Abstract
Sub-aerial biofilms (SAB) are ubiquitous, self-sufficient microbial ecosystems found on mineral surfaces at all altitudes and latitudes. SABs, which are the principal causes of weathering on exposed terrestrial surfaces, are characterised by patchy growth dominated by associations of algae, cyanobacteria, fungi and heterotrophic bacteria. A recently developed in vitro system to study colonisation of rocks exposed to air included two key SAB participants - the rock-inhabiting ascomycete Knufia petricola (CBS 123872) and the phototrophic cyanobacterium Nostoc punctiforme ATCC29133. Both partners are genetically tractable and we used them here to study weathering of granite, K-feldspar and plagioclase. Small fragments of the various rocks or minerals (1 to 6 mm) were packed into flow-through columns and incubated with 0.1% glucose and 10 µM thiamine-hydrochloride (90 µL.min-1) to compare weathering with and without biofilms. Dissolution of the minerals was followed by: analysing (i) the degradation products in the effluent from the columns via Inductively Coupled Plasma Spectroscopy and (ii) by studying polished sections of the incubated mineral fragment/grains using scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray analyses. K. petricola/N. punctiforme stimulated release of Ca, Na, Mg and Mn. Analyses of the polished sections confirmed depletion of Ca, Na and K near the surface of the fragments. The abrupt decrease in Ca concentration observed in peripheral areas of plagioclase fragments favoured a dissolution-reprecipitation mechanism. Percolation columns in combination with a model biofilm can thus be used to study weathering in closed systems. Columns can easily be filled with different minerals and biofilms, the effluent as well as grains can be collected after long-term exposure under axenic conditions and easily analysed.
Highlights
As life began to spread onto land at least 1.2 Ga ago, the first settlers were oxygenic cyanobacteria along with various organotrophic microorganisms
Patchy growth dominated by associations of algae, cyanobacteria, fungi and heterotrophic bacteria are amongst the principal characteristics of Sub-aerial biofilms (SAB)
Abiotic experiments remained aseptic while the number of cells of both organisms increased during 180 d experimental period in the inoculated columns (Figure 2)
Summary
As life began to spread onto land at least 1.2 Ga ago, the first settlers were oxygenic cyanobacteria along with various organotrophic microorganisms. Present day sub-aerial (bare) rock surfaces are inhabited by similar microbial communities. Sub-aerial biofilms (SAB) are ubiquitous, self-sufficient, miniature microbial ecosystems that are found on mineral surfaces at all altitudes. Patchy growth dominated by associations of algae, cyanobacteria, fungi and heterotrophic bacteria are amongst the principal characteristics of SABs. In addition to being one of the principal causes of weathering, SABs accelerate the decay of cultural heritages (Dornieden et al, 2000a,b; Warscheid and Braams, 2000; de los Ríos and Ascaso, 2005). Microbial colonization of architectural monuments and works of art causes discoloring as the SAB inhabitants often produce highly colored pigments including carotenoids, chlorophylls and melanins (Gorbushina et al, 1993; Diakumaku et al, 1995; Urzì and Realini, 1998; Gorbushina and Broughton, 2009). Limiting or preventing biodeterioration is critical to conserving objects of cultural value
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