Biology Reference
In-Depth Information
The algal partner is known as photobiont or phycobiont. It may be either a green alga and/
or a cyanobacterium. In the vast majority of lichens four genera of algae are known to participate
in the symbiosis. Of these, two are green algae (
Trebouxia
sp. and
Trentepohlia
sp.) and two belong
to cyanobacteria, i.e.
Nostoc
sp.
and
Scytonema
sp. (Ahmadjian, 1967; Büdel and Hensen, 1983;
Tschermak-Woess, 1988; Nyati
et al
., 2007). Lichens with a single photobiont (either a green alga or
cyanobacterium) are known as bipartite lichens. In some lichens, the green alga constitutes the main
photobiont and the cyanobacterium is restricted to reproductive structures known as cephalodia. Such
lichens are known as tripartite lichens. According to Laundon (1995) when the nature of photobiont
is not known it is designated as photomorph. The unidentifi ed green and blue-green algae are thus
called as chloromorph and cyanomorph, respectively.
Exceptionally,
Petroderma maculiforme
is the only brown alga that enters into symbiotic association
with an undescribed species of ascomycetous fungus
Verrucaria
(Wynne, 1969). This lichen was
collected from intertidal rock surfaces in northern California and the fungal species was identifi ed
as
V
.
tavaresiae
(Moe, 1997). The thallus organization of this lichen with the nature of symbiont
interaction (Sanders
et al
., 2004) and ultrastructural studies on the photobiont in free-living and
in lichen symbiosis (Sanders
et al
., 2005) have been described. Two photobionts belonging to
Xanthophyta have also been reported in some other lichen thalli (Tschermak-Woess, 1988).
Cyanolichens , i.e. lichens possessing cyanobacteria constitute 10% of the lichen species. In the order
Lecanorales of lichens, the most commom photobiont is
Nostoc
(Friedl and Büdel, 1996). About half
of the cyanolichens have green algae as the main photobiont. A few other fi lamentous, heterocystous
cyanobacterial photobionts belong to the genera
Calothrix
,
Dichothrix
,
Fischerella
,
Stigonema
and
Tolypothrix
(Tschermak-Woess, 1988; Oksanen, 2004). Cyanolichens are generally restricted to or are
most abundant in old growth and mature forests. Some of the examples of cyanolichens belong to the
species of
Coccocarpia
,
Lobaria
,
Leptogium
,
Nephroma
,
Peltigera
,
Pseudocyphellaria
and
Sticta
(Figs. 2 to 5).
The dominance of lichens in a wide variety of habitats refl ects on their ability to tolerate extremes of
environmental conditions such as cold, desiccation, heat, UV radiations and visible light and other
harsh environmental conditions. High levels of UV radiations are frequently met within the Polar
and higher mountain regions and so the lichens occurring in these regions must be able to cope up
with these radiations. The differences in the physiology of green algae and cyanobacteria may be
useful in explaining the differences in the physiology of the photobionts of lichens. Cyanolichens
differ from green algal lichens in their photochemical apparatus in at least two important aspects
that make them more susceptible to high light levels. Cyanobacteria lack zeaxanthin-violoxanthin
cycle that acts as a screen, for high light intensity, present in green algae (Demming-Adams, 1990).
Secondly, PSII reaction center protein D1 has a lower resistance to photoinhibition in cyanobacteria
than green algal D1 protein (Clarke
et al
., 1993). Thus the cyanobionts of cyanolichens are weaker for
defense against excess light. Further, cyanobacteria and cyanolichens are not able to photosynthesize
without liquid water (Lange
et al
., 1986). Accordingly, light and availability of liquid water appear
to be crucial factors for the viability of cyanolichens. Combination of high wind speed and high
irradiance levels cause higher rates of evaporation leading to thallus desiccation. Combined effects of
desiccation and high irradiance will also increase risk for thallus damages (Demming-Adams, 1990;
Gauslaa and Solhaug, 1996). However, the generation of UV absorbing compounds, quenching of
toxic intermediates and damage repair are some of the strategies evolved by the lichens. Increased
production of lichen phenolics by the mycobiont in
Cetraria islandica
in response to UV-B radiation
exhibited no perceptible changes in chlorophyll and carotenoid concentrations (Bachereau and
Asta, 1997). Although a negative correlation between UV-B radiation and phenolics has been made
in certain lichens (Swanson and Fahselt, 1997), due to the absorption of UV-B and UV-C bands by
