Geoscience Reference
In-Depth Information
CLASSIFICATION OF ROCK- and SOIL-DWELLING ORGANISMS
LITHOPHYTES
(rock dwelling organisms)
ENDOLITHS
(inside rocks)
HYPOLITHS
(under pebbles)
EPILITHS
(on rock surfaces)
CRYPTOENDOLITHS
(in cavities in porous
rocks)
CHASMOENDOLITHS
(in open cracks)
EDAPHOPHYTES
(soil dwelling organisms)
EPEDAPHS
(on soil surface)
ENDEDAPHS
(within the soil)
Figure 7.4
Classification of rock and soil niches inhabited by desert microorganisms.
environment, and the temperature can be up to 8
◦
C cooler
than the surrounding soil during the day (Azua-Bustos,
2008). On soil and rock surfaces, desiccated microphytic
plants can survive extreme temperatures. At several hyper-
arid BSC sites in Chinese deserts, all having mean annual
rainfalls
water within each niche than on temperature or meteo-
rological rainfall (Warren-Rhodes
et al.
, 2006; Pointing
et al.
, 2007). Warren-Rhodes
et al.
(2006) studied the
abundance of hypolithic cyanobacteria along a rainfall
gradient in the Atacama Desert in Chile. By counting a
minimum of 1000 stones at each sample location, they
found that along a transect where mean annual rainfall
declined from 21 mm/a to about 2.5 mm/a, the fraction
of quartz pebbles colonised on the sides or base declined
from 27.6 to
24 to
54
◦
C (Pointing
et al.
, 2007). Many crust organisms are
able to source at least a part of their moisture require-
ments from very minor rain events, or from fog, dew or
atmospheric moisture (De Vries and Watling, 2008). In
the western Negev, it has been estimated that dew and
fog may support 3.2-9.4% of the total annual time for
which BSCs on dunes are sufficiently wet to be able to
photosynthesise (Kidron, Herrnstadt and Barzilay, 2002).
Owing to their capacity to exploit even minor sources
of moisture, crust organisms are able to survive in harsh
conditions where vascular plants cannot and BSCs are of-
ten found in the open interspaces between dryland shrubs
and grasses. It is common experience that many of these
organisms can begin photosynthesis within minutes of wa-
ter becoming available, green coloration from chlorophyll
rapidly becoming visible. Despite their ability to inhabit
very harsh environments, the diversity of organisms in-
habiting soil and rock environments in the drylands is
<
25 mm, soil temperatures range from
−
<
0.1%. A major decline occurred for annual
rainfalls less than 5 mm/a or where periods of around
10 years with no rain were experienced. With increasing
hyper-aridity, the size of stone that was colonised also
increased. It is interesting to note from the findings of
Warren-Rhodes
et al.
(2006) that at the driest sample site
in the Atacama the hypolithic organisms had on average
only 75
15 hours per year during which there was suf-
ficient water and light for photosynthesis to proceed. The
organisms therefore spend most time in a dormant state.
±
7.5.7
The organisms forming biological
soil crusts
