Comparison of Rock Varnish Bacterial Communities with Surrounding Non-Varnished Rock Surfaces: Taxon-Specific Analysis and Morphological Description

Abstract

Rock varnish is a thin layer of Fe and Mn oxyhydroxides with embedded clay minerals that contain an increased Mn/Fe ratio compared to that of the Earth's crust. Even if the study of rock varnish has important implications in several fields, the composition of epilithic bacterial communities and the distribution of taxa on varnish surfaces are still not wholly described. The aim of this study was (i) to identify the bacterial taxa which show the greatest variation between varnish and non-varnish environments, collected from the same rock, and (ii) to describe the morphology of epilithic communities through scanning electron microscopy (SEM). Triplicate samples of rock surfaces with varnish and triplicate samples without varnish were collected from five sites in Matsch Valley (South Tyrol, Italy). The V4 region of 16S rRNA gene was analyzed by Illumina sequencing. Fifty-five ubiquitous taxa have been examined to assess variation between varnish and non-varnish. Cyanobacteria, Chloroflexi, Proteobacteria along with minor taxa such as Solirubrobacterales, Conexibaxter, and Rhodopila showed significant variations of abundance, diversity, or both responding to the ecology (presence/absence of varnish). Other taxa, such as the genus Edaphobacter, showed a more marked spatial variation responding to the sampling site. SEM images showed a multitude of bacterial morphologies and structures involved in the process of attachment and creation of a suitable environment for growth. The features emerging from this analysis suggest that the highly oxidative Fe and Mn-rich varnish environment favors anoxigenic autotrophy and establishment of highly specialized bacteria.

Mineral Types and Tree Species Determine the Functional and Taxonomic Structures of Forest Soil Bacterial Communities

“…Notably, while Acidobacteria are very abundant in the bulk soil compartment, the relative abundance of sequences assigned to Acidobacteria strongly decreases on mineral surfaces. Their low abundance was previously empha-sized on rock substrate of the Matsch Valley, but no driving factor was identified (45). At the experimental site of Breuil-Chenue, the abundance of Acidobacteria was also shown to be lower in the rhizosphere or on the surfaces of minerals than in the surrounding bulk soil.…”

Microbial communities and primary succession in high altitude mountain environments

“…had a marked spatial variation according to the sampling site. A highly oxidative Fe and Mn-rich varnish environment seemed to favor an anoxigenic autotrophy on rock coating (Esposito et al 2015). Despite the harsh environmental conditions of high mountain rock surfaces, autotrophs (Duc et al 2009a, b) and heterotrophs (Tscherko et al 2003;Bardgett et al 2007) may follow a succession, playing crucial roles in ecosystem development.…”

Mn-Fe-Enhancing Budding Bacteria in Century-Old Rock Varnish, Erie Barge Canal, New York

“…Budding bacteria able to precipitate manganese and iron oxides grow on rock varnishes found in warm deserts, as noted through in situ observations using a combination of secondary electron (SE) imagery and energy-dispersive spectroscopy of X-rays (EDS; Krumbein 1969;Dorn andOberlander 1981a, 1982;Krumbein and Jens 1981). Budding bacterial forms concentrating Mn and Fe on rock varnishes also occur in cold desert settings such as Tibet (Krinsley et al 2009;Wang et al 2011), in Italy (Esposito et al 2015), and along tropical rivers (Dorn et al 2012;Keim et al 2015). In situ observations also reveal that budding bacteria produce Mn-rich deposits on rocks in such varied environments as mountain soils (Khak-mun 1966(Khak-mun , 1968, acid mine drainage (Tebo et al 2005), and caves (Northrup et al 2003;Lozano and Rossi 2012).…”