untitled

and the partnership is only established when the
cyanobacteria are present in a specific stage of their life
cycle, termed the primordium stage. When cyanobac-
teria in this stage make contact with the fungus, the
tip of the hypha bulges and surrounds some of the

cyanobacterial cells, which are then incorporated into

the fungus by endocytosis. This leads eventually to the

development of a symbiosome– a membrane-bound

structure containing the cyanobacteria but separating

them from the rest of the cell contents (Fig. 13.25). It

is notable that each bladder represents the result of a

single incorporation of cyanobacteria, so this would

have to happen many times in the colony. Once

formed, the individual bladders can live for more than

half a year.

The cyanobacterial symbiont in these associations

(Nostoc punctiforme) can be grown quite easily in

laboratory conditions, and several strains of this

cyanobacterium can be used to establish the Geosiphon

symbiosis – even cyanobacterial strains from other

symbiotic associations such as the liverwort Blasiaand

the higher plant, Gunnera.

There is strong evidence that Geosiphonrepresents

a symbiotic and mutualisticassociation. The fungus

benefits primarily from a supply of photosynthate

from the cyanobacterium, while the cyanobacterium

probably depends on the fungus for a supply of pho-

sphate. Consistent with this, the Geosiphonbladders

have been shown to be impermeable to even small

organic compounds such as sugars, so the fungus

FUNGAL SYMBIOSIS 275

Fig. 13.23(a) The desert crust lichen,
Peltula, which typically grows as a
thallus composed of several squa-
mules. (b) The same lichen seen from
below, showing a mass of branched
rhizinae that “root” into the desert
sand. (c) Part of Fig. 13.22, enlarged
to show the mass of cyanobacterial
filaments (Scytonemasp.). (d) A single
filament of Scytonemaencased in a
mucilaginous sheath with soil particles.

(a) (b)

(c) (d)

Fig. 13.24Bladders of Geosiphon pyriformegrowing on
the surface of soil. Scale bar =1 mm. (Courtesy of A.
Schuessler.)