lichens

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)