Stone 227
and organic compound complexes; and take up water from air and release it
under low RH conditions. Stone moisture and nutrients are thereby increased
while porosity, water-uptake capacity and evaporation are reduced. Notably rich
and homogeneous biofilms, composed mostly of bacterial rods, are often
observed on weathered stone substrates from sheltered areas (Figure 8).
Microorganisms may degrade stone mechanically, chemically and aesthet-
ically through metabolic activities and biomineralisation processes in these
biofilms. The mechanical stress induced by shrinking and swelling of the col-
loidal biogenic slimes inside stone pores may damage stone and it may cause
changes in the circulation of moisture to further enhance chemical dissolu-
tion and mineral loss from stone.
3.5 Interactions of Microbes with Stone Salts
Salts acting on their own are very important decay agents and can attack
stones, mainly mechanically in pore spaces during RH and temperature
changes. Efflorescences present a niche for salt-tolerant and salt-loving bac-
terial populations which are osmotically well-adapted to an extreme exis-
tence (typical of an ancient group of bacteria known as the Archaea). High
concentrations of sodium chloride and magnesium sulfate (up to 25%) may
be appropriate for studying bacteria from efflorescences on stone monuments.
It has also been shown that microorganisms can enhance the physical or chemical
Figure 8Biofilm on weathered stone