Lichens
Lecturer: Asst. Prof. Dr. İsmail EKER
Lichen internal structure
An association of a fungus and a photosynthetic symbiont
resulting in a stable vegetative body having a specific structure
Chlorophyta or Cyanobacteria
+
Ascomycota or Basidiomycota or Deuteromycota
Symbiotic association
• Both partners gain water and mineral nutrients
mainly from the atmosphere, through rain and dust.
• Benefits for fungus are clear, it receives
– organic compounds as C and energy source
from algae or receives N source from
cyanobacteria
– O2 source
• Benefits for algae are less clear-cut
– Fungus produces substances that absorb water
which is provided to alga
– Protects algal cells from mechanical injury,
predation, and high light intensities
– Association allows alga to achieve a wider
distribution than if free-living and alga may use
the fungus to anchor to a substrate
– CO2 source
• Extended periods of high moisture lead to the
fungus killing the alga
CO2, H2O
↓
fungus
↑
O2, food
alga
↓
↑
CO2, H2O
fungus
General characteristics of Lichens
• Lichens are unique as a symbiotic relationship because they look and
behave quite differently from their component organisms. Lichens
are regarded as organisms in their own right and are given generic
and species names.
• about 30,000 species of lichen worldwide, covering 8% of the land
surface.
• Most are in the Ascomycota – ratio of lichenized to nonlichenized
Ascomycota is 14,000 : 15,000
• Ca. 23 genera of algae and 15 genera of cyanobacteria found in
lichens, 90% of lichens contain Trebouxia, Nostoc or one other genus
• Green algae – Trebouxia sp. is a common genus, found in 75% of
lichens in temperate zone. Trebouxia is not very successful as a freeliving alga
• Cyanobacteria – Nostoc sp. is a common
genus
• Classification–based on morphology of fungal
symbiont: Ascolichens and Basidiolichens
• Widely distributed – grow on soil, rocks, trees, marine or
intertida – cold to hot, arid to moist
• May exist where other organisms can’t – surface of desert
rocks, alpine, arctic, etc.
• When lichen thallus is wetted, absorbs water quickly by
gelatinous matrix in the cortex. As thallus dries, growth process
slows and stops
• Exhibit low growth rates – many grow at rates of 1-4 mm/yr, up
to 9 cm/yr
• Light – variable – some prefer low light intensities, others high
• Lichens are many times the first organism to settle in an area
with no soil
Thallus morphology
The "body" of a lichen is termed the
thallus, and its general shape is
used to group lichens into four
broad categories.
1. Fructicose (shrubby)
• branched, strap shaped or
threadlike thallus, upright or
hanging
Usnea sp.
(Ascolichenes)
2. Foliose (leafy)
• flattened branching
lobes loosely attached
to the substratum,
leaflike
• Have upper and lower
surfaces
Evernia sp.
(Ascolichenes)
Pseudoevernia sp.
(Ascolichenes)
Lobaria spp.
(Ascolichenes)
Xanthoria sp.
(Ascolichenes)
(Ascolichenes)
Cora sp.
(Basidiolichenes)
3. Crustose (crustlike)
• flattened, scalelike,
• No lower surface,
tightly bound to
substratum
Lecanora sp.
(Ascolichenes)
4. Squamulose
• intermediate
between foliose and
crustose
• Scales, lobes
smaller than in
foliose
Cladonia sp.
(Ascolichenes)
Reproduction
• Sexual reproduction – characteristic of fungal symbiont. Thought that after
ascospores germinate, they make contact with algal cells.
• Asexual reproduction – fragmentation (soredia - algal cells enveloped by hyphae)
Ascocarp of fungus
Fungal hyphae
Soredia
Algal layer
Fungal hyphae
Algal cell
Scanning electron micrograph (SEM) of a lichen.
The round structures are the fruiting bodies (apothecia),
which contain the reproductive spores of the fungi
Apothecia
Ecological importance
Air pollution
• Lichens can grow in extreme climate
conditions (there are lichens that survive from
-198 0C to 50 0C). 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
Economical importance
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)
• as food for many arctic animals, such as reindeer. Cladonia rangiferina
(Reindeer Lichen) is aeten by many arctic animals and sometimes by people.
A desert lichen, Lecanora (Manna Lichen) is collected and eaten in Libya.
• dyeing yarns and as acid-base indicators such as litmus (Roccella tinctoria
and Rocchella fuciforme (Litmus lichens)).
