Even though obviously not readily observable, microorganisms inhabit all desert areas and in the extreme arid zones are often the only life forms present. Relatively little is known about the diversity within the lower three kingdoms (Fungi, Protista, Monera) in general and even less is known about the species richness of these groups in deserts. A recent survey that uses 'DNA fingerprinting', aiming at resolving bacterial ribo-somal DNA, indicated that soils of semiarid sites can harbor higher bacterial richness then mesic sites. Since factors other than water availability are more important (chiefly soil pH) in determining microbial diversity, it can be assumed that true desert can be quite rich as well.
Mycorrhizal fungi seem to be quite important in desert ecosystems, as in more mesic ecosystems. It appears that mycorrhizae of desert plants not only supply the plants with nutrients but also supply moisture during the dry season, at times taking the place of root hairs. Studies conducted in the Chihuahuan Desert indicated that most dominant, perennial species have high arbuscular (AM) fungal infection rates in their coarse roots system, while fine-rooted annual species in comparison show much lower infection rates and are also much less dependent on mycorrhizal associations in general. Worth mentioning are mycorrhizal desert truffles (Terfezia and Tirmania: Ascomycetes), that are host specific to Helianthemum species in the arid region of the Middle East and the Mediterranean zones of the Old World. The desert of the American West supports an elusive community of aboveground observable fungi in which the Gasteromycetes (puffballs and allies) figure predominately. Another common example is Podaxis psi-tillaris (desert shaggy mane), a species most common in sandy deserts.
Except for their crucial part in mycorrhizal associations, desert microorganisms are noteworthy for their role in three typical desert phenomena: desert crusts, desert varnish, and interstitial communities. Desert crusts are microbiotic communities composed of drought and heat-tolerant algae, cyanobacteria, fungi, lichen, and mosses. These often species-rich communities are held together by sticky polysaccharide secretions and thus form surface crusts. Desiccated crusts are often indiscernible until rainfall or dew moistens the surface and microbial communities become active and green. Under extreme conditions, such crusts can form below the surface. This is possible especially under the protection of semitransparent calcareous or siliceous stones (quartz is a good example) that enables transmission of light up to a depth of 5 cm. The most common life form in crusts (and in some areas also in hot deserts in general) is cyanobac-teria. Among their roles in the desert ecosystem are atmospheric fixation of nitrogen and the binding of soil particles. Together with mineral-reducing bacteria, the cyanobacteria are important in soil fertilization and soil formation and thereby have clearly important effects on vascular plants and dependent animal consumers. In hot deserts, cyanobacterial crusts often form smooth surfaces, while in cold deserts, where crust forming interacts with frost heaving, a very rough surface is typical. These different surface types clearly affect vascular plants differently.
Even exposed desert rocks can support life. Clearly the most visible organisms are crustose lichens. However, when conditions become too extreme for growth of lichens, bacteria can still survive on the surface of rocks. Desert varnish, the dark and shiny surface found on sun-exposed, porous stones in hot deserts, is the result of bacterial activity. These bacterial colonies obtain energy from inorganic and organic substances and trap submicro-scopic, wind-borne clay particles. These particles accumulate in a thin layer and act as sun protection. Over very long time periods, estimated at thousands of years, these bacterial communities oxidize wind-blown manganese and iron particles and when baked together with clay particles form the dark desert varnish. The color of desert varnish varies depending on the relative proportion of oxidized manganese (dark black) to iron (reddish).
Environmental conditions even more extreme than those that support surface bacterial growth can still allow the formation of interstitial communities. These communities consist mostly of algal species that inhabit the matrix of sedimentary rocks in depth up to 4 mm. These communities can stay dormant for long periods of time and inhabit hot and cold deserts alike (they are known to exist on exposed rocks in Antarctica).
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