Biology Reference
In-Depth Information
Other non-heterocystous and heterocystous cyanobacteria that tolerate extremes of desiccation
have been reported from biological soil crusts all the world over. Generally, soil crusts dominated
by cyanobacteria are dark coloured or black. The most common cyanobacterium that occurs world-
wide in the soil crusts is
Microcoleus vaginatus
. The fi laments are equipped with a thick external
polysaccharide sheath (Fig. 22). This cyanobacterium generally occupies 0.2 to 0.5 mm beneath the
soil surface where suffi cient light is available for its growth and harmful effects of UV radiation
are minimized. When suffi cient moisture is available it migrates upwards and grows extensively
and the populations of
Scytonema
and
Nostoc
with coloured sheaths make their appearance. When
dry spells return,
M
.
vaginatus
again becomes subterranean (Belnap
et al
., 2001b). The intertidal
marine mats are dominated by the growth of
Microcoleus
sp. (Pentecost, 1985; Potts and Whitton,
1977, 1980) whereas terrestrial crusts are dominated by species of
Tolypothrix
,
Calothrix
and
Nostoc
(Whitton
et al
., 1979; Potts and Whitton, 1980; Whitton, 1982). In the soil crusts of Sonoran hot
Figure 22:
Transmission electron micrographs of cryo-fi xed
Microcoleus
sp. in cross section (A) and longitudinal sections (B,
C, and D). The arrangement of thylakoids and the the thick extracellular polysachharide mucilaginous sheath can be clearly
seen. The magnifi cation bars in represent 200 nm (B) and 500 nm (A, C and D). CW, cell wall; R, Ribosomes; G, polysaccharide
(glycogen) granules; C, carboxysomes; asterisk, perforations; EPS, extracellular polysaccharides; V, vesicles. Pictures courtesy
A. Kaplan, I. Ohad and R. Nevo, Department of Biologiclal Chemistry, Hebrew University, Jerusalem, Israel 972-2-658-5423.
