Windows on the desert floor

When scientists at the National Aeronautics and Space Administration (NASA) began thinking seriously about the possibility of life elsewhere in the universe, they looked to Earth's own desert soils for some idea of what might exist on less hospitable planets. It was not an illogical choice. Many desert surfaces--from frozen polar rockscapes to the sun-baked salt crusts of Death Valley--come dose to matching the extreme conditions imagined to exist on other orbiting bodies. So desiccating are these desert soils that, at times, plants let their, roots die back to avoid losing water to the very ground that gave them life. Compounding this stress are drastic temperature swings. In warmer regions, desert soils easily reach 150 [degrees] - 160 [degrees] F (in Death Valley, soil temperatures of 190 [degrees] have been recorded). Then, at night, when heat is radiated to the cold vault of the heavens, temperatures often plummet one hundred or more degrees, sometimes dropping close to the freezing mark. For water-dependent, temperature-constrained, soil-bound organisms, the desert floor may well represent the extreme limit, the hard edge of terrestrial life.

What might-seem at first a ground almost devoid of life, however, turns out on dose inspection to harbor a surprising diversity of some of the toughest organisms on earth--a microbiota of remarkably drought-resistant and heat-tolerant algae, cyanobacteria (formerly called blue-green algae), fungi, lichens, and mosses. In the best of situations, these organisms thrive on the surface, forming a distinct crust held together by sticky polysaccharide secretions and constituting a true pioneering community--the only living ground cover in some deserts of the world. These crusts are sometimes a conspicuous feature of the soil surface, as in the sagebrush deserts of the American West, where they may produce pillars several inches high and resemble miniature canyon lands. In the hot Sonoran Desert, the brown and desiccated crust usually escapes notice altogether by the untrained eye. But when it rains, the photosynthetic members of the community respond quickly, greening within hours.

Under the harshest conditions, these same organisms persist out of sight, protected in a unique microhabitat--a special niche provided by calcareous or siliceous stones, most commonly quartz and chalcedony, scattered about the desert surface. As I sprawled flat on my belly one day in the searing heat of the Sonoran Desert to photograph this phenomenon, I carefully pried from the ground a piece of milky quartz that was partly embedded in the soil. A wisp of powder drifted upward as I disturbed the dusty, dry silt trapped by the veneer of pebbles on the ground surface. Beneath the quartz was a miniature garden--an oasis amid the parched and barren rock rubble. Only a tiny sprig of moss poking through the sand grains was recognizable to my naked eye; other inhabitants of this garden were microscopic, their abundance and photosynthetic activity evident as a striking green tint against the dull, inanimate background.

In an environment where unusual life forms are common, and extraordinary adaptations appear everywhere, the ability of this community to persist underground was perhaps no more remarkable than the ability of, say, aquatic beetles to survive in the desert by living within the watery pulp of downed and rotting saguaro cacti. Still, as I lay there in the formidable heat examining the quartz, the emerald-colored band staining the white mineral contradicted my anthropocentric sense that this ground was too inhospitable for such life. How could this diminutive community continue to function under such conditions?

All stones on the desert surface preserve moisture in the soil beneath them, at least for a time. This is ancient knowledge: prehistoric Hohokam farmers of the Sonoran Desert used rocks as mulch around their agave plantings. In addition, translucent calcareous and siliceous minerals can transmit visible light, in some cases to depths approaching two inches, thus supporting photosynthesis below ground and under complete cover, much like a greenhouse. Measurements made by plant physiologist Frank Salisbury, of Utah State University, showed that about 1.5 percent of the sunlight striking the surface of a milky quartz stone penetrated a thickness of one inch; this means that as much energy reaches an algal colony beneath this stone on a sunny autumn day as is available to a potted plant in a typical, well-lighted office.

These minerals offer another advantage. When I slipped the thin wire probe of my thermocouple thermometer just below the surface of an algae-encrusted soil lacking a cover of stones, my digital meter read 146.1 [degrees] F. (The air temperature in the shade of my body was 106.5 [degrees] F.) Under a dark volcanic rock lying on the surface, my instrument registered a temperature of 148.5 [degrees] F. Beneath an adjacent quartz stone of similar size, however, the soil temperature was only 134.1 [degrees] F, indicating that the light-colored quartz reflected a significant amount of the sun's energy, reducing the temperature of the soil below it. By midafternoon, the soil surface hit 153 [degrees] F; temperatures beneath the quartz remained on average fourteen degrees cooler.