The Soil…It's Alive!

How do desert plants survive baking heat and constant drought? With the help of cryptobiotic friends!

Cryptobiotic? It sounds like something out of a graveyard horror movie. Actually, the word means “hidden life” (from the Greek words kruptos and biotikos). Invisible to the casual observer, dense mats of cryptobiotic organisms, including cyanobacteria, lichens, mosses, green algae, and microfungi, mix with sand and clay to form a life-sustaining soil in desert regions. Without it, a lot of desert plants wouldn't survive.

Imagine yourself as a desert plant exposed to constant heat that threatens to bake you dry and wind that blows sand out from under your roots. You would have a hard time making it. Desert heat and sand are not hospitable to vegetation that needs stable, moist soil for root growth. But weave together a combination of tiny organisms, sand, and clay, and you get a mat of living soil that resists erosion and holds on to water like a sponge—enough to make a difference to a lot of desert plants.

The tiny organisms at work in this cryptobiotic soil range from single-celled bacteria and algae to multicellular lichens, mosses, and fungi. Cyanobacteria, sometimes called blue-green algae, are one-celled microbes that often grow in long, thin tube shapes called filaments. As they grow, their stringy cells push through sand and clay, binding these particles in a crusty mat. Likewise, lichen, moss, and fungi put out tiny root-like projections that do the same thing. This crusty mix is ideally suited for dealing with the ups and downs of water availability in the desert. During wet spells, water is soaked up and held by both the organisms and spaces within the crust. During dry spells, evaporation is slowed because the crust tends to hold on to its bound water.

Take the pinyon pine and Utah juniper, two evergreen shrubs that grow abundantly in the desert region of the U.S. Southwest. It's not just the seedlings of these shrubs that germinate easily in the shelter of their cryptobiotic crust—their vulnerable young roots also get a strong start. This same protection allows the growth of an amazing array of unusual plants like the narrow-leafed yucca and Mormon tea plant. The cryptobiotic ground cover helps these plants by acting like a layer of mulch to prevent their roots from drying.

These same plants depend on their cryptobiotic friends to keep from starving. All plants obtain nitrogen-containing compounds from soil. Since deserts don't have much natural fertilizer, their plants have to rely on the soil organisms for help. Cyanobacteria and certain lichens are able to carry out a series of reactions that convert atmospheric nitrogen into ammonia. Called nitrogen fixation, this process allows these organisms to act like tiny fertilizer factories, spewing out ammonia and related compounds into the soil, where plant roots can take it in.

Plants aren't the only ones to benefit from this arrangement. Cryptobiotic organisms also profit from their plant pals. As pinyon pines, Utah junipers, and other plants grow, their root systems spread deeper and wider and act as stabilizers. By providing root structure to soil, shade from sun, and protection from wind, they promote cryptobiotic growth.

You can find this amazing living soil in deserts worldwide. In the United States, it is estimated that the bumpy crust covers up to 70 percent of deserts stretching from Mexico up through most of the Southwest and Northwest. If you travel in any of these areas, look for it—you can recognize it by its ridged, lumpy appearance.

It's a good thing to recognize, too, since this crust is quite fragile and is easily destroyed by hikers, off-road vehicles, cattle, and anything that crushes it. Once crushed, the cryptobiotic crust breaks up and blows away, leaving exposed the sand beneath. It can take more than 50 years to replace the damage done by a few minutes of careless hiking. By recognizing and protecting these hidden cryptobiotic creatures, you can make the “friendships” come full circle—you included.

Vocabulary

algae:
Various chiefly aquatic, eukaryotic (containing a membrane-bound nucleus) photosynthetic organisms, ranging in size from single-cell forms to large kelps. They are not considered land plants because they lack true roots, stems, leaves, and embryos.

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Activity

  1. What are cyanobacteria, and what do they do?
  2. Why is this crust particularly useful for nitrogen fixation in desert climates?
  3. What might happen if the crust over an entire desert were destroyed?