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
characterized by patchy growth dominated by associations of algae,
cyanobacteria, fungi and heterotrophic bacteria. A recently developed in vitro
system to study colonization 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 (1–6 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: (i) analysing the degradation products in the effluent from the columns
via Inductively Coupled Plasma Spectroscopy and (ii) by studying polished
sections of the incubated mineral fragments/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 favored 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 analyzed