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

The Negev Desert in southern Israel is famous for its unique rock art. Since at least the third millennium BCE, hunters, shepherds, and traders who traveled through the Negev left thousands of carvings (petroglyphs) on the rocks. These figures are mostly carved in desert lacquer: a thin black coating on limestone rock that forms naturally. Many of them depict animals, such as ibex, goats, horses, donkeys, and domestic camels, but there are also abstract forms.

Now a study published in Frontiers of Fungal Biology revealed that the petroglyphs are home to a community of rare, specialist fungi and lichens. Unfortunately, these species could pose a serious threat to the 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 doctoral student at the Academy of Fine Arts in Vienna, Austria, and the study’s first author. “They can secrete different types of acids that can dissolve the limestone in which the petroglyphs are carved. In addition, the fungi can penetrate and grow in the grains of the stone, causing additional mechanical damage.”

Extreme conditions

Rabbachin and colleagues collected samples from petroglyphs in the central-western Negev highlands, where an average of just 87 mm of rain falls per year, and temperatures on 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 rocks from which the samples were taken. They also left open petri dishes near the rocks to capture 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 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 prior culture. The latter method can detect strains that do not grow in culture.

Few but Destructive Species in Petroglyphs

Both methods showed that the diversity and abundance of species on the petroglyph rocks is low compared to the soil, suggesting that few species are able to survive the local extremes of drought and temperature.

DNA coding of cultured isolates revealed that the petroglyphs contained many species of fungi from the genera Alternaria, Cladosporium, and Coniosporium, while direct sequencing detected many species from the genera Vermiconidia, Knufia, Phaeotheca, and Devriesia. All except Alternaria and Cladosporium are so-called microcolonial fungi, known to thrive in hot and cold deserts worldwide. Lichens from the genus Flavoplaca were also abundant.

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

“Lichens are also known to cause the destruction of rocks and thus pose a potential threat to the cultural heritage of stones.”

In the surrounding soil and air, scientists found primarily a variety of cosmopolitan fungi known to be able to survive 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? It is unlikely, the authors warn.

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