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

The Negev Desert in southern Israel is known for its unique rock art. Since at least the third millennium BCE, hunters, herders, and traders roaming the Negev have left thousands of rock carvings (petroglyphs). These figures are mostly cut in desert varnish: a thin layer of black on the limestone rock, which forms naturally. Many of them represent animals such as idols, goats, horses, donkeys and domestic camels, but abstract forms also appear.

Now, a study published in Frontiers in fungal biology has 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 can significantly contribute to the gradual erosion and damage of petroglyphs,” said Laura Rabbachin, a Ph.D. student at the Vienna Academy of Fine Arts in 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 inside the stone grains, causing additional mechanical damage.”

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Extreme conditions

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

The authors identified the fungi and lichens collected by two complementary methods. First, they repeatedly cultured fungal material or spores from rocks or soil on plates 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 latter method can detect strains that do not grow in culture.

Few species but destructive in petroglyphs

Both methods showed that species diversity and abundance in petroglyph-bearing rocks was 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 harbor multiple fungal species within the genera Alternaria, Cladosporium, and Coniosporium, while direct sequencing further revealed multiple species in the genera Vermiconidia, Knufia, Phaeotheca, and Devriesia. All but Alternaria and Cladosporium are so-called microcolonial fungi, known to thrive in hot and cold deserts around the world. Also abundant were lichens in the genus Flavoplaca.

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“Microcolonial fungi are considered very dangerous for stone artefacts. For example, they have been implicated as a possible cause of the deterioration of stone cultural heritage in the Mediterranean,” said Rabbachin.

“Lichens are also known to cause rock deterioration and are thus a potential threat to stone cultural heritage.”

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

Documenting 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? This is unlikely, the authors warned.

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