Windows on the desert floor

Natural History, May, 1998 by Peter J. Marchand

The literature is rich with taxonomic lists of microbiota found under these translucent stones, much of it going back to the efforts of Roy Cameron, of NASA's Jet Propulsion Laboratory, who conducted exhaustive studies of desert soils in the 1960s. Cyanobacteria are usually the most prevalent life form in the hot deserts. Some, like the filamentous Nostoc muscorum, convert atmospheric nitrogen in the soil into a nutrient form more readily available to other plants, thus increasing soil fertility. Another filamentous species, Microcoleus vaginatus, grows like a bundle of optic fibers within an outer sheath of mucilage that binds soil particles together even long after the cyanobacteria die, thereby stemming erosion. Spherical cells of green algae are present in some soils, along with an assemblage of bacteria, some of the latter deriving their energy from the metabolism of sulfur compounds in the soil. Hyphal strands of fungi probe through the community, sometimes parasitizing the algae and cyanobacteria and sometimes uniting with them to form lichens. Tiny mosses, too, often find refuge beneath these translucent stones, their rootlike rhizoids branching profusely, weaving the community together.

Like the moist duff of a forest floor, the organic matter accumulated by the living and dying of these organisms, sometimes building a layer one-half inch thick, holds more moisture than the surrounding soil, reinforcing conditions that favor growth in this desert refugium. And within this Lilliputian garden roam protozoa and nematodes, as well as mites and other minute arthropods--grazers and predators alike feeding on bacteria, algae, fungi, plants, detritus, or each other. It is a microcosm of the desert at large in the shadow of a translucent stone. Behind these windows on the desert floor, the life cycles of a community continue to turn long after the last raindrop falls, while other desert organisms mark time, waiting for the next season of scant rain.

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In a land scorched by sun and wind, exposed rock would seem an unlikely refugium for life of any kind. Yet even in the most hostile of climates, a single rock can provide a variety of living situations for microbiota adapted to fife at the extremes. In addition to the environment beneath translucent stones, both the surface and subsurface of porous rocks often support a surprising diversity of microorganisms.

Most conspicuous of the surface rock-dwellers in hot deserts are the crustose lichens, which have an extraordinary capacity to endure months of almost complete dehydration and then resume growth quickly when wet. They are also able to continue photosynthesizing beyond the temperature thresholds at which most plants shut down. Resilient as they are, however, crustose lichens have their limits, and where rock surfaces become too inhospitable for the lichens, they are often replaced by bacteria.

Commonly referred to as "desert varnish," the dark, often shiny surfaces of sun-baked rock (favored sites of some prehistoric artists) found in numerous desert environments are actually large colonies of bacteria. Able to obtain energy from inorganic as well as organic substances, these hardy colonists adsorb submicroscopic bits of wind-transported clay to their cellular surfaces to build a thin layer of protection from direct sunlight. Minute quantities of manganese and iron, also collected from atmospheric dust and subsequently oxidized by the bacteria, combine with the clay minerals to form a tough coating of dark manganese oxide or reddish iron oxide.