Desert Varnish Goes Biological

What was thought to be a geological phenomenon
turns out to be the work of photosynthetic bacteria

If you have driven any time in the southwest USA, you have undoubtedly noticed the dark, glossy sheen on sandstone cliffs. It’s called “desert varnish.” But did you know it is biological in origin?

Desert varnish (dark stain on rock) is commonly observed on sandstone cliffs.

Desert varnish, or rock varnish, is found around the world in desert environments. A press release from Stanford’s SLAC National Accelerator Laboratory points out that Charles Darwin observed “glittering rocks” on his voyage on the Beagle, as did explorer Alexander von Humboldt before him. What is it? How does it form, and why?

Using advanced accelerators, 14 researchers including those at SLAC, Los Alamos and Caltech have finally solved the “long-standing mystery” of desert varnish. The dark coating on the rock surface, just several 100 micrometers thick or less, is laid down by photosynthetic bacteria: “rock varnish is left behind by microbial communities that use manganese to defend against the punishing desert sun.” Their paper in the Proceedings of the National Academy of Sciences calls it an “ecophysiological explanation for manganese enrichment in rock varnish.” (Lingappa et al., PNAS, 22 June 2021).

Desert varnish is enriched in the element manganese (Mn, atomic number 25) by as much as 100 to 1,000 times over levels in the surrounding soil. Earlier researchers suspected that biology had something to do with it, but didn’t know how, or which species were involved. This team identified the species and explained how and why the microbes concentrate it. Manganese reduces stress from exposure to harsh sunlight, reactive oxygen species, heat and wind. The species Chroococcidiopsis, a cyanobacterium that performs photosynthesis, makes for itself a manganese ‘sunscreen’ as it lives on sunlit desert cliffs. Manganese oxides (which can be colorful like iron oxides) turn dark as they are further oxidized when old cells die and new ones take over. In this way, the varnish builds up in layers like stromatolites – another geological phenomenon mediated by bacteria.

The authors figure that wind-borne dust brings up the manganese the bacteria need. The microbes fasten the manganese ions not to proteins, but to other organic and inorganic compounds, using molecular machines directed by their genetic code. With their highly-complex systems for photosynthesis (which scientists still are trying to understand), the cells intake carbon dioxide from the air and make food for themselves, which they share with other species of microbes that join them on the wall. Commenting on the paper in PNAS , Culotta and Wildeman summarize the discovery:

A model can now be built to explain the formation of manganese-enriched desert varnish (Fig. 1). The Cynanobacteria Chroococcidiopsis chose to populate sunlit areas of desert rock and evolved to survive the harsh attacks of desert heat, desiccation, and radiation in part through the microbe’s ability to concentrate intracellular manganese and use this manganese in antioxidant redox chemistry. As the organism dies it leaves behind its footprint of manganese, originally as Mn²⁺, which is then converted to the Mn³⁺ or Mn⁴⁺ oxides through either neighboring microbes or through abiotic oxidation. Additionally, the presence of Chroococcidiopsis and other photosynthetic microbes at sites of varnish help feed other local microbes by fixing carbon from carbon dioxide through the Calvin cycle, thereby promoting microbial growth in the otherwise nutrient-sparse environment of the desert. Darwin himself would certainly be pleased to learn of this invisible intricate ecosystem that over thousands of years created his beautiful glittering rocks.

The papers indicate two purposeful functions that this ecosystem accomplishes for the good of the earth. One is the ability to fix nitrogen. Breaking apart the diatomic molecule N₂ with its triple bond is a feat only accomplished at ambient temperatures by microbes. Agricultural engineers would love to learn the secrets of nitrogenase enzymes in bacteria that know how to do it (10 Sept 2013). The other function is one that will please climate scientists: desert varnish bacteria have the ability to sequester carbon by converting carbon dioxide into useful compounds for life.

The writers in PNAS suggest that rock varnish might exist on Mars, based on a few lander/rover observations. Their bias toward hydrobioscopy leads them to speculate about rock varnish as a possible proxy for microbes there. As with stromatolites, though, there could be geophysical explanations for the Martian counterparts. Since Mars has no protective magnetic field and very little atmosphere, microbial life above the surface would be subjected to deadly radiation. That’s why astrobiologists expect life, if any, to exist only under the surface. No biological interpretations for dark stains on Martian rocks should be accepted, therefore, without extraordinary evidence.

How long does it take for desert varnish to form? The authors and commentators think that it only takes centuries or millennia. That’s because new desert varnish has been seen formed in petroglyphs chipped into the rock varnish by human graffiti artists, like the Native American cliff dwellers who inhabited the American southwest around 800 or more years ago. A paper we reported on 3 Feb 2013 says that varnish growth can be rapid. The new growth rate estimate is 16 times faster than the previous estimate (see also CMI article by Michael Oard, April 2014).

Newer rock drawings chipped into desert varnish are clearly distinguishable from those hundreds of years old. Photos by DFC.

The microbes did not “evolve” their marvelous mechanism “to survive,” and Darwin had nothing to do with it; those throw-away comments about Darwin and his Stuff Happens Law add nothing to the story. Instead, people should stand in awe of a Creator who endowed these tiny cells with abilities our best scientists are struggling to imitate. When admiring desert landscapes, give a word of thanks for the ecosystem up on the walls that transforms geology into biology, fixes nitrogen and turns greenhouse gas into food – some of the many good things that bacteria do for the earth.

Isn’t it interesting that desert varnish does not take a long time to form. This should be a problem for old-earthers. They think some of these rock strata are tens or hundreds of millions of years old. If 100 micrometers formed in approximately 6,000 years since the Flood, how much should be there after one million years? Over half an inch—something clearly not observed. In ten million years, there should be 6.5 inches of the stuff. In 100 million years, 65 inches! Expect some scrambling for theory-rescue devices from the old-earthers. Meantime, Biblical creationists should investigate this further as another reasonable, empirical witness for a young earth.