The Slowest Painters on Earth
The striking, dark sheen on the sandstone formations of Bardenas Reales is not a simple stain left by water or weather. It is desert varnish, a thin, layered veneer created by a patient community of microorganisms over thousands of years. This coating is a living biofilm, composed primarily of clay particles cemented to the rock by oxides of manganese and iron. The architects of this natural art are colonies of bacteria and fungi living on and within the rock surface.
The process relies on wind-blown dust, which delivers clay and trace elements to the rock. Specialized manganese-oxidizing microbes, including cyanobacteria from the Xenococcaceae family, are important. These microorganisms absorb manganese from the dust and, through their metabolic processes, oxidize it. This biological process concentrates manganese at levels 50 to 60 times greater than in the surrounding earth's crust. The resulting manganese oxides, along with iron oxides, act as a glue, binding the fine clay particles to the sandstone and creating the dark, shiny patina.