Desert fungi and lichens pose a deadly threat to 5,000-year-old rock art

The Negev Desert in southern Israel is known for its unique rock art. From at least the third millennium BC, hunters, shepherds and traders roaming the Negev left thousands of carvings (petroglyphs) on the rocks. These figurines are usually ground into desert varnish: a thin black coating on the limestone rock that forms naturally. Many represent animals such as ibex, goats, horses, donkeys and domestic camels, but there are also abstract forms.

Now a study published in Frontiers in Fungal Biology revealed that the petroglyphs are home to a community of unusual specialized fungi and lichens. Unfortunately, these species may pose a serious threat to rock art in the long term.

“We show that these fungi and lichens could significantly contribute to the gradual erosion and damage of the petroglyphs,” said Laura Rabbachin, Ph.D. student at the Academy of Fine Arts in Vienna, Austria, and first author of the study. “They are able to secrete different types of acids that can dissolve the limestone in which the petroglyphs are carved. In addition, fungi can penetrate and grow in the stone grains, causing further mechanical damage.”

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Extreme conditions

Rabbachin and colleagues took samples from a petroglyph site in the central-western Negev highlands. An average of only 87 mm of precipitation falls here annually, and temperatures on the rock surfaces can rise to 56.3 °C in summer. The researchers scraped samples from desert varnish next to the petroglyphs, from rocks without desert varnish, and from soil near the sampled rocks. They also left open petri dishes near the rocks to catch airborne spores.

The authors identified the collected fungi and lichens using two complementary methods. First, they repeatedly cultured fungal material or spores from rocks or soil in dishes with one of two different growth media until they obtained pure isolates for DNA barcoding. Second, they directly sequenced the DNA of fungal material present in rock or soil samples without first culturing them. The second method can detect strains that do not grow in culture.

Few, but destructive species on petroglyphs

Both methods showed that species diversity and abundance on petroglyph-bearing rocks were low compared to soil, suggesting that few species are able to withstand local extremes of drought and temperature.

DNA barcoding of the cultured isolates revealed that the petroglyphs contain multiple fungal species within the genera Alternaria, Cladosporium, and Coniosporium, while direct sequencing further detected multiple species within the genera Vermiconidia, Knufia, Phaeotheca, and Devriesia. All but Alternaria and Cladosporium are so-called microcolonial fungi known to thrive in both hot and cold deserts around the world. Lichens of the genus Flavoplaca were also abundant.

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“Microcolonial fungi are considered highly dangerous for stone artifacts. For example, they have been considered a likely cause of the deterioration of stone cultural heritage in the Mediterranean,” said Rabbachin.

“Lichens are also well known to cause rock degradation and are therefore a potential threat to stone cultural heritage.”

In the surrounding soil and air, the scientists found mainly various cosmopolitan fungi known to be able to survive the harsh desert conditions by producing drought-resistant spores.

Documentation of endangered rock art is a must

Can anything be done to protect the petroglyphs from the slow but destructive work of the observed microcolonial fungi and lichens? That’s unlikely, the authors warned.

“These natural weathering processes cannot be stopped, but their rate of weathering depends largely on whether and how the climate changes in the future. What we can do is monitor microbial communities over time and, most importantly, document these valuable works of art in detail,” said Rabbachin’s academic supervisor, Professor Katja Sterflinger, lead author of the study.