The desert has its own palette, distinctive and at the same time subtle yet dramatic. There are many factors at work creating the patterns and hues of arid lands - obviously the kind of sand, the kind of rock, the vegetation, minerals and salts, desert varnish - but there are also artists at work that we can't see and barely understand: microbial communities.
We can see the mosses and the lichens, but the vast ecosystem of bacteria and fungi operates essentially invisibly; we are only beginning to scratch the surface of the desert to reveal the ubiquity and importance of microbial life in environments we often describe as "lifeless." These are communities labelled "cryptic" by biologists, a useful term disguising the fact that they are something we really don't understand very well. I found this definition helpful, from a piece in Nature a few years ago titled "Body doubles" by Alberto G. Sáez and Encarnación Lozano:
Have you ever approached someone whom you thought you knew, talked to him with familiarity, only to find out later that he was a complete stranger, albeit remarkably similar in appearance to the person you had in mind, such as a twin brother? Well, taxonomists are similarly puzzled when they come across two or more groups of organisms that are morphologically indistinguishable from each other, yet found to belong to different evolutionary lineages. That is, when they discover a set of cryptic species.
The microscopic cryptic communities of arid lands form the well-known "desert" or "cryptobiotic" crusts that we now realise play key roles in the ecosystem, cycling carbon dioxide and nitrogen, providing resources for plant life, controlling drainage and the hydrologic behaviour of the soil, and reducing erosion - and hence, atmospheric dust.
Among the leading researchers shedding light on these cryptic communities is Ferran Garcia-Pichel at Arizona State University's School of Life Sciences. For example, in 2013, togather with international colleagues, he published a paper titled "Temperature Drives the Continental-Scale Distribution of Key Microbes in Topsoil Communities." Science summarises the work as follows:
Soil microorganisms make up a substantial fraction of global biomass, turning over carbon and other key nutrients on a massive scale. Although the soil protects them somewhat from daily temperature fluxes, the distribution of these communities will likely respond to gradual climate change. ... [We] surveyed bacterial diversity across a range of North American desert soils, or biocrusts—ecosystems in which photosynthetic bacteria determine soil fertility and control physical soil properties such as erodability and water retention. Most of the sites were dominated by one of two cyanobacterial species, but their relative proportions were controlled largely by factors related to temperature. Laboratory enrichment cultures of the two species at different temperatures also showed temperature as a primary determining factor of bacterial diversity. It is unknown if temperature will affect the distribution of other soil microorganisms, but the marked shifts of these two keystone bacterial species suggest further change is in store for these delicate ecosystems.
The work, only available behind the Science paywall, was helpfully reported by Live Science. The two dominant "keystone" bacterial species are Microcoleus vaginatus and M. steenstrupii, the former preferring cooler conditions whereas the latter likes things hot. As temperatures vary, things become competitive and warming conditions result in the mysterious steenstrupii taking over. Now, because these communities are microscopic and cryptic, we can only measure such effects - and detect which organisms are in the soil - through sophisticated DNA analysis. It is further results of this kind of painstaking and careful work that Garcia-Pichel and his colleagues have just published in Nature. With Estelle Couradeau, also at Arizona State, as the lead author, the paper describes - startlingly - how "Bacteria increase arid-land soil surface temperature through the production of sunscreens." Microcoleus vaginatus and M. steenstrupii are far from alone, and, amongst their companions are tribes of cyanobacteria such as the hundreds of species belonging to the genera Scytonema and Tolypothrix. These little critters dislike the sun and apply a biosynthetic sunscreen, scytonemin, an alkaloid pigment that strongly absorbs solar radiation and dissipates this energy as heat. This sunscreen can be seen as patches of darker colour covering areas of desert crust, as in this photo by Garcia-Pichel from the recent report on Science Daily.
This pigmentation may protect some members of the bacterial community, but it can locally warm up the surface by as much as 10 degrees C (18 degrees F). This has a dramatic effect on the health of the cool-loving Microcoleus vaginatus, but is welcomed by M. steenstrupii, who come to dominate as the sunscreen develops, at the expense of vaginatus. As Garcia-Pichel comments:
... we can show that the darkening of the crust brings about important modifications in the soil microbiome, the community of microorganisms in the soil, allowing warm-loving types to do better. This warming effect is likely to speed up soil chemical and biological reactions, and can make a big difference between being frozen or not when it gets cold... On the other hand, it may put local organisms at increased risk when it is already quite hot.
And this has to be happening on a global scale. As Estelle Coradeau suggests, "Because globally they cover some 20 percent of Earth's continents, biocrusts, their microbes and sunscreens must be important players in global heat budgets. We estimate that there must be some 15 million metric tons of this one microbial sunscreen compound...warming desert soils worldwide."
But because we have only a poor understanding of what exactly these desert crusts are and how they work, their roles in local ecology and global systems are impossible to define. It is only through the meticulous work of Ferran Garcia-Pichel and his team, together with others such as Jayne Belnap of the USGS in Moab, Utah, that we can begin to unravel the extraordinary nature and contributions of these long-ignored microbial desert communities. As Belnap has commented:
These are the only game in town to prevent dust storms and erosion, so they're really, really critical parts of this ecosystem. Yet we've never asked the question, 'who's really in there, and what's going to happen there as things shift?'
and, as reported in a piece on Belnap in High Country News, the palette and patterns of our arid lands owe much to an invisible living world:
She also remains convinced that the dark shadows on the desert are the true — and fragile — foundation of the Colorado Plateau. "Whenever we pull on the thread of what makes the system tick," she says, "we end up with soil crusts on the other end."