Biocrusts: A Hidden World at Our Feet

By Andrew Stuart

Its appearance can be intimidating, but West Texas is anything but lifeless. Indeed, this land sustains diverse habitats and ecosystems. Yet there’s one ecosystem here that even seasoned desert explorers are often unaware of. 

Its inhabitants include the planet’s oldest life forms. And our region’s extreme conditions suit these beings just fine. 

Biological soil crust, or biocrust – also called cryptogamic soil – is the “living skin of the desert.” Once you’ve been introduced to this hidden world, you’ll be echoing the plea: “Don’t bust the crust.”

With an untrained eye, it’s easy to overlook. Biocrust often looks like nothing more than “dirty dirt.” But there’s a lot going on in these patches of dark, knobby ground.

The process begins with organisms called cyanobacteria. They’ve existed for at least 2 billion years – and account for the oldest known fossils of life on Earth. 

Dr. Nicole Pietrasiak, of New Mexico State University, studies biocrust. Unlike other bacteria, she said, cyanobacteria perform photosynthesis – converting sunlight into food. The byproduct is the basis for biocrust. 

“What is really fascinating about cyanobacteria,” Pietrasiak said, “is they can excrete, as a community, these compounds that are glue-like – sugary compounds that are sticky, where the soil particles, the physical component, can actually be bound together by them, weaving and meshing through the soil environment – that’s how a crust gets built.”

“Ecosystem engineers” are organisms that create or modify a habitat, such that it sustains other creatures. Think prairie dogs – whose burrows provide shelter and safety for other animals, bringing the prairie to life. Cyanobacteria play a similar role – at a microscopic level. 

The cyanobacteria create colonies on bare desert ground. Remarkably, there’s a division of labor among these single-celled creatures – some create food through photosynthesis, while others store nitrogen, another essential resource.

Once the cyanobacteria are in place, other organisms arrive – to feed on their byproducts and dead cells, or to prey on them. Those organisms include “micro-animals” – creatures like rotifers, roundworms and tardigrades.

“It’s kind of like the food-web interaction,” Pietrasiak said, “where you need to have primary producers that start the whole system, and then consumers that rely on their presence. It’s just like the big system – just shrink it down.”

“Succession” is the process of species change in an ecosystem over time. It’s familiar from forests – where pioneering trees create a canopy, which in turn allows shade-tolerant trees to move in. The principle applies to biocrusts as well. Cyanobacteria are the “pioneers,” stabilizing the soil. But over time, algae, mosses and lichens arrive.

Lichen itself is a symbiotic relationship – between a fungi and a cyanobacteria or algae. The fungi relies on its partner for nutrients – but in turn provides the physical structure in which the “primary producer” lives. And the fungi offers another service: the darkened appearance of biocrust comes from melanin or “sunscreen” compounds produced by fungi, which protect algae and cyanobacteria from ultraviolet radiation in the brutal desert sun.

Biocrust organisms spend most of their time in a dormant state, “sleeping.” But come that blessed event – a desert rain – and they waste no time. 

“They start metabolism within seconds of rehydration,” Pietrasiak said. “They start cranking photosynthesis within 10 minutes or less. They green up and start doing their job. In that way, they’re just incredible – it’s way faster than plants could do this, and they have to, because they only have limited time. They have to do that metabolism and reproduce themselves before it becomes dry again.”

Biocrusts play critical roles in arid places. They stabilize soils, sequester carbon and create fertile soil – which benefits plants.

They’ve been extensively disturbed by human activity. But they can still be found across our region. The gypsum soils in and around Guadalupe Mountains National Park abound in biocrusts.

Scientifically, biocrusts are a terra incognita. In her research, Pietrasiak has identified a dozen new species of cyanobacteria. 

“We have to do a better job of documenting where they are and who they are,” she said, “and then we can ask those cool questions – what drives their biogeography? Can we see patterns in their evolutionary history?”

Biocrusts can take decades, even centuries, to recover from disturbance – and when they’re dry, they’re easily damaged. When you’re hiking the desert, avoiding stepping on the “dirty dirt” and “busting the crust.” In complex communities, the Earth’s most primal life forms are flourishing at your feet.