Lichens and higher plants on stone: a review - AseanBiodiversity.info

Fig. 2. Di erent forms of lichen growth on stone: (a) crust-like lichen;
(b) foliose lichen; (c) fruticose lichen; (d) endolithic lichen.
According to Savoye <strong>and</strong> Lallemant (1980), however,
lichens establish on a substrate after it has been partly transformed
by substances in the air <strong>and</strong> then by bacteria. This
hypothesis seems veri ed by the fact that lichens grow more
readily on archaeological ruins on which mould facilitates
bacterial growth <strong>and</strong> on stone surfaces that have become
porous.
Although the role of primary colonizer seems doubtful,
lichens, especially the crust-like <strong>and</strong> endolithic forms, nevertheless
transform stone substrates, causing de-cohesion
<strong>and</strong> biocorrosion. Certain alterations were demonstrated by
Gehrmann et al. (1988) to be typical of crust-like lichens.
Mechanical damage is due, in the rst place, to penetration
of the substrate by the hyphae (Fig. 2). The depth to which
the thallus penetrates depends on the lichen species <strong>and</strong> the
nature of the substrate. A study by Syers (1964) showed that
penetration may range from a minimum of 0:3 mm to a maximum
of 16 mm. Loss of cohesion may also occur as a result
of the expansion <strong>and</strong> contraction of the thallus as it undergoes
uctuations in water supply (Fry, 1924; 1927). <strong>Lichens</strong>
may also damage a substrate by means of acid substances
suchas carbon dioxide, lichenic acids <strong>and</strong> oxalic acid. Carbon
dioxide, produced by respiration of the thallus, dissolves
calcareous rocks in the presence of moisture, leading to the
formation of soluble bicarbonates that may be washed away
or cause encrustation. This type of alteration is characteristic
of endolithic lichens (Syers <strong>and</strong> Isk<strong>and</strong>er, 1973). A large
variety of substances produced by the lichen symbiosis are
grouped under the name of lichenic acids. Shatz (1963) <strong>and</strong>
later Isk<strong>and</strong>er <strong>and</strong> Syers (1972) showed that some of these
substances are chelating agents <strong>and</strong> in aqueous suspensions
of basalt, granite <strong>and</strong> biotite, form metal complexes of low
solubility. Ascaso et al. (1976) detected mineral complexes
that were not present on the naked rock surface, at the
lichen-substrate interface. Fragments of thallus incubated in
suspensions of the same pulverized rock formed minerals
similar to those found at the interface. Further con rmation
of the capacity of lichenic acids to produce alteration was
provided by Galvan et al. (1981) <strong>and</strong> this enabled a series
of minerals associated with the presence of lichens to be
identi ed.
It has long been known that lichens produce oxalic acid
(Bracconot, 1825). Unlike lichenic acids, oxalic acid is not
an exclusive product of lichens. It is also found in fungi as
a nal product (Chiari et al., 1989) <strong>and</strong> is common in all
living organisms as an intermediate of the Krebs cycle. It
reacts withsubstrate minerals to form various oxalates, according
to the cations available <strong>and</strong> the state of hydration
of the rock (Corball et al., 2001). Calcium oxalate is by
far the most common of these; usually it crystallizes as a
monohydrate (whewellite), but may also occur in the dihydrated
form (weddelite) (Jones et al., 1980). Due to their low
solubility, oxalates tend to accumulate inside the thallus (Enderman
et al., 1977), or at the lichen-substrate interface (Ascaso
et al., 1976), sometimes forming a crystalline layer on
the upper surface of the thallus (Jackson, 1981). Del Monte
<strong>and</strong> Sabbioni (1987) suggested that oxalate patinas on ancient
monuments (Guidobaldi et al., 1984) are the result of
the metabolic activity of lichens of the past that have disappeared
from towns due to pollution. This hypothesis caused
much controversy <strong>and</strong> stimulated research which, far from
settling the dispute about the origin of the patinas, nevertheless
revealed that they can protect the underlying stone by
virtue of their low solubility (Aless<strong>and</strong>rini, 1989).
2.3. Recognition of biodeterioration of a material
As for <strong>plants</strong>, the identi cation of lichens is based on
the diagnostic characters of the di erent species. The main
lichen characters used are form of growth, color, thallus
size, surface structures (isidia, soredia <strong>and</strong> so forth), fruiting
bodies, number <strong>and</strong> form of spores, <strong>and</strong> speci c substances
in the thallus. Identi cation is generally performed in
situ. When microscopic observation is necessary, fragments
of thallus <strong>and</strong> fruiting bodies are removed with a scalpel
without damaging the rock surface. In the laboratory, special
reagents are used that give a color reaction with speci c
lichen chemicals.
To study the distribution of lichens over vast areas, phytosociological
surveys can be performed according to the
method of Braun-Blanquet (1964). Each survey consists of
a list of species identi ed in a de ned area. Eachspecies is
assigned a substrate cover value in a scale of several classes.
The type of substrate, its inclination <strong>and</strong> aspect are noted for
eachsurvey. The data is classi ed <strong>and</strong> ordered to identify
clusters <strong>and</strong> relationships between groups by the method of
Wildi <strong>and</strong> Orloci (1988). The ecological characterization of
the various groups can be performed by associating the indices
of pH, nitrophytism, hygrophytism <strong>and</strong> photophytism
proposed by Wirth(1980).
Deterioration can be observed at the rock-lichen interface
by combined methods such as the study of translucent or