Detection of Mn oxidizing microorganisms in Desert Varnish from Chihuahuan Desert, Mexico

Abstract

Desert varnish is a microlayer that forms on rocky surfaces that is usually associated with arid and desert environments. It consists mainly of clay minerals (~70%) and Fe and Mn oxides (~30%) (Dorn & Oberlander, 1981). The film is 50 to 200 μm thick and its growth rate is very slow, 1 to 40 μm/ky. The development of vanishes could be due to biomineralization processes, dust precipitation events or pH fluctuations (Goldsmith et al., 2014). Microbial communities have been found associated with varnishes, mostly fungi (Esposito et al., 2015) and bacteria (Fagliarone et al., 2017; Lang-Yona et al., 2018). In Sonora, Mexico microorganisms have been detected inside the varnishes (Martinez- Pabello et al., 2020). The Sierra Samalayuca of the Chihuahuan Desert contains large extensions of varnishes which have not been thoroughly characterised. In this work we identify the presence of Mn oxidising microorganisms in patinas from the municipality of Juárez, Chihuahua. Rock samples were washed with a phosphate buffer and sterilised for 30 min under UV light. The patinas were pulverised in a mortar and a powder fraction (20 μg) was suspended in sterile peptone water [0.01%]. 10 μL of the suspension was planted on Czapek-agar, Thorton-agar and water-agar culture media. The Petri dishes were incubated at room temperature (21 ± 3, 25 ± 1 and 37 ± 1 oC). Fungi and bacteria colonies grow on all media after four incubation days. The cell morphology corresponded to septate and coenocytic hyphae, cocci and Gram-positive rods. After 21 days of growth, the Mn oxides colony production was determined with benzidine [1%] (Nealson, 2006). Some fungi species from varnishes were identified by rRNA18S. The presence of a microbial biofilm on the patinas was observed as well by SEM. Therefore, rock varnishes from the Sierra Samalayuca contain microbial communities that could oxide Mn.