Some types of

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.fix nitrogen from the atmosphere. This process relies on the presence of

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

as a partner species within the lichen. The ability to fix nitrogen enables lichen to live in nutrient-poor environments. Lichen can also extract nitrogen from the rocks on which they grow. Nitrogen fixation, and hence the abundance of lichen and their host plants, may be decreased by application of nitrogen-based agricultural

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

and by atmospheric pollution.

The nitrogen cycle

The

nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biologi ...

is one of the Earth's

biogeochemical cycle A biogeochemical cycle (or more generally a cycle of matter) is the pathway by which a chemical substance cycles (is turned over or moves through) the biotic and the abiotic compartments of Earth. The biotic compartment is the biosphere and the ...

s. It involves the conversion of nitrogen into different chemical forms. The main processes of the nitrogen cycle are the fixation, ammonification, nitrification, and denitrification. As one of the macronutrients, nitrogen plays an important role in plant growth. The nitrogen cycle is affected by environmental factors. For example, in the subarctic heath, increase in temperature can cause nitrogen fixation to increase or decrease based on season, while overall climate warming indirectly caused the vegetation change which in turn affected the nitrogen fixation process.

Lichens

Lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.heath A heath () is a shrubland habitat found mainly on free-draining infertile, acidic soils and characterised by open, low-growing woody vegetation. Moorland is generally related to high-ground heaths with—especially in Great Britain—a cooler ...

. While it does not occur often, formation of

akinete An akinete is an enveloped, thick-walled, non-motile, dormant cell formed by filamentous, heterocyst-forming cyanobacteria under the order Nostocales and Stigonematales. Akinetes are resistant to cold and desiccation. They also accumulate and stor ...

s (type of cell formed by cyanobacteria which are resistant to cold and desiccation) was observed in nitrogen fixing lichen. Depending on its partner, lichens derive the carbon and nitrogen from algal and cyanobacteria

photobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. Lichen fungi can fix nitrogen during the day and night, as long the dark period is not too long.

Nitrogen-fixing and non-nitrogen-fixing lichens

Both nitrogen-fixing lichens and non-nitrogen-fixing lichens take up nitrogen from the environment as a nutrient. Both type of lichens secrete many different organic compounds to absorb minerals from the substrates. Main difference between nitrogen fixing lichen and non-nitrogen fixing lichen is their photosynthetic partner: nitrogen fixing lichen partner with cyanobacteria which can fix nitrogen from the air, while green alga, partner of non nitrogen fixing lichen, does not perform the same process. The nitrogen fixation is energetically costly due to chemical transformation and only about 10% of lichen are partnered with cyanobacteria. In agricultural regions, non nitrogen fixing lichen reflect uptake of ammonia emission indicating that it have lower nitrogen value. Some lichens such as Placopsis gelada contain both nitrogen fixing

phototroph Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a common misconcep ...

s and non nitrogen-fixing phototrophs in which

Nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in vari ...

(cyanobacteria, the phototrophic nitrogen fixer) was dwelling within cephalodia (small gall like structure within lichen; contains cyanobacteria symbionts). In such cases,

heterocyst Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as '' Nostoc punctiforme'', ''Cylindrospermum stagnale'', and ''Anabaena sphaerica''. They fix nitrogen fr ...

differentiation was greater in cephalodia when compared to having Nostoc as the primary symbionts in lichens, showing that, in the presence of non nitrogen-fixing phototroph, Nostoc specialize for nitrogen fixation.

Interaction with lichen

Response to nitrogen and phosphorus

A lichen's response to nutrient enrichment depends on not only on species and environmental factors but also partially on

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms wer ...

concentrations of nutrients such as nitrogen and phosphorus. Ammonium, nitrate and organic nitrogen can be assimilated by lichen along with phosphorus as an important stimulant for

cyanolichen Cyanolichens are lichens that apart from the basic fungal component ("mycobiont"), contain cyanobacteria, otherwise known as blue-green algae, as the photosynthesizing component ("photobiont"). Overall, about a third of lichen photobionts are cyan ...

s. The photobiont will become less dependent on fungal nutrient supply when nitrogen deposition increases as it will be able to access its own nitrogen and it will stimulate the photobiont, causing it to build up, resulting in increased photosynthesis which increases carbon input. However, for lichens that cannot increase their photobiont growth, nitrogen deposition can be damaging due to higher nitrogen concentration than their biological requirements. Generally, when a lichenized algal cell is nitrogen limited, the addition of nitrogen caused the growth of algal cells. Under nitrogen limiting condition, chlorophyll concentration was positively correlated with the growth of algal cells indicating that should the concentration of chlorophyll increase, the photobiont population will also increase. As lichens absorb nitrogen through fixation, it will have a very strong negative reaction if the nitrogen availability changes, indicating its sensitivity to environmental changes. According to the experiment by Sparrius et al., when nitrogen fertilizer was added into the soil, lichen cover was reduced by ~50%, while the addition of phosphorus showed opposite result. In the region such as

boreal forest Taiga (; rus, тайга́, p=tɐjˈɡa; relates to Mongolic and Turkic languages), generally referred to in North America as a boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces, ...

, where nitrogen and phosphorus are limiting nutrients and for symbiotic interaction to occur properly, their ratio must be balanced. General pollution of climate that is indicated by the concentration of

nitrogen oxide Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or n ...

s can also affect the growth of lichen. When compared to

bryophyte The Bryophyta s.l. are a proposed taxonomic division containing three groups of non-vascular land plants (embryophytes): the liverworts, hornworts and mosses. Bryophyta s.s. consists of the mosses only. They are characteristically limited in ...