• Orcein, derived from a lichen, is used extensively in cytological preparations
for staining chromosomes.
• Synthetic dyes have now replaced lichen dyes for commercial and
laboratory use.
• Usnic acid derived from lichens (especially Usnea sp.) has antibiotic
properties againist a number of fungi and bacteria, including human
pathogens.
• Some lichens (Evernia prunastri (Oak lichen); Pseudoevernia furfuraceae
(tree lichen)) are the sourch of an oily substance used in perfume industry.
Mycorrhizae
Lecturer: Asst. Prof. Dr. İsmail EKER
Mycorrhizzae Symbiosis
• Mycorrhiza (plural, -ae or –as) - Greek - “mycos (fungus)” + “rhiza
(root)”
• Mycorrhizae is a symbiotic relationship with a plant’s roots and
a fungus that is primarily Mutualistic (benefits both) but can be
Pathogenic (causing harm to plant).
• Mycorrhizae is most useful in nutrient poor soil. Unlike normal roots,
Mycorrhizae are thinner and are generally more wide-spread (up
to 8m away!), which allows them to search for nutrients (P, Zn, Cu in
particular) and water more effectively.
• While Mycorrhizae has a far more effective method of gathering
nutrients and water for the plant, the fungi also has access to the
Glucose and Sucrose provided by the plant.
• Some mycorrhizae can protect roots from other fungi
• Common: 90% of plants do this! Few higher plants do not form
mycorrhizal associations, 10-20% including some aquatic vascular
plants and members of the Brassicaceae, Cyperaceae, and
Juncaceae.
How Do Mycorrhizae Function?
• Fungal hyphae release enzymes
(chitinase, peroxidase, cellulase,
protease) which allows them to
digest and penetrate substrates.
• Secretion of enzymes breaks down
tough organic substrates that can
then be absorbed and used by the
fungus and/or host plant as energy
and nutrient sources for growth and
reproduction.
Both members benefit
from their interaction.
Plants receive mineral
nutrients and water;
mycorrhizal
fungi
receive carbon and a
place to live.
Benefits to Plants
• Hyphae increase surface area of
roots for increased absorption of
soil nutrients. Increase water uptake
and aid drought resistance to plants
• Resistance to some root
pathogens due to thick hyphal mantle
• Increase plant tolerance to soil
temperature extremes, pH
extremes, toxic heavy metals, and
transplant shock
Benefits to World Agriculture
• Aid in plant establishment on
nutrient poor soils (mining
reclamation and revegetation projects)
• Increase plant size in short time
period (forestry)
• Reduce fertilizer requirements
• Cut down production costs
Plant on left grown without mycorrhizal fungi
Types of mycorrhizae
Endomycorrhizae (Vesicular
arbuscular mycorrhiza) – hyphae
penetrate cells of plant
Glomeromycetes form symbiotic
endomycorrhizae with plant roots:
supplying minerals to the roots and
obtaining carbohydrates in return.
Specialized hyphae (arbuscules)
perform this exchange by pushing in
the plasma membrane.
Ectomycorrhizae (ectotrophic,
sheathing) – hyphae of fungus do not
penetrate cells of plant root
1. Endomycorrhizae: Vesicular arbuscular mycorrhiza (AM)
(Arbuscular mycorrhizal fungi)
• All are in the Zygomycota in the
Glomales – or newly proposed phylum
Glomeromycota
• Include ca 130 species in 6 genera –
infects 300.000 plant species
• Found in species in all divisions of
terrestrial plants – widely distributed in
annuals, perennials, temperate and
tropical trees, crop and wild plants
• All are obligate biotrophs
• Form extensive network of hyphae even
connecting different plants
• Appear to be the most common type
of mycorrhizal association with
respect to the number of plant species
that form them
• Typically disintegrate after ca 2 weeks
in plant cell and release nutrients
Arbuscular mycorrhizal fungi
2. Ectomycorrhizae (EM)
• 2000 plant species – primarily temperate trees and
eucalyptus
• Over 5000 species of fungi have been shown to
form
ectomycorrhizae.
