Physiology of lichens: water relations and
nutrient exchange
Many lichens will tolerate prolonged drought and
resume activity rapidly after rewetting. Lichens that con-
tain green algae can recover by absorbing water from
humid air and then begin to photosynthesize within
a short time. However, lichens that contain cyanobac-
teria can only resume photosynthesis after absorbing
free (liquid) water. In all cases the drought tolerance
of lichens is likely to be conferred by water-repellent
hydrophobins that coat the hyphal walls of the
medulla.The hydrophobins seem to be produced only
by the fungus, because cells of Trebouxia, which have
a naturally hydrophilic surface, become covered with
a hydrophobic material when grown in the presence
of a lichen fungus.
The principal roles of the lichen fungi are to provide
physical protection for the photosynthetic cells and
to absorb mineral nutrients from the underlying sub-
strate or from rainwater. Lichen fungi are especially
adept at accumulating nutrients from trace amounts in
the environment. They are so efficient in this respect
that they tend to accumulate atmospheric pollutants,
particularly sulphur dioxide, to levels that are toxic. This
is why many lichens are rarely seen in major towns and
cities. Only a few species, such as Hypogymnia and
Xanthoria parietina, are found commonly in towns,
and even then they grow poorly, compared with their
growth in nonpolluted environments.
The principal role of the photosynthetic partner
is to provide sugars for growth of the lichen.
Radiolabeling studies have shown that green algae
and cyanobacteria can release up to 90% of their
photosynthate to the fungal partner. In lichens with
Trebouxiaas the photobiont, and presumably with
other green algae, the fungal hyphae can produce
short branches that penetrate through the algal wall
to act as nutrient-absorbing structures (haustoria;
Fig. 13.18). In lichens with cyanobacterial symbionts
the hyphae similarly can form protrusions that pene-
trate the hydrophilic gelatinous sheaths around the
cyanobacterial cells (Fig. 13.18).
The major soluble carbohydrates in lichens are
sugar alcohols(polyols). These are present in the
form of mannitol and, to a lesser degree, arabitol in
the fungal hyphae. The green algae also produce sugar
alcohols as their main photosynthetic products – for
example, ribitolin the case of Trebouxia. However, the
cyanobacteria seem to release glucoseto the fungal
partner, apparently through a glucose carrier in the cell
membrane after intracellular glucans have been enzy-
matically degraded. It is notable that the maximum
rates of nutrient release from the photobionts occur in
optimal moisture conditions, whereas the photobionts
retain most of their carbohydrate in conditions of
water stress. On this basis it has been suggested that
periodic cycles of wetting and drying might be advant-
ageous in maintaining a lichen symbiosis, each
partner gaining carbohydrate at different stages of the
wetting–drying cycle.FUNGAL SYMBIOSIS 271
Fig. 13.17(a) Tightly packed tissue of the upper cortex of the lichen Peltigera canina. (b) Thick-walled, hydrophobic
hyphae of the medulla of Peltigera.
(a) (b)