(non-vascular land plant), which is also sensitive to nitrogen fertilizer, lichen showed a much stronger response.

Nitrogen metabolism

There are many different species of lichens and each has its own way of allocating nitrogen. The non nitrogen fixing lichen invests a large amount of nitrogen into photosynthetic tissue, whereas nitrogen fixing lichen will invest into the fungal tissue. Nitrogen-fixing lichen species can only attain a certain amount of nitrogen, as the addition of ammonium decreases its rate of nitrogen-fixation, which decreases the amount of nitrogen that is exported into the adjacent

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

e. Nitrogen fixation is energy dependent and very costly for lichens. In a region where nitrogen deposition is high, lichens have a lower uptake of nitrogen in comparison to the Antarctic green algal lichen, which takes up 90% of nitrogen deposition in both

nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...

and

ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...

form. Some lichen species are able to refrain from assimilating excessive amount of nitrogen in order to maintain a balanced tissue concentration. Majority of lichen species absorbs more NH4+ than NO3- and the impact of temperature on the rate of fixation is "consonant to the normal enzymatic kinetics of them".

Effects of nitrogen fixation

Nitrogen fixing lichens actively fix atmospheric nitrogen using the

nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in vari ...

, located in the cephalodia. Lichens are sensitive to nitrogen availability. Upon nitrogen fixation, there will be an increase of algal cell growth, chlorophyll concentration, and photobiont population. While costly, in regions where nitrogen availability is low, fixation process is the main way for the lichen to absorb nitrogen which is

macronutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

(essential nutrient).

Importance

Ecology

Nitrogen, as a macronutrient and a biogeochemical cycle, also affects the ecology. Through the nitrogen cycle, it breaks down into the chemical form that allows plants to absorb as nutrients. There are certain regions in the world that most plants cannot live due to harsh environments as well as lack of nutrients such as nitrogen. That means that in some regions, the biogeochemical cycle (including nitrogen cycle and

carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...

) is unlikely to run smoothly. Lichen is able to absorb nitrogen in multiple forms from soil, rock, and air, taking a part in carbon cycle at the same time. Even though only a small fraction of lichens have the ability to fix nitrogen, it helps the lichen to spread throughout the world and survive even in the harsh environment. The industrial nitrogen fertilizer greatly affected the vegetation and agriculture throughout the world, resulting significantly increased the amount of food with better quality, but it has a negative impact on ecology in the long run. Deposition of nitrogen causes soil acidification, and the nitrogen in the fertilizer are often leached through soil and water, running off the different area. Soil acidification increases toxicity of the soil which reduces plant biodiversity and based on the toxic level of soil acidification, heavy metal such as aluminum and iron can be related to soil water.

Rock and soil

Earth's mantle contains non-atmospheric nitrogen in the form of rocks and in the soil. Weathering of the rocks and stone are normally caused by physical, chemical and biological processes. Plants cannot absorb nitrogen from rocks, but fungi can. Fungi within lichens can extract nutrients from mineral surfaces by secreting organic acids. The organic acids (e.g. phenolic acids) are important in solubilizing nutrients from inorganic substrates. A study was conducted to test rock phosphate solubilization by lichen-forming fungi. Bacteria that were attached to biotic or abiotic surfaces stimulate exopolysaccharide synthesis. While lichens have the ability to absorb nitrogen from rock, this only accounts for a small portion of the nitrogen cycle compared to the conversion of atmospheric nitrogen as it is more easily available.

Effects on vegetation

Photobionts will become less dependent on fungal nutrient supply when nitrogen deposition increases, as it will be able to access its own nitrogen, and primary producers' nutrient limit will also be reduced. Nitrogen is one of the more limiting nutrients and the addition of nitrogen stimulates the photobiont, building up its cell, which subsequently increases its photosynthesis and its carbon input. Multiple nitrogen compounds can be assimilated by lichens, such as NH₄⁺, NO₃⁻ and organic nitrogen compounds. Nitrogen deposition reduces the nutrient limitation of primary production. Increase in nitrogen deposition will allow the photobiont to access its own nitrogen which makes it less fungal dependent but only up to certain point. Depending on the environmental nitrogen availability, the addition of nitrogen can either increase and decrease the growth of the lichen. If the lichen cannot increase its photobiont growth, high nitrogen uptake may result in a higher concentration than it physiologically requires which will negatively affect the lichen and its host plant as the other nutrients are too limiting. Lichen's response to nutrient enrichment is both species-specific and dependent on environmental factors such as nutrient concentration, light availability and water supply.

Nitrogen stress

Lichen is nitrogen sensitive and change in nitrogen availability can affect its health greatly. Two main nitrogen stress factors for lichens are nitrogen deficiency and high nitrogen deposition. Both types of nitrogen stress result in the reduction of the rate of thallus expansion in lichen. Nitrogen stressed lichen did not show a significant change in chitin:chlorophyll ratios, but

ergosterol Ergosterol (ergosta-5,7,22-trien-3β-ol) is a sterol found in cell membranes of fungi and protozoa, serving many of the same functions that cholesterol serves in animal cells. Because many fungi and protozoa cannot survive without ergosterol, the ...

concentration showed significant increase indicating a higher demand on the respiratory system. According to an experiment, the ammonium toxicity due to nitrogen deposition reduced the vitality of lichen greatly at different regions such as inland dunes, boreal conditions, and subarctic heaths.

References

{{reflist Lichenology Nitrogen cycle