Colonies of microbes create heat islands within deserts, areas hotter than their surroundings. These ecosystems help fight erosion and fertilize the limited soil, but they also play a neglected part in warming the planet.
As the image above reveals, the microbial communities create a dark crust on the soil, increasing the absorption of light. According to Arizona State University's Dr. Estelle Couradeau, the dark colors are a by-product of the microorganisms' need to protect themselves from ultraviolet light.
"We have found that the presence of sunscreen-bearing crusts can actually raise local surface temperature by as much as 10°C (18°F). Because globally they cover some 20 percent of Earth's continents, biocrusts, their microbes and sunscreens must be important players in global heat budgets," Couradeau said in a statement. "We estimate that there must be some 15 million tonnes [16.5 million tons] of this one microbial sunscreen compound, called scytonemin, warming desert soils worldwide."
"Biocrusts, while cryptic, deserve more consideration from us," Couradeau said. "We need to include them in our climate models and speak about them in the classroom." She has begun this process with a paper in Nature Communications, in which she investigated what makes biocrusts sampled at a high altitude location in Utah so dark. Although other compounds influence darkening, Couradeau found that at her study site scytonemin is six times as effective at absorbing light as chlorophyll a, the next most important contributor.
A close up of a biocrust microbial community. Estelle Couradeau/ Arizona State University
By heating the local environment so much these biocrusts affect what is able to live there, tilting the balance towards species that love the warmth. Couradeau previously discovered that rising temperatures put some of these microbes at risk along with macroscopic lifeforms. Microcoleus vaginatus, one of the two types of cyanobacteria that form the base of biocrust foodchains, is far more sensitive to heat than its competitor and becomes rare as temperatures rise. Unsurprisingly Couradeau found M. vaginatus declines as scytonemin levels rise.
It is a sort of small scale version of Daisyworld, James Lovelock's example of an imaginary world where black flowers warm the planet and white flowers cool it. However, where Lovelock's daisies self-regulate, with black ones doing better in cold climates and white preferring the heat, these biocrusts produce run-away local warming until dark colors saturate.
"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," explained senior author Ferran Garcia-Pichel. "On the other hand, it may put local organisms at increased risk when it is already quite hot."
The paper notes that biocrusts, "Represent the major type of ecosystem on the continents before the advent of land plants in the Devonian." They may have played a major role in shaping the climate and atmosphere of the Earth at the time, providing even more motivation to understand their influence.
