The surfaces of deserts are not as bare as they may appear — they get their crusty nature from the dense growth of many organisms in the upper millimetre of dry soil. These organisms not only influence the behaviour of water in the soil, but they also contribute substantially to nitrogen and carbon fixation. Research by O. L. Lange and his colleagues over the past few years, including their latest report in Functional Ecology1, has shown that deserts differ in their crustal communities. And we now know that these highly active layers of desert soils contain productive species that are finely tuned in their physiology to the nature and the timing of their water supply, whether in the form of dew or rain showers.
Desert crusts consist of a tangled mesh of various types of organism that bind, in a mucilaginous, fragile concretion, the fine-textured inorganic particles that form their matrix. Their living components include algae, mosses (including protonemal forms), lichens, fungi, bacteria and cyanobacteria. These organisms occupy just the upper crust, because many of them need light for their photosynthetic activities and also because water is briefly available in these superficial layers, derived from dew, fog and showers. The ecological value of the layer in protecting the soils from wind erosion and as an absorptive organ for water is generally appreciated, as is its role in providing germination sites for the seeds of flowering plants2.
The presence of free-living, nitrogenfixing cyanobacteria in the upper layer, together with lichens that contain cyanobacterial symbionts, suggests a pathway by which atmospheric nitrogen enters the desert ecosystem. Moreover, the extensive cover of even a thin layer of photosynthetic organisms is an important source of carbon fixation in an environment where primary production is low. In their studies of the Namib Desert of southern Africa (Fig. 1), Lange and colleagues have shown3 that the chlorophyll density of the microbial mat is around 200-500 mg m−2 — of the same order as in the leaf of a higher plant. So the crust has considerable photosynthetic potential, although this is limited by the need for hydration before it can be exploited.
In the Negev Desert of Israel, Lange and his team showed4 that they could induce maximal photosynthesis from the crust by moistening it with the equivalent of just 0.2-0.3 mm of precipitation. These photosynthetic rates were around 20% of those achieved by local desert shrubs, such as Zygophyllum dumosum, illustrating the ability of the crust to act as a carbon sink. In the Negev, as in the Namib, moisture mainly comes in the form of condensation — as dew and fog during the cold night. So, although these moisture levels are often achieved, the rising sun soon desiccates the productive crust into a dormant state, and crustal photosynthesis is limited to an early-morning window when moisture and light briefly coincide.
Now that general taxonomic surveys of crust communities and their physiological properties have been carried out, questions of geographical and ecological variations are being tackled. For example, not all deserts depend on condensation. The intermontane deserts of the western United States are watered by occasional showers or storms rather than by fog and dew — do their crusts have a different species composition and physiology? To answer these questions, Lange et al. have done their latest work1 in southern Utah.
One evident taxonomic feature of the Utah desert is that, although there are many lichens with green algae as their symbionts (as in the Negev and Namib deserts), the most abundant crustal constituent is a gelatinous lichen, Collema tenax, with a cyanobacterial symbiont. This lichen requires a greater water supply (equivalent to over 1 mm of precipitation) than do the green-algal lichens to attain its maximum photosynthetic rate. But, it then shows the highest photosynthetic capacity of any arid-zone soil lichen yet studied. Moreover, C. tenax can maintain photosynthetic activity over a wide range of temperatures, from near freezing to over 40 °C, with an optimum around 30 °C. It also requires a high light intensity for maximal photosynthesis, and it has a high light-compensation point (the minimum light intensity at which it can break even in terms of carbon balance). All of these are features that plant physiologists would associate with a ‘sun plant’.
Collema tenax has a distinctive physiology that equips it well for the water regime in Utah, where erratic showers bring larger quantities of rain at any given time than are experienced in the ‘fog’ deserts. The gelatinous nature of its thallus means that C. tenax serves as a reservoir, maintaining its hydrated state long enough to provide for extended photosynthesis even in the hot conditions, when the light is high, in the middle of the desert day. Lichens with cyanobacterial symbionts are already known to need more water to set photosynthesis in motion than lichens with algal symbionts. So, perhaps the features described for C. tenax will be of wide taxonomic, physiological and biogeographical significance.
Recognizing that such niche differentiation exists among the primary producers in desert crusts is an important step towards appreciating the complexity of crustal micro-ecology. Because these soil features are so important in the water relations and nutrient cycling of desert soils, and because we know that they are sensitive to disturbance by the trampling pressures of humans, domestic animals and motor vehicles, further understanding of crustal ecology should soon provide a basis for conservation of desert crusts.
Lange, O. L., Belnap, J. & Reichenberger, H. Funct. Ecol. 12, 195–202 (1998).
West, N. E. Adv. Ecol. Res. 20, 179–223 (1990).
Lange, O. L., Meyer, A., Zellner, H. & Heber, U. Funct. Ecol. 8, 253–264 (1994).
Lange, O. L. et al. Funct. Ecol. 6, 519–527 (1992).
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