. Questioning the identity of curious checkerboard blotches on the ground, the mircobial ecologist found that they were a mixture of endemic lichen and cyanobacteria. This community sips on daily fog instead of guzzling brief spells of rain like traditional biocrusts do.
The grit crusts are so unlike their earthier cousins elsewhere that scientists debate on whether they count as biocrusts at all. But Jung argues, “it's a completely new perspective on the term.” Including grit crusts as biocrusts would broaden the scientific notion on the forms and functions that of these communities plus fertilize ideas on what other unlikely places biocrusts might lurk.
Beyond unearthing surprising new varieties, the research community is moving toward understanding biocrusts’ collective footprint. To that end, remote sensing is the latest tool in the research arsenal that is helping advance biocrust science. The open-sky living requirement of biocrusts makes them amenable to aerial observations. In recent years, scientists have used drones, balloons, and satellites to read the telltale wavelengths of the electromagnetic spectrum that biocrusts reflect or emit in order to map their locations. With the rise of remote sensing, no longer do scientists have to travel long distances to examine biocrusts one patch at a time. Now they can survey biocrust fields from a bird's-eye view, no matter how inaccessible their location.
These modern methods open the door for scientists to better understand the planetary influence of biocrusts. For all their puny statures, biocrusts can shape climate forecasts and dictate geochemical cycles through photosynthesis and nutrient uptake. Bridging the gap between microscopic and planetary-scale processes, biocrust scientists are entering these tiny organisms into their rightful place in mathematical models of the pulse of the Earth, from which they had long been neglected.
Biocrusts at our service
It's already easy to marvel at these communities of microorganisms for their extraordinary biology. But more than that, they're also important planetary stewards for the key ecosystem services they provide.
Biocrusts stabilize the soil in drylands, reduce erosion, and wrangle dust emissions under control. Controlling dust emissions is one example of how human health and prosperity is embedded within the environment, aka One Health. www.usgs.gov/media/images/biocrusts-provide-soil-stability-and-prevent-erosion.
As the first colonizers of barren ground, biocrusts stabilize the soil surface. Members restore soil fertility by locking nutrients from the air in the ground. Through the work of biocrusts, the earth becomes better at retaining moisture and resisting wind erosion. In one study, researchers found that the world has biocrusts to thank for firming up the Great Wall of China and warding off erosion. The function of biocrusts depends on the unique mixture of species present, which, in turn, depends on their habitat. In the case of grit crusts, weathering is hastened: The gravel they live on disintegrates into a more familiar-looking kind of soil, one more amenable for future plants to grow on. Researchers even think we have biocrusts to thank for colonizing dry land more than half a billion years ago, when life was still confined to the oceans—the terraforming services of biocrusts may have literally paved the way for ancestral marine plants and animals to move in.
Even arthropods exploit biocrusts. Ants construct underground nests among the microbial folds, which helps prevent their colonies from being buried by sand. Bristletails and nematodes feed off the lichen and moss from biocrusts in China's deserts. In the Atacama, Jung suspects that a subterranean snail also relies on grit crust for sustenance. During drizzly weather, the mollusk comes to the surface to seek out ephemeral shrubs taking advantage of the rainy reprieve. While waiting for shoots to grow after this moisture, the snail likely snacks on lichen-smeared grit, Jung says.
Given their land-locking abilities, biocrusts are bulwarks against the burgeoning challenges of desertification and dust. Without biocrusts, it is estimated that the globe would be more than twice as dusty, with the mass equivalent of all the buildings in New York City aerosolized into the atmosphere every year. Dust has been known to hasten glacier melting, disrupt water quality in lakes and rivers, choke plants’ photosynthetic machinery, and impair human health.
Scientists are putting biocrusts to work. In the crumbling desert landscapes of the Tengger, the Hobq, and the Gurbantunggut, in China, workers have sprayed cyanobacteria, the founders of biocrusts, onto the terrain as a way to combat land degradation. It's the desert equivalent of tree planting using tinier native ecosystem engineers. Reports have shown that this introduction of new biocrusts, when mobilized alongside other antierosion strategies, has successfully stabilized sand dunes and prevented grasslands from being buried under sand.
A 10-by-10 cm biocrust slab from a sand dune in Diamantina National Park, Australia. Photograph: David Elliott. www.flickr.com/photos/drelliott0net/20555116241.
“It's an amazing demonstration of the power of microbes to have macroscopic effects,” says David Elliott, a microbial ecologist at the University of Derby, in the United Kingdom.
What's more, biocrusts play a crucial role in our fight against climate change. Soil itself is a crucial carbon sink, and photosynthesizing biocrusts can enhance soils’ carbon-storage function by over twofold.
But climate change is making it harder for these biocrusts to do their jobs. Biocrusts may be able to withstand the desert's harsh climate, but rising temperatures are pushing their physiologies to the limit. Changing weather patterns alter the composition and diversity of biocrusts, throwing off their valuable ecosystem functions. One model predicted that climate change will shrink the global cover of biocrusts by 25% or more by 2070.
