Lichens

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)

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.

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

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

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

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

Thallus morphology• 4 basic types• Fruticose – branched, strap shaped or

threadlike thallus, upright or hanging

Thallus morphology

• Foliose – flattened branching lobes loosely attached to the substratum, leaflike

• Have upper and lower surfaces

Thallus morphology• Crustose – flattened,

scalelike, • No lower surface,

tightly bound to substratum

Thallus morphology

• Squamulose – intermediate between foliose and crustose

• Scales, lobes smaller than in foliose

• Intermediates exist

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

Internal structure

Internal structure

Internal structure

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

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

Isidia

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

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

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

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

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

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

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

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

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)