Lichens are a symbiotic association between a fungus and a photosynthetic symbiont such as an alga or cyanobacterium. This stable vegetative structure exhibits characteristics that neither organism has alone. Lichens are widely distributed and can withstand a variety of extreme environmental conditions. While resistant to natural extremes, lichens are extremely sensitive to air pollution like sulfur dioxide. The fungus provides structure and protection while obtaining organic compounds from the photosymbiont.

1. Lichens

2. Lichen
• An association of a fungus and a photosynthetic
symbiont resulting in a stable vegetative body
having a specific structure
• Unique entity formed by a fungus and a
photoautotroph (alga or cyanobacterium) that
has characteristics that neither has alone
• Widely distributed – grow on soil, rocks, trees,
marine or intertidal
• Variety of habitats – cold to hot, arid to moist
• Withstand environmental extremes
• May be dominant vegetation (e.g. tundra)

3. Lichens
• Are primary colonizers
in succession
• Involved in weathering
of rock and formation of
soil
• May exist where other
organisms can’t –
surface of desert rocks,
alpine, arctic, etc.

4. Air pollution
• Even though lichens are very resistant to
natural environmental extremes – they are
extremely sensitive to air pollution –
particularly SO2
• Obtain nutrients from atmosphere, not soil
• Both species composition and numbers of
thalli decline from edge to center of
industrialized areas
• Some are useful as indicator species

5. Classification
• Classified to species –based on
morphology of fungal symbiont
• Classification of the lichenized fungus
• Fungal symbiont never found free-living
• Are currently ca. 14,000 species of
lichenized fungi
• Most are in the Ascomycota – ratio of
lichenized to nonlichenized Ascomycota is
14,000 : 15,000

6. Fungal symbiont
• Most lichenized fungi are
Ascomycota – most form
apothecia, some form perithecia
and pseudothecia
• 12 orders include mostly
lichenized members
• Some are Basidiomycota –
Aphyllophorales, few Agaricales
• Some are Deuteromycota

7. Autotrophic symbionts
• Green algae – Trebouxia is a common
genus, found in 75% of lichens in
temperate zone
• Cyanobacteria – Nostoc is a common
genus
• Ca. 26 genera of algae and cyanobacteria
found in lichens, 90% of lichens contain
Trebouxia, Nostoc or one other genus
• Autotroph may be free living

8. Thallus morphology
• 4 basic types
• Fruticose – branched, strap shaped or
threadlike thallus, upright or hanging

9. Thallus morphology
• Foliose – flattened
branching lobes
loosely attached to
the substratum,
leaflike
• Have upper and lower
surfaces

10. Thallus morphology
• Crustose – flattened,
scalelike,
• No lower surface,
tightly bound to
substratum

11. Thallus morphology
• Squamulose – intermediate between
foliose and crustose
• Scales, lobes smaller than in foliose
• Intermediates exist

12. Internal structure
• Contain various zones or regions of fungal
tissue
• In most, the autotrophic symbiont is
restricted to a definite layer (in some, it is
dispersed throughout)
• Crustose thalli generally have simpler
structure than foliose or fruticose thalli
• May contain various fungal tissues and
cell types

13. Internal structure

14. Internal structure

15. Internal structure

16. Reproduction
• Sexual reproduction – characteristic of
fungal symbiont
• Ascospores are discharged, algal cells are
not discharged with them
• Thought that after ascospores germinate,
they make contact with algal cells
• Asexual reproduction – variety of
mechanisms – e.g. fragmentation

17. Asexual reproduction
• Specialized structures
• Soredia - algal cells
enveloped by hyphae,
no cortex, form
powdery masses on
surface of thallus,
detach from thallus
• Isidia – column like
structures with cortex

18. Isidia

19. Physiology
• Autotrophic associations – algal cells carry out
photosynthesis, lichen depends on net
production of organic compounds by
photosynthesis
• Most of the photosynthate (70-80%) produced
by alga is incorporated into the fungus
• Green algae secrete polyalcohols like ribitol,
cyanobacteria secrete glucose
• Photobiont becomes leaky of carbohydrates
when associated with fungus – not so when
grown alone

20. Growth
• Exhibit low growth rates – many grow at
rates of 1-4 mm/yr, up to 9 cm/yr
• Makes studies difficult
• Factors affecting growth
• Light – variable – some prefer low light
intensities, others high
• Temperature – variable
• Moisture – appears to be an important
variable, do not have water absorbing
organs, depend on moisture in air

21. Moisture
• When lichen thallus is wetted, absorbs water
quickly by gelatinous matrix in the cortex
• Starts growth process
• As thallus dries, growth process slows and
stops
• Dew and humidity are important sources of
moisture
• Thalli are inactive when dry – only grow
when wetted – may be responsible for slow
growth rate

22. Separation of symbionts
• Fungal symbionts grown in culture exhibit
slow growth rates (1-2 mm/yr)
• Many exhibit requirements for vitamins
• Algae also grow slowly in culture,
Trebouxia prefers organic N and low light

23. Resynthesis of lichens
• Difficult process – requires extreme
environmental conditions to maintain
• Periodic drying and low nutrients
• Extended periods of high moisture lead to
the fungus killing the alga

24. Symbiotic association
• Traditionally been classified as a
mutualistic symbiosis where both
symbionts benefit
• Fungus appears to be chief benefactor,
receives
– Organic compounds as C and energy source
– With cyanobacteria, N fixation may occur so
that the fungus also receives N source

25. Symbiotic association
• Benefits for autotrophic symbiont are less
clear-cut
– Fungus produces substances that absorb
water which is provided to alga
– Fungus takes up inorganic nutrients from
atmosphere
– Protects algal cells from mechanical injury,
predation, and high light intensities
– Association allows alga to achieve a wider
distribution than if free-living

26. Symbiotic association
• Trebouxia is not very successful as a free-
living alga
• Alga pays a high price for these benefits
• Lichen might be a better example of
controlled parasitism than mutualism
• Lichens are long term, close symbioses
• Together the organisms express different
characteristics than individually

27. Symbiotic association
• Sexual reproduction by
fungus only occurs in the
association
• A number of chemicals are
only synthesized by the
symbiosis – dyes,
antibiotics, essential oils,
litmus (over 600 different
chemicals unique to lichens
have been identified)