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= Lichens and Nitrogen Cycling =

Lichens and Nitrogen Cycle
The nitrogen cycle is one of the Earth’s biogeochemical cycle that 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. Both nitrogen-fixing lichens and non-nitrogen-fixing lichens absorb the nitrogen as one of the nutrients. Different environments have different factors that contribute or affect the nitrogen cycle. For example, in the subarctic heath, increased temperature can cause N2 fixation to fluctuate based on the season while overall climate warming indirectly caused the vegetation change which in turn affected of the N2 fixation process.

Lichens are symbiotic organisms that play an important role in the biogeochemical cycle on Earth. Lichen's characteristic, such as strong resistance to factors such as desiccation, ability to grow and break down rocks allow lichen to grow in different types of environment including highly nitrogen limited area such as subarctic health. While it does not occur often, formation of the akinetes (a type of cell formed by cyanobacteria which are resistant to cold and desiccation) was observed in N2-fixing lichen. Depending on its partner, lichens derive the carbon and nitrogen from algal and cyanobacteria photobionts (which fixes N2 from the air). Lichen fungi can fix dinitrogen during the day and night, as long the dark period is not too long.

Nitrogen-Fixing Lichens vs. Non-Nitrogen-Fixing Lichens
Both N2-fixing lichens and non-N2-fixing lichens take up nitrogen from the environment as a nutrient. Both types of lichen secrete many different organic compounds to absorb minerals from the substrates.

The main difference between N2-fixing lichen and non-N2-fixing lichen is their photosynthetic partner. N2-fixing lichen is partnered with cyanobacteria which can fix N2 from the air while green alga, partner of non-N2-fixing lichen, can't 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 the agricultural region, non-N2 fixing lichen reflects uptake of NH3 emission indicating that it has lower nitrogen value.

Some lichens such as Placopsis gelada contains both N2-fixing phototrophs and non-N2-fixing phototrophs in which Nostoc (cyanobacteria, the phototrophic N2 fixer) was dwelling within cephalodia (small gall like structure within lichen; contains cyanobacteria symbionts). In such cases, heterocyst differentiation was greater in cephalodia when compared to having Nostoc as the primary symbionts in lichens, showing that, in the presence of non-N2-fixing phototroph, Nostoc specialize for N2 fixation.

Response to Nitrogen and Phosphorus
Lichen's response to nutrient enrichment depends on not only on species and environmental factors but also partially on thallus concentrations of nutrients such as nitrogen and phosphorus.

Ammonium, nitrate and organic nitrogen can be assimilated by lichen along with phosphorus as important stimulants for cyanolichens. Photobiont will become less dependent of fungal nutrient supply when nitrogen deposition increases as it will be able to access its own nitrogen and it will stimulate photobiont, causing it to build up. Thus, resulting in the increase in 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 is positively correlated with the growth of algal cells indicating that the concentration of chlorophyll and the photobiont population should 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 an opposite result. In a region such as boreal forest, 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 NOx can also affect the growth of lichen. When compared to bryophyte (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-N2-fixing lichen invests a large amount of nitrogen into photosynthetic tissue, whereas N2-fixing lichen will invest into the fungal tissue. N2-fixing lichen species can only attain a certain amount of nitrogen as the addition of ