Perhaps the hardiest assemblage of lichens, fungi and algae yet found has been hiding in plain sight in northern Chile’s Atacama Desert called grit crust.
This newly discovered “grit-crust,” as we have named it, coats tiny stones and draws moisture from daily pulses of coastal fog that roll across the world’s driest nonpolar desert. These communities are optimized to photosynthesize using less than half of the water that other known desert biological soil crusts use, as we reported in the January Geobiology.
Biological soil crusts, or biocrusts, are conglomerations of algae, cyanobacteria, lichens, fungi or mosses that cover an estimated 12 percent of the land on Earth. They are commonly found in deserts, where they blanket the soil and prevent erosion. They also shape ecosystems by drawing atmospheric carbon and nitrogen into the ground and producing oxygen via photosynthesis.
Only a few millimeters of rain dampen the Atacama on average each year. But some areas experience daily cycles of fog and dew. In one such “fog oasis,” about 2.5 kilometers from the Pacific Coast in Pan de Azúcar National Park north of Santiago, we spotted odd markings.
We got there with our cars and saw these blackish and whitish patterns in the landscape.
Previous surveys have identified other biocrusts in the Atacama. But the new crust samples weren’t like those — analyses revealed lichens, fungi, algae and cyanobacteria enveloping tiny, 6-millimeter pebbles and keeping the pebbles stuck together atop the soil, like a rock-based peanut brittle. Unlike other biocrusts, which form on soil surfaces, grit-crust is “something different that we’ve not seen before.
In lab experiments, we measured the rate at which the crust collectives consumed carbon dioxide with varying amounts of moisture. Photosynthetic activity peaked when a sample had just 0.25 millimeters of water — equivalent to 250 milliliters of water for one square meter of grit-crust — which is within the range expected for deposits from daily fog banks near the coast. By comparison, biocrusts in the Sonoran Desert in Mexico and the U.S. Southwest are most photosynthetically active when saturated with between 0.5 and 1 millimeter of water.
Detailed microscopy of the rocks showed fungi associated with the grit-crust tunneling in from the surface. These fungi’s tubular growth structures, or hyphae, swell and shrink with the flow of fog, creating cracks that eventually break up the stones. This “biological weathering” is the only known process to create new soil in the Atacama Desert. Such grit-crusts may have transformed the harsh surface of ancient Earth before photosynthesizing plants arose by breaking down stones and contributing to nutrient cycling.
I'm now focusing on a new funding to continue my research on this novel, unique and fascinating biocenosis.