References

Microorganisms of Biological Crusts on Soil Surfaces 311
Fig. 3. Scheme of
a Nostoc-dominated soil
crust. While adult Nostoc
colonies are on the soil
surface, the motile phase
(hormogonia) lives inside
the soil; not drawn
to scale
Further frequent filamentous cyanobacteria in soil crusts are the false
branched genera Scytonema and Calothrix (Fig. 1b), both characterized by
a brown to bright yellow sheath, with the color being created by the UV-
A protecting indol-alkaloid scytonemin embedded within the gelatinous
sheath (Garcia-Pichel and Castenholz 1991; Proteau et al. 1993). The species
of both genera form small, cushion- or turf-like colonies emerging from
theuppermostpartofthesoilandarevisiblewithahandlensassmall
brown or black patches when wet. In humid savanna soil crusts, the genus
Schizothrix, together with other cyanobacteria of the Oscillatoriales, are the
major constituents in terms of biomass and crust formation (San José and
Bravo 1991; Büdel et al. 1994). Further filamentous cyanobacteria occurring
in soil crusts are listed in Table 1.
Among unicellular cyanobacteria, the coenobia-forming genera Gloeocapsa,
Gloeothece,andChroococcus occurfrequentlyinsoilcrusts,whilethe
genera Chroococcidiopsis (Fig. 1e) and Pleurocapsa dominate in hypolithic
crusts (Fig. 4; Büdel 2001d; Ullmann and Büdel 2001a). Species of Chroococcidiopsis
and the species Pleurocapsa minor Hansgirg are very sensitive to
high irradiance and the hypolithic habitat seems to provide just the appropriate
light climate in addition to a less extreme water regime (Vogel 1955).
The genus Chroococcidiopsis not only occurs inside soils and underneath
translucent rocks, but also can be found in lichens of soil crusts. From
a recent study on their molecular phylogeny and systematics, we know
that they do not belong to the pleurocapsalean group of cyanobacteria, but
are the closest living relatives to the filamentous, heterocyst-differentiating
cyanobacteria (Nostocales; Fewer et al. 2002). Other unicellular genera
found in biological soil crusts are listed in Table 1.