Lichens

Lecturer: Asst. Prof. Dr. İsmail EKER

Lichen internal structureAn 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

↑O2, food

↓

↑CO2, H2O

CO2, H2O

↓

alga

fungus

fungus

• 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 free-living alga

General characteristics of Lichens

• 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 morphologyThe "body" of a lichen is termed the

thallus, and its general shape is used to group lichens into four broad categories.

• branched, strap shaped or threadlike thallus, upright or hanging

1. Fructicose (shrubby)

Usnea sp.(Ascolichenes)

• flattened branching lobes loosely attached to the substratum, leaflike

• Have upper and lower surfaces

2. Foliose (leafy)

Evernia sp.(Ascolichenes)

Pseudoevernia sp.(Ascolichenes)

Lobaria spp.(Ascolichenes)

Xanthoria sp.(Ascolichenes)

(Ascolichenes)

Cora sp.(Basidiolichenes)

• flattened, scalelike, • No lower surface,

tightly bound to substratum

3. Crustose (crustlike)

Lecanora sp.(Ascolichenes)

• intermediate between foliose and crustose

• Scales, lobes smaller than in foliose

4. Squamulose

Cladonia sp.(Ascolichenes)

Ascocarp of fungusFungal hyphae

Algal layerSoredia

Fungal hyphae

Algal cell

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)

Scanning electron micrograph (SEM) of a lichen. The round structures are the fruiting bodies (apothecia), which contain the reproductive spores of the fungi

Apothecia

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

Ecological importance

• 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.

Economical importanceA number of chemicals are only synthesized by the symbiosis – dyes, antibiotics, essential oils, litmus (over 600 different chemicals unique to lichens have been identified)

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.

Plant on left grown without mycorrhizal fungi

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

• 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

Benefits to World Agriculture

Types of mycorrhizae

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.

Endomycorrhizae (Vesicular arbuscular mycorrhiza) – hyphae penetrate cells of plant

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?

Root hair Smallest hyphae

• Roots and root hairs cannot enter the smallest pores

• Hyphae is 1/10th diameter of root hair

• Increased surface area

• Surface area/volume of a cylinder:

SA/vol ≈ 2/radius

Inoculated with mycorrhizae

Not inoculated with

mycorrhizae

Why mycorrhiza?

• 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

• Hyphae is 1/10th of root hair

• Increased surface area

• Extension beyond depletion zone

• Breakdown of organic matter

C – C – NH2 --> C – C + NH3

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 out-

planted 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

Basis for fungal species richness on plant biodiversity and production

No symbionts One symbiont Two symbionts Four symbionts

Increasing diversity

Increasing productivity

Mycorrhizae and Plant diversity

Mutualistic Nitrifying Cyanobacteria

• 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.

• 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

Nitrogen fixation

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