an endosymbiont. In the mature stage, Geosiphonpro-
duces transparent bladder-like structures on the soil sur-
face, each about 1 mm high, with the cyanobacteria
located towards the top of the bladders (Fig. 13.24).
Hyphae radiate into the soil from the base of the
bladders, and it is possible (but as yet unproven) that
these hyphae interact with plant roots to form arbus-
cular mycorrhizas. When the partnership is fully
established, the cyanobacteria are photosynthetically
active, and the cyanobacteria produce heterocysts,
which can fix atmospheric nitrogen.
Experimental studies have revealed several stages
in the development of this unique symbiosis. The two
partners initially live independently on the soil surface,274 CHAPTER 13
Fig. 13.21A dried soil crust community
of lichens, cyanobacteria and fungal
hyphae that form a thin covering
which binds the surfaces of semiarid
soils. Such a community would be
more than 100 years old and represents
a relatively early stage in soil formation.Fig. 13.22A small fragment of a desert
crust community when remoistened,
showing several small lichen thalli
(Peltula sp.) and one small cyanobac-
terial lichen (Collema). Most of the soil
surface is covered with filaments of the
cyanobacterium Scytonema.