Geomicrobial Investigations of Colored Outer Coatings from an Ethiopian Rock Art Gallery

Abstract

The open rock shelter of Yabelo in Ethiopia hosts diverse Holocene paintings of great cultural importance. The paintings are characterized by the presence of different mineral coatings, whose features have not been studied yet. Our goal was to understand whether different rock samples from the Yabelo paintings collected in close proximity may reveal coatings with different minerology and biology. Thus, elemental analyses combined with microscopic and molecular investigations were performed on two coatings, one whitish (sample 1) and one reddish (sample 2). Although both samples were dominated by heterotrophic bacteria, the two coatings showed distinct mineralogical and microbiological characteristics. Sample 1 contained higher amounts of Ca and P than sample 2, which was likely related to the presence of organic matter. Sample 1 hosted bacterial genera that are potentially involved in biomineralization processes, metal redox cycles and metal resistance. In contrast, sample 2 showed mainly pathogenic and commensal bacteria that are characteristic of animal and human microbiota, and other microorganisms that are involved in nitrogen and metal biogeochemical cycles. Overall, our results indicated that the bacterial communities were particular to the coating mineralogy, suggesting a potential role of the biological components in the crust genesis.

Microscopic and biomolecular complementary approaches to characterize bioweathering processes at petroglyph sites from the Negev Desert, Israel

“…Manganese and iron oxyhydroxides, together with clay minerals and various trace elements, were found previously as major components of the dark coating (Krumbein and Jens, 1981;Dorn, 1991;Northup et al, 2010;Lang-Yona et al, 2018). Microbial contribution to the formation of this dark coating via controlled or induced mineralization processes has been proposed (Grote and Krumbein, 1992;Gorbushina, 2007;Wu et al, 2020). Nevertheless, the results of our microscopic study suggest that lichens (e.g.…”

The Sustainability of Rock Art: Preservation and Research

“…Starting from this consideration, we suggest a biogeomorphological approach to weathering processes based on in situ and laboratory analyses of rocks and pigments, and the multidisciplinary investigation of SABs [5,27,33,102]. As most of the rock decay processes act at the micro-scale-thus representing a sort of 'phantom menace'-it is almost unrealistic to identify and describe threatening processes without sampling SABs, pigments, and the rock surface.…”

“…Early research was conducted in Atlanterra shelter (south Spain) using cultured methods [138], whereas in recent years, next-generation sequencing allowed the identification of non-culturable bacteria, which are the majority in environmental samples. This includes research from Ethiopia that identified bacterial communities from rock art panels by 16S rRNA gene sequencing, which found bacteria with mineralization potentials that could form patinas, and animal microbiomes possibly resulting from herding activities at the site [27]. The researchers used confocal laser scanning microscopy (CLSM) to reconstruct 3D images of the SABs colonizing the colored outer coatings from the Ethiopian rock art gallery.…”

“…In our recent experience in rock art research in a variety of remote locations, we tried to establish a procedure to reduce sampling and, at the same time, increase the possibility of mining information using different methodological approaches [5,27,33,148]. In remote locations, it is hard to transport (or export) portable instrumentation; thus, we decided to proceed with several steps: (1) biogeomorphological survey of the rock substrate to assess the potentially many and diversified types of surface rock decay and SABs formation at the rock surface and/or below it; (2) micro-sampling (using small sterile chisels) of each type of deterioration evidence on the rock surface, far from rock art representations; (3) very microinvasive sampling of painting (pigments) or rock decay evidence related to representations.…”

“…Moreover, such processes oversee the preservation of rock art as well as its rocky support [5]; for that reason, we cannot distinguish between the stability and decay of the host rock and the preservation of rock art pigments. The host rock, mineral, and organic elements of pictographs and the biological community living at the interface between the two [27,28] interact in terms of biogeochemical cycles and thus belong to the same system. From this perspective, we must introduce the concept of 'rock art ecosystem' (Figure 5), meaning the complex and multidirectional interplay between the host rock, pigments, environmental parameters (humidity, light, pH, Eh, alkalinity, etc.…”