Basidiomycota
–
Agaricales (many mushroom species) and
Ascomycota – Pezizales – cup fungi and truffles
• Major species of coniferous and deciduous trees
• Rare to find uninfected trees
• In some trees, the association is obligate, in others
facultative
• Mycorrhizal association important in forestry
• Plant roots are enclosed by a sheath of fungal
hyphae – fungal mycelium penetrates between
cells in cortex of the root
• Fungal tissue may account for up to 40% mass of
root
• Hyphae also extend out into the soil –
extramatrical hyphae. Forms extensive network of
hyphae even connecting different plants
• Ectomycorrhizal root: contains a fungal sheath
Parenchyma of root cortex is surrounded by hyphae
Why mycorrhiza?
• Roots and root hairs
cannot enter the smallest
pores
Why mycorrhiza?
• Roots and root hairs
cannot enter the smallest
pores
• Hyphae is 1/10th
diameter of root hair
• Increased surface area
Root hair
Smallest hyphae
•Surface area/volume of a
cylinder:
SA/vol ≈ 2/radius
Why mycorrhiza?
Not inoculated
with
mycorrhizae
Inoculated with
mycorrhizae
• Roots and root hairs
cannot enter the smallest
pores
• Hyphae is 1/10th of root
hair
• Increased surface area
• Extension beyond
depletion zone
Why mycorrhiza?
• Roots and root hairs
cannot enter the smallest
pores
C – C – NH2 --> C – C + NH3
• Hyphae is 1/10th of root
hair
• Increased surface area
• Extension beyond
depletion zone
• Breakdown of organic
matter
Benefits of Mycorrhizae
• Tolerant of harsh conditions
– fungi are more tolerant of acidity, elemental toxicity and high soil
temperatures than are higher plants and able to, in some cases
(ectomycorrhizae), shield the root from these condition.
– Lower levels of heavy metals generally found in mycorrhizal plants than
nonmycorrhizal plants.
• Increased seedling survival
– Mycorrhiza promotes plant survival, whether new seedlings or outplanted container stock.
– Survival of inoculated plants can be up to five times the survival of
uninoculated plants.
– Improved survival is no doubt due to a combination of mycorrhizal
benefits, including faster growth to help overtop weeds, protection from
pathogens, and improved drought tolerance.
Mycorrhizal vs non-mycorrhizal plant
Mycorrhizal and non-mycorrhizal plant
Mycorrhizae and Plant diversity
• Biodiversity of belowground fungal symbionts increases biodiversity of
above ground plants
• Increased access to nutrients becomes restricted under competitive
conditions
• Differences in functional capacity of a specific fungus-plant combination
appear to explain the effect
• In ecosystems, increased functional capacity allows one plant species to
perform better than others
• Restored plant communities have been found to be more diverse when
mycorrhizal fungi are present when both inoculated and uninoculated areas
receive the same seed mix.
Mycorrhizae and Plant diversity
Basis for fungal species richness on plant biodiversity and production
No symbionts
One symbiont
Increasing diversity
Mycorrhizae and Plant diversity
Basis for fungal species richness on plant biodiversity and production
No symbionts
One symbiont
Two symbionts
Increasing diversity
Increasing productivity
Mycorrhizae and Plant diversity
Basis for fungal species richness on plant biodiversity and production
No symbionts
One symbiont
Two symbionts
Four symbionts
Increasing diversity
Increasing productivity
Mutualistic Nitrifying Cyanobacteria
Nitrogen fixation
•
•
•
Nitrogen: lots in atmosphere (79% N2), but plants can’t use that
Nitrogen fixation: Cyanobacteria and the other nitrification bacteria use N2 to form NH3
(ammonia) or NO3- (nitrate)
About 90% of nitrogen fixation is done by bacteria
• The aquatic fern Azolla sp. is the only fern
that can fix nitrogen. It does so by virtue of a
symbiotic association with a cyanobacterium
(Anabaena azollae).
• Nitrification cannot occur in the presence of
oxygen, so nitrogen is fixed in specialized cells
called heterocysts. These cells have an
especially thickened wall that contains an
anaerobic environment.
• This has been used to great advantage in
the cultivation of rice, where the floating fern
Azolla is actively distributed among the rice
paddies. The fern houses colonies of the
Anabaena in its leaves, where it fixes nitrogen.
The ferns then provide an inexpensive natural
fertilizer and nitrogen source for the rice plants
when they die at the end of the season.
Another cyanobacterium on the palm (Welfia regia) in
an epiphyllic relationship
It is believed that these bacteria transfer some %
of fixed N to the plants through the leaf surfaces