IV. DESERT ALGAE, LICHENS, AND FUNGI
201
tic. Nicot (1960) summarized their general features and points out the
two prevalent biological types of adaptation to desert conditions :
The first type is characterized by brown-pigmented mycelia and/or
spores or conida, such as Dematiaceae, also Sphaeropsidales with carbonaceous linings, and Mycelia sterilia with chlamydospores and bulbils.
It is believed that the pigments act as a light filter. Also, the thick cell
walls in many forms may be considered as a light reducing device. The
second type shows an extraordinarily rapid development which may exploit
short periods of favorable growth conditions. This, together with multicellular spores ("multiplication of reproductive structures") results in a "vast
reproductive capacity" of the organisms.
Durrell and Shields (1960) showed in illumination experiments that not
all dark melanin pigments in fungal spores or conidia act as an effective
light filter against excessive solar irradiation. When irradiated with UV
light of 253.7 nm wavelength, the survival time of Stemphylium ilicis Tengwall (a typical desert form) was 60 minutes as against 2 minutes for the
equally dark conidia of A s per gill us niger Van Tiegh. They also found that
the pigment produced by Stemphylium absorbs light between 200 and
2000 nm and is insoluble in organic solvents, while the pigment of Aspergillus has a wide transmission band and is soluble in methyl alcohol.
Borut (1960) tested some of the fungi isolated from Negev soils for
their temperature tolerance. Most species had a growth optimum at about
26°C, some species around 30°C, and Aspergillus fumigatus Fres. and
A. niveus Blochw. at 36°C. With some exceptions, the majority of species
does not grow at 40°C. Borut points out, however, that soil temperatures,
with the obvious exception of surface layers, do not generally reach this
value.
Mahmoud et al. (1964) studied the effect of the rhizosphere on the
microflora of desert soils in Egypt and found a significantly higher density
of microbial population (fungi and actinomycetes) in the rhizosphere than
in the soil apart. While Pénicillium sp. was dominant in the soil, Alternaria
sp., Fusarium sp., and Aspergillus sp. were characteristic of the rhizosphere. These effects are attributed partly to the organic matter and root
secretion furnished by the growing roots of desert plants, and partly to
the higher moisture contents.
Microscopic soil fungi have a significant biological role in deserts. As
these soils are generally of low organic content, the decomposing activity
of fungi is especially important (Borut, 1960; Khudairi, 1969). Fungal
hyphae (together with filaments of blue-green algae) also play a basic role
in the stabilization of desert soils and soil crusts. The stabilized soil crusts
reduce erosion (Fletcher and Martin, 1948) and also affect the biology
of certain desert animals.
201
tic. Nicot (1960) summarized their general features and points out the
two prevalent biological types of adaptation to desert conditions :
The first type is characterized by brown-pigmented mycelia and/or
spores or conida, such as Dematiaceae, also Sphaeropsidales with carbonaceous linings, and Mycelia sterilia with chlamydospores and bulbils.
It is believed that the pigments act as a light filter. Also, the thick cell
walls in many forms may be considered as a light reducing device. The
second type shows an extraordinarily rapid development which may exploit
short periods of favorable growth conditions. This, together with multicellular spores ("multiplication of reproductive structures") results in a "vast
reproductive capacity" of the organisms.
Durrell and Shields (1960) showed in illumination experiments that not
all dark melanin pigments in fungal spores or conidia act as an effective
light filter against excessive solar irradiation. When irradiated with UV
light of 253.7 nm wavelength, the survival time of Stemphylium ilicis Tengwall (a typical desert form) was 60 minutes as against 2 minutes for the
equally dark conidia of A s per gill us niger Van Tiegh. They also found that
the pigment produced by Stemphylium absorbs light between 200 and
2000 nm and is insoluble in organic solvents, while the pigment of Aspergillus has a wide transmission band and is soluble in methyl alcohol.
Borut (1960) tested some of the fungi isolated from Negev soils for
their temperature tolerance. Most species had a growth optimum at about
26°C, some species around 30°C, and Aspergillus fumigatus Fres. and
A. niveus Blochw. at 36°C. With some exceptions, the majority of species
does not grow at 40°C. Borut points out, however, that soil temperatures,
with the obvious exception of surface layers, do not generally reach this
value.
Mahmoud et al. (1964) studied the effect of the rhizosphere on the
microflora of desert soils in Egypt and found a significantly higher density
of microbial population (fungi and actinomycetes) in the rhizosphere than
in the soil apart. While Pénicillium sp. was dominant in the soil, Alternaria
sp., Fusarium sp., and Aspergillus sp. were characteristic of the rhizosphere. These effects are attributed partly to the organic matter and root
secretion furnished by the growing roots of desert plants, and partly to
the higher moisture contents.
Microscopic soil fungi have a significant biological role in deserts. As
these soils are generally of low organic content, the decomposing activity
of fungi is especially important (Borut, 1960; Khudairi, 1969). Fungal
hyphae (together with filaments of blue-green algae) also play a basic role
in the stabilization of desert soils and soil crusts. The stabilized soil crusts
reduce erosion (Fletcher and Martin, 1948) and also affect the biology
of certain desert animals.