IV. DESERT ALGAE, LICHENS, AND FUNGI
183
E. INSOLATION: TEMPERATURE AND LIGHT
Although desert algal habitats are often exposed to extreme fluctuations
of temperature, it is clear from the preceding discussion that, due to insulation and the slow evaporation of their water reservoirs, their microenvironment is often cooler and more balanced in temperature than the
surrounding macroclimate. Vogel (1955) points out that heat resistance
is less of a problem for these organisms, since desiccation generally occurs
before temperature rises to extreme values, and heat resistance in dry and
inactive cells is much higher than it would be in a liquid medium (see
Section I,D above).
In most desert algal habitats mechanisms exist which prevent exposure
to excessive illumination. The limitation of algal growth to certain horizontal zones with well-defined vertical boundaries (see Sections I,C,1,
d, I,C,2,a, and I,C,2,b above) indicates that algae exist within the confines
of tolerable light limits. The dark pigmentation of sheaths such as it occurs
in the upper levels of endolithic growth (see Section I,C,2,b) is obviously
a light-filtering device.
F. NITROGEN FIXATION
Numerous reports exist about the nitrogen-fixing activity of desert algae,
and their role in the biology of nitrogen-deficient desert soils is considerable. The nitrogen-fixing activity of endedaphic and epedaphic blue-green
algae (the latter both in algal crusts and lichen crusts) has been reported
by Fletcher and Martin (1948), Shields et al (1957), Cameron and Fuller
(1960), Fuller et al (1961), Shields and Drouet (1962), Mayland and
Mclntosh (1966a,b), and Mayland et al (1966). Fuller et al (1961)
and Mayland and Mclntosh (1966a) presented experimental evidence that
the nitrogen fixed by algae is available to higher plants. Tchan and Beadle
(1955) described nitrogen-fixing blue-green algae from endadaphic and
hypolithic habitats in Australia. Soviet research on N-fixing soil algae is
summarized by Bolyshev (1968) and by Gollerbakh and Shtina (1969).
Odintsova (1941) reports on the nitrogen-fixing activity of chasmolithic
algae in the cold desert of Western Pamir. In a zone which stretches from
about the margin of the permanent snow to 4000 meters above sea level,
several rock types contain considerable amounts of nitrates. The rocks
composed of large hexagonal quartz crystals and small plates of a carbonate matrix.
Single cells and colonies of a large-celled endolithic Gloeocapsa sp. are attached to
the surface of the quartz crystals. A grape-like cell group on the left side became
separated from the substrate and the flat base adhering to the crystal surface is
exposed. χ2000. After Friedmann (1971).
183
E. INSOLATION: TEMPERATURE AND LIGHT
Although desert algal habitats are often exposed to extreme fluctuations
of temperature, it is clear from the preceding discussion that, due to insulation and the slow evaporation of their water reservoirs, their microenvironment is often cooler and more balanced in temperature than the
surrounding macroclimate. Vogel (1955) points out that heat resistance
is less of a problem for these organisms, since desiccation generally occurs
before temperature rises to extreme values, and heat resistance in dry and
inactive cells is much higher than it would be in a liquid medium (see
Section I,D above).
In most desert algal habitats mechanisms exist which prevent exposure
to excessive illumination. The limitation of algal growth to certain horizontal zones with well-defined vertical boundaries (see Sections I,C,1,
d, I,C,2,a, and I,C,2,b above) indicates that algae exist within the confines
of tolerable light limits. The dark pigmentation of sheaths such as it occurs
in the upper levels of endolithic growth (see Section I,C,2,b) is obviously
a light-filtering device.
F. NITROGEN FIXATION
Numerous reports exist about the nitrogen-fixing activity of desert algae,
and their role in the biology of nitrogen-deficient desert soils is considerable. The nitrogen-fixing activity of endedaphic and epedaphic blue-green
algae (the latter both in algal crusts and lichen crusts) has been reported
by Fletcher and Martin (1948), Shields et al (1957), Cameron and Fuller
(1960), Fuller et al (1961), Shields and Drouet (1962), Mayland and
Mclntosh (1966a,b), and Mayland et al (1966). Fuller et al (1961)
and Mayland and Mclntosh (1966a) presented experimental evidence that
the nitrogen fixed by algae is available to higher plants. Tchan and Beadle
(1955) described nitrogen-fixing blue-green algae from endadaphic and
hypolithic habitats in Australia. Soviet research on N-fixing soil algae is
summarized by Bolyshev (1968) and by Gollerbakh and Shtina (1969).
Odintsova (1941) reports on the nitrogen-fixing activity of chasmolithic
algae in the cold desert of Western Pamir. In a zone which stretches from
about the margin of the permanent snow to 4000 meters above sea level,
several rock types contain considerable amounts of nitrates. The rocks
composed of large hexagonal quartz crystals and small plates of a carbonate matrix.
Single cells and colonies of a large-celled endolithic Gloeocapsa sp. are attached to
the surface of the quartz crystals. A grape-like cell group on the left side became
separated from the substrate and the flat base adhering to the crystal surface is
exposed. χ2000. After Friedmann (1971).