Rock varnish in the Sonoran Desert: microbiologically mediated accumulation of manganiferous sediments

Abstract

Rock varnish occurs in virtually all environments, most commonly in arid and semi‐arid climates, including Antarctica. Rock varnish consists of thin layers of intimately mixed aeolian and chemical sediments often showing botryoidal and more rarely stromatolite‐like morphologies. Typical rock varnish samples collected at Twin Peak Mountain Park, near Phoenix, Arizona, consist of abundant quartz, with plagioclase, illite and a mixed layer, Fe‐clay mineral, probably corrensite. EDS, SEM (BSE) and TEM analyses revealed that the typical Mn, Fe minerals occur as minute particles; some of these particles and other mineral grains are attached to filaments. XRD and electron diffraction showed that the Mn.Fe‐bearing particles are poorly crystalline. The filaments, based on morphological criteria, are virtually indistinguishable from fungal filaments. Most filaments are fragments, probably broken by scraping during sample collection. Coccoid and rod‐shaped forms, resembling cyanobacteria and other bacteria, respectively, are also present. Unlike definitive minerals, these filaments disintegrated in the concentrated energy of the SEM electron beam at the instrumental and experimental conditions used. In addition, no filamentous, rod‐shaped or coccoid forms were observed in samples hydrolysed with 6 N HCl for 24 h at 100°C. Bacteria and fungi in powdered rock varnish were cultured on four media, incubated aerobically in the dark at 25°C. The culture media yielded dense growths of spore‐forming bacteria and filamentous fungi. One fungus and two Bacillus isolates oxidized and concentrated manganese. Control experiments revealed that fungi and bacteria are present on and below the surfaces of rock varnish. Free and hydrolysed, peptide/protein‐bound amino acids were identified in the rock varnish. Amino acids showed virtually no racemization with the exception of D/L asp = 0.1. Relatively high molecular weight humic matter was also separated from the rock varnish. High‐resolution mass spectrometry revealed non‐hydrocarbon moieties, similar to a Suwannee River (FL) humic acid standard. Micro‐organisms and their original biochemical compounds do not seem to be preserved for long in the accreting varnish layer. The studies showed that the filaments helped to trap mineral particles of rock varnish, and that bacteria and fungi abetted Mn concentration. Some structures in the layers of rock varnish resemble stromatolites and present definitions would allow them to be termed as such.

Silica- and sulfate-bearing rock coatings in smelter areas: Products of chemical weathering and atmospheric pollution I. Formation and mineralogical composition

“…Numerous studies identified incorporated clay minerals in aerially exposed silica-rich coatings (e.g. Potter and Rossman, 1977;Dorn and Oberlander, 1982;Nagy et al, 1991;Krinsley, 1998;Perry et al, 2006). On the contrary, only a few synthesis experiments showed that clay minerals can form in situ in a silica-rich matrix.…”

Spatial, temporal and geographic considerations of the problem of rock varnish diagenesis

“…Analyses of 'bulk samples' (i.e., those requiring the scraping of varnish and underlying weathering rind and rock material) have included analysis of amino acids (Warsheid, 1990;Nagy et al, 1991;Perry and Kolb, 2003;, isotopic analysis (Dorn and DeNiro, 1985), analysis of fatty acid methyl esters (Schelble et al, 2005), phospholipid fatty acids (Kuhlman et al, 2006a), and extraction of genetic material for phylogenetic insight (Eppard et al, 1996;Kuhlman et al, 2005;Spilde et al, 2005;Benzerara et al, 2006;Kuhlman et al, 2006a,b;Northup et al, 2010).…”

Bacillus Endospores Isolated from Granite: Close Molecular Relationships to Globally Distributed Bacillus spp. from Endolithic and Extreme Environments

“…Endolithic Bacillus spp. have been obtained from a wide variety of sites within the lithosphere, such as the interiors of mined amber (6), halite crystals (58), marine manganese nodules (36), rock varnishes from Sonoran, Mohave, and Negev desert rocks (23,35,45), deep subsurface boreholes (2,4), Sonoran desert basalts (3), and Florida limestones from intertidal zones (1). In addition, several Bacillus spp.…”