The most important macro-elements can be identified, e.g. carbon, nitrogen,
sulfur, phosphorus, iron, sodium, potassium and magnesium, which are the main
components of the so-called cell dry-mass, but other microelements such as
manganese, cobalt, copper, nickel, molybdenum, selenium, zinc and vanadium
are also required. The carbon sources that can be utilized by different groups of
microorganisms can be used to distinguish between autotrophic and heterotrophic
organisms. The most important species on wall paintings are autotrophic
algae, cyanobacteria and nitrifying bacteria, while the chemoorganotrophic
(heterotrophic) bacteria and fungi, which depend on organic carbon compounds,
also occur.
In relation to the microbial requirements for humidity, the influence of the
climate becomes especially obvious for objects that at some stage have been inac-
cessible, creating a stable climate. Recently, newspapers reported on microbial
problems in a cave in France that had been recently opened to the public. Even
if only a limited number of visitors are allowed, it is necessary to consider the
amount of humidity that is introduced by people. Therefore, in most cases, wall
paintings opened to the public are subject to enhanced humidity, due to both
human influences and the effects of the external climate after opening. Only
very rarely can investigations of the climate and air contamination be carried
out before documentation, investigation and consolidation are started. One of the
exceptions to be described is the tomb of Queen Nefetari, in the Queens Valley,
Thebes. Caves like Altamira or Lascoux have now been completely closed to
tourists and are now only accessible to selected visitors with specific interests.
To enable visitors to observe the paintings and people to work on site, elec-
tric lights have to be installed in places which may not have been illuminated
for centuries or longer. Light in damp areas inevitably leads to infestation by
algae and cyanobacteria, which are photosynthetically-active microbes. Sub-
sequently, chemoorganotrophic bacteria and fungi can grow on the organic
compounds synthesized by the light-dependent phototrophs. These succes-
sive growths may induce biodeterioration of the paintings. The influence of
humidity can be clearly observed in the lower parts of wall paintings inside
old churches where, in most cases, algal growth is restricted to areas that are
subject to rising damp or other places of enhanced humidity, such as seen in
Figure 1 after rainwater infiltration through the vault.
To control the availability of moisture to microbes, ancient techniques of
preservation introduced sugar (fruit preservation) or salt (vegetable, fish, meat,
hides) to reduce the amount of water available for the organisms. Therefore, in
early investigations of biodeterioration of wall paintings, areas with obvious
salt problems were expected to be without infestation. However, it was shown
that a considerable number of microorganisms isolated from wall paintings are
adapted to growth under high salt concentrations. Even pH-values about 11 can
support fungal growth. Under such conditions on wall paintings, especially in
248 Chapter 10