DANIN, AVINOAM. Department of Evolution, Systematics, and Ecology, The Hebrew University of Jerusalem, Israel 91904. - The impact of lithobionts on biodeterioration of desert rocks and the diversity of their microhabitat-dependant communities.
In the desert areas of Israel there is hardly any place where rock
surface is devoid of one or another kind of poikilohydric lithobiont.
When dry, their thallus is dormant, but when wetted they rapidly
resume physiological activity. Microscopic organisms such as
cyanobacteria, lichens, and non-lichenized fungi, have constant
specific impact on weathering of rocks. Typical kinds of rock surfaces
are thus formed in association with each lithobiont community. In each
climatic zones of Israel specific morphological types of weathered
surfaces are confined to certain geomorphologic positions. Hard
limestone rocks, with water holding capacity of 0.5-2.0%, at the Negev
Highlands, subjected to 100-150 mm mean annual rainfall, are reported
here. Epilithic crustose lichens cover most of the rocks at
north-facing slopes. They protect the surface from direct
deterioration by splashing raindrops. Efficient solar radiation over
the south-facing slopes induces harsh microclimatic conditions. In the
newly exposed surfaces the drought resistant fungi Lichenothelia
species cause minute pits by dissolving the rock around their 0.1- 0.2
mm globular body. Establishing in the micropits, cyanobacteria and
cyanophilous lichens, which cause faster weathering in their close
vicinity, lead to the formation of hemispheric depressions, 2-10 mm
diameter, in the rock. More porous limestone, with 10-15% water
holding capacity, looks like the previous, but supports
cryptoendolithic filamentous cyanobacteria. Chasmoendolithic coccoid
cyanobacteria cause or accelerate deterioration in mainly south-facing
slopes. The specific morphology in these places is known as
exfoliation. Detached stones in that area enjoy moisture of dew in
addition to the rainfall water. They support euendolithic lichens,
which cause jigsaw puzzle-like pattern on the stone surface. When
rolled on slopes these stones display all their faces to the
atmosphere and thus become weathered in a jigsaw pattern throughout.
Pitting organisms on are common on man-made marble monuments in more
mesic areas of Europe.
Key words: cryptoendolithic filamentous cyanobacteria, cyanobacteria, jugsaw puzzle-like pattern, Lichens, pitting