Dark Secrets of the Cryptobiotic Crust
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A tiny world of colorful lichens, courtesy of the Canyonlands National Park Service
Think of a desert and you probably picture dry, dead dirt. If so, surprise! Much of the world’s most arid ground is alive with a tiny ecosystem, no more than a few inches thick. It’s so tiny, in fact—so thin and delicate, so apparently parched and lifeless—that this “secretly living” layer of soil is called a cryptobiotic crust.
Take the Colorado Plateau, 130,000 square miles of high desert that extends into the Four Corners region of northwest New Mexico. The region may be most famous for the bared red rock of the Grand Canyon across the Arizona border, but 70% of its surface is actually crunchy with its own cryptobiotic crust. That’s 91,000 square miles of delicately networked bacteria, fungi, and lichens teeming with activity.
Or at least, they teem in the hours following rainfall. When the ground is truly parched, most of these species enter a dormant state, alive but essentially hibernating. Like mammals elsewhere who beat the starvation of winter by slowing their biological systems to a bare minimum, these tiny organisms are perfectly adapted to wait out droughts. In response to a bit of moisture they’ll return to their main activity: photosynthesis.
The starring role is played by cyanobacteria, the artist formerly known as blue-green algae. Its trademark color comes from chlorophyll, the same pigment found in leafy greens across the globe. Unlike full-fledged plants, whose complex systems of roots, stalks, leaves, and reproductive systems require abundant organic material and water, each cyanobacterium is a single self-sufficient packet of chlorophyll capable of performing photosynthesis. All it takes is a little sunlight, water, and CO₂ for them to thrive, which explains their ability to colonize miles of land too barren to support vascular plant-life.
Cyanobacteria’s sticky polysaccharide sheath at 950x magnification
’Along the way they have a happy side-effect for the land. The dirt they inhabit is primarily granulated minerals, easily eroded by wind and the occasional flash flood. Cyanobacteria produce a protective coating of polysaccharides, i.e. sticky sugars, which they slough off like a snake shedding its skin as they slide through the dirt one micrometer at a time on damp days. Over the years this builds a network of brittle filaments that lace the soil and provide some structure, protecting it from erosion.
Old-growth soil crust draped in lichens, courtesy of the Canyonlands National Park Service
Between weaving that stabilizing web, trapping a bit of moisture, and infusing the ground’s upper layer with organic material, cyanobacteria and their tiny companions render desert ground ever-so-slightly hospitable for larger organisms like cacti, arthropods, and small mammals. They may go unrecognized most of the time, but they’re critical to maintaining the health of wide swaths of land across New Mexico.
If you’re hiking in the Colorado Plateau or another desert region in the state, keep an eye out for ground that looks particularly rough or dark. In addition to a bluish tinge from cyanobacteria, living soil may be pigmented with melanin produced by free-living fungi as a defense against UV radiation. That’s right: one of the dark secrets of the cryptobiotic crust? It can develop a genuine tan while lying out in the desert sun.
My view of crusty soil atop white gypsum rock on a recent hike of White Ridge in northwest New Mexico
This post was brought to you by Jayne Belnap, Soil Ecologist for the U.S. Geological Survey; by Jane Rodgers, Vegetation Specialist at Joshua Tree National Park; and by a weekend hike along the White Ridge Bike Trails.
