Valeria C. Culotta and Asia S. Wildeman; PNAS July 13, 2021; 118 (28) e2109436118

Figure: The microbial environment within desert varnish. Desert varnish formed on sunlit areas of desert rock is highlighted with a small magnifying glass. Desert varnish is composed of stromatolites, or vertical stacks of remnants of the Cyanobacteria Chroococcidiopsis shown in green. Wind carries Mn-containing dust to live Chroococcidiopsis cells (dark green) that hyperaccumulate manganous Mn²⁺. When cells die (light green) the hyperaccumulated Mn²⁺ is deposited in the environment and is converted to Mn³⁺ or Mn⁴⁺ oxides (MnO₂) by abiotic methods or neighboring microbes (purple). Chroococcidiopsis also fixes atmospheric CO₂ into organic carbon (C _org) that serves as an energy source to feed other bacteria (orange). The varnish generated by the life and death of Cyanobacteria Chroococcidiopsis represents a nutrient- and Mn-rich environment for microbial life within the harsh desert.

During his expeditions on the HMS Beagle from 1832 to 1836, the young Charles Darwin described specific rock formations off the coast of South America as “black glittering rock” that “glitters most brilliantly when turned before light” (1). The shiny rock is now commonly known as desert varnish or rock varnish and is found across the globe in arid climates, largely in desert landscapes. The varnish is a metal coating several hundred microns deep, composed largely of black oxides of manganese or orange-colored iron oxides (2). Many ancient petroglyphs of Native Americans were created by etching into the black or orange coat of varnish, exposing the lighter rock underneath. The blackened manganese-rich varnish contains Mn³⁺ or Mn⁴⁺ oxides at concentrations two to three orders of magnitude higher than manganese levels in neighboring soils or rock (2, 3). These varnishes develop very slowly over time, requiring thousands of years in the making (4). In a paper by Lingappa et al. (5) the secrets of rock varnish genesis are unveiled by the discovery of microbes that inhabit desert rock and deposit manganese footprints as their legacy.

See more: https://www.pnas.org/content/118/28/e2109436118