The unseen engineers
The famous sailing stones of Racetrack Playa slide across a surface that is not lifeless. The dry lakebed is coated in a dark, bumpy layer of biological soil crust, a complex community of microorganisms dominated by cyanobacteria. An important organism in this ecosystem is Microcoleus vaginatus, a filamentous bacterium that forms bundles sheathed in a sticky polysaccharide slime. This secretion, known as an extracellular polymeric substance (EPS), binds the fine clay particles of the playa together. The resulting cohesive mat stabilizes the ground, preventing the fierce desert winds from carrying the soil away as dust.
This microbial crust practices a survival strategy called anhydrobiosis, or life without water. The organisms can enter a dormant, desiccated state for years or even decades, enduring the extreme temperature swings and intense ultraviolet radiation of Death Valley. In this state, their metabolic activity becomes undetectable. They are, for all practical purposes, in suspended animation, waiting for the rare event that brings them back to life: rain. The playa receives only about 3 to 4 inches (76 to 102 millimeters) of precipitation annually, making these revivals infrequent.
A slick solution to a geological mystery
When rain finally falls, the microbial crust undergoes a rapid transformation. The dormant cyanobacteria absorb water and can resume photosynthesis within an hour. This reanimation causes the crust to swell and turn a darker, greenish-black color as pigments become active. The once-dry, dusty surface becomes slick with the reactivated EPS slime produced by the microbial community.
This slippery biofilm was long hypothesized to contribute to the mystery of the moving rocks. While direct observation in 2014 confirmed that thin sheets of floating ice driven by light winds are the primary force, the underlying microbial mat provides the necessary low-friction surface. A rare combination of events is needed: the playa must flood with just enough water to form ice that is thick enough to be strong but thin enough to move freely. As the sun melts the ice into large panels, gentle winds of about 4-5 meters per second can push these panels, which then shove the rocks across the slick, microbially-lubricated mud. The rocks, some weighing hundreds of pounds, slide across the surface, leaving the famous tracks that can persist for years.