The bigger threat to biocrusts, however, is direct human activity. Their invisible stature makes them prone to trampling by unwitting tourists and ranch animals. Under the heel, biocrusts are buried alive or flipped upside down like an overturned turtle, robbing them of the exposure to sunlight they need to photosynthesize.
Biocrust scientists are painfully aware that their research activities—which may involve stepping on some biocrusts—can also harm their study targets, so they do their best to minimize the impact. When Reed and her colleagues need to venture among biocrusts, they devise fixed trails that overlap with natural areas already devoid of biocrusts, such as dry stream beds and weathered sandstone called slickrock. “We can't go for a zero effect, but we try really hard to be thoughtful about how we move on the landscape,” she says.
Livestock grazing is particularly damaging to biocrusts. On the Patagonian steppe, in Argentina, roaming sheep have annihilated biocrust cover, says Sergio Velasco Ayuso, a University of Bueno Aires ecologist who surveyed grazing's impacts on biocrusts in his home country. His team estimated that the biocrusts there would require up to 58 years to bounce back. Interestingly, native grazers are less of a danger to biocrusts—for example, guanacos, llama cousins that live in the Atacama, usually do not congregate in numbers as large as that of livestock. They also tend to stick to existing migratory routes, often moving in single file.
Velasco Ayuso recognizes, though, that banning ranching to save biocrusts isn't a solution, because ranching is an important local livelihood. He thinks the appropriate path forward is proper land management, by triaging areas for herding or the preservation of biocrusts.
Land use and climate change are a dangerous duo. Studies have shown that the combined pressures of grazing and increasing aridity in Australia and South Africa are dealing a double whammy to desert biocrusts.
There is growing awareness that biocrusts, like any other biome, require protection. And people are stepping up. Hiking trails in the western United States are now often adorned with signage such as “Don't bust the crust” and “In crust we trust” to urge visitors against impulsive wandering. Reed says that, throughout the decades of her career, she's observed a rising appreciation among land managers in the American Southwest for these inconspicuous defenders of the drylands. These people have frequently sought her out for advice on public outreach and biocrust restoration efforts on public lands that have been disrupted by development projects, sometimes even as minor as a parking lot expansion.
On occasion, biocrusts have even helped spark a larger environmental movement. Jung says his discovery of grit crusts foiled a proposed railroad construction project for mining through the Atacama Desert in 2017. As more people recognize the importance of biocrusts, the preservation of dryland ecosystems as a whole might hinge on these slender but critical foundations.
Restoration
Less than nine miles from Utah's Arches National Park, under the watch of looming mesas, sits the world's first and largest biocrust farm. The Mayberry Native Plant Propagation Center grows a variety of desert fauna, but when it comes to biocrusts, Mayberry has big plans beyond its 30-acre holdings.
Leaving biocrusts alone is the first and easiest step toward preserving them. Damaged biocrusts can recover, given ample time and space. To that end, scientists are giving them a boost by reseeding disturbed fields with biocrusts grown elsewhere, such as from Mayberry.
Restoring biocrusts, it turns out, isn’t technically complicated. Simply dispersing crushed-up biocrusts from a healthy patch nearby can kickstart new growth. Usually, scientists source biocrusts from nearby, because the similar microclimates are likely to produce well-adapted cultivars, therefore increasing the chances of replanting success. The downside though, is that new biocrusts come at the expense of older stock, so this method isn't scalable. This is why Mayberry was established—its purpose is to raise biocrusts for export rather than sacrificing nature's original growth.
Some of the early restoration efforts, starting around the turn of the millennium, saw scientists cultivating biocrust inoculums indoors—in greenhouses and laboratories—under strictly controlled settings. These coddled crusts grew quickly, but none of them survived after relocation. “One of our hypotheses was [that] we made life too easy,” says Reed, a frequent collaborator of Mayberry. “We didn't allow the organisms to prepare themselves for the challenges of the real world.” Now, biocrusts are grown in outdoor nurseries like at Mayberry so that cultivars experience the harsh desert elements right from the start and grow more resilient.
New innovations in biocrust growth are helping active restoration efforts become more scalable. Instead of dispersing inoculum crumbs, scientists have explored transplanting entire biocrust communities in one go. For that, Mayberry's workers have experimented with growing swaths of biocrusts on biodegradable weed cloth, which can then be rolled up like sod and transported elsewhere.
Scientists create conditions of elevated temperatures in outdoor testing plots to study how biocrusts would respond to global warming. Photograph: USGS. www.usgs.gov/media/images/biocrust-outdoor-testing-plots or www.usgs.gov/media/images/usgs-scientist-sasha-reed-studys-outdoor-biocrust-testing-sites.
In the face of climate change, Mayberry is looking into restoration with nonnative biocrusts from hotter climes. Scientists think this might inoculate the landscape against global warming if its keystone inhabitants are already well-adapted to higher temperatures.
During transplantation, scientists have to take care not to introduce harmful species. A few years ago, scientists discovered a natural predator of biocrusts: the bacterium Candidatus Cyanoraptor togatus. Its modus operandi is grisly: It targets cyanobacteria and “sucks out their insides, like a vampire,” Giraldo-Silva says. Where this pathogen strikes, spots of white appear on the biocrust fields, not unlike the phenomenon of coral bleaching, creating ghosts that cannot be revived.
But Cyanoraptor isn't a major threat to biocrust survival. “It's a part of the natural ecosystem,” says Ferran Garcia-Pichel, a microbiologist at Arizona State University. He led the research that first identified the existence of Cyanoraptor.
In his view, it is still humans, not Cyanoraptor, that represent the greatest peril to biocrusts. Nevertheless, some human enterprises could inadvertently bring biocrust salvation.
A community effort
In Arizona's Sonoran Desert, rows of solar panels stretch across the landscape like black sails. The ground beneath them has been disturbed during their construction, so it's in need of biocrust restoration. But, as scientists have learnt, these partially covered plots are also ideal nursery sites for cultivating biocrust Inoculants.
In the last few years, Garcia-Pichel's team has partnered with solar companies on exploring this novel strategy of biocrust restoration. The shady area beneath a solar panel is slightly milder than the open air for a young biocrust struggling to assert itself. Shade provides cooler temperatures and allows the ground to retain water for longer.
These gentler conditions would otherwise have needed to be replicated in a greenhouse or farm. But solar panels are already doing exactly that by fiat. So, taking advantage of this accidentally nursery is “a very low hanging fruit,” says Garcia-Pichel. He coined the term “crustovoltaics” to describe the venture of harnessing solar panel infrastructure for mass producing biocrusts for export.
The so-called crustovoltaics initiative by Garcia-Pichel's team comes at a critical time for urban Arizona. About 40% of the farmland in Maricopa County, nearly 100,000 hectares in all, is left fallow due to water scarcity. These fields are prone to dust emissions that menace the Phoenix metropolitan area. Among the public health hazards associated with dust is valley fever, a disease caused by fungal spores riding on wind-blown particles. Cases of valley fever have more than doubled in the last 15 years, and Arizona has reported the highest number of infections of any state in the United States. Furthermore, dampening the dust could boost the efficiency of the solar farms, preventing the panels being obscured. Garcia-Pichel calls it a win–win situation for solar companies.
“It's really cool,” says Reed, who wasn't involved in the project, regarding the concept of crustovoltaics. “As we turn more of the desert landscape into solar farms, thinking about other co-occurring uses could help stabilize the system or make the most out of that land cover type.”
In a pilot study, biocrusts under the solar panels regenerated in as rapidly as two years after Garcia-Pichel's team harvested a portion of them. According to calculations, if the region's three largest solar farms doubled as biocrusts nurseries, all the fallow plots in Maricopa County could be treated within five years. Garcia-Pichel is currently in talks with multiple solar companies to expand the team’s crustovoltaics trial.
Roping in solar companies for biocrust restoration has the potential to make the movement more scalable. This collaborative approach is emblematic of the philosophy of teamwork from biocrusts themselves, which are a living example of overall resilience against the kind of environmental adversity that no individual species alone can overcome. The biocrust “is such a perfect union of different microorganisms,” Giraldo-Silva says. “They coordinate and synchronize in such a way that brings all these consistent services to the soil that otherwise wouldn't be possible.” In general, to save and preserve biocrusts, the best bet is a all-hands-on-deck kind of effort involving different players, as embodied by Garcia-Pichel's crustovoltaics strategy.
Restoration efforts are now expanding rapidly. More broadly, biocrust research as a whole is an exploding field. Just two decades ago, few researchers focused on biocrusts, Reed says. Now a growing group of dedicated specialists are tapping the mysteries of the desert's quiet sentinel. “It's having its moment.”
And what researchers have discovered so far about biocrusts is dizzyingly rich—and just the tip of the iceberg. The humble appearance of the biocrust belies the complex interactions between its microscopic members and its critical contributions to the overall health of the desert. As scientists unearth more about these marvelous communities and begin to grasp biocrusts’ impact on the planet, that knowledge is also helping scientists to give these crucial species a boost from the threats of our own making—for our own sake. So, the next time you find yourself on a desert trail—hopefully watching where you tread—spare a moment to respect and marvel at the extraordinary microbes underfoot and all around you. Our climate future may depend on them.
Author Biography
Shi En Kim (@goesbykim.bsky.social) is a freelance science journalist based in Washington, DC. She’s one of the cofounders of digital startup magazine, Sequencer. Find more of her work on www.shienkim.wordpress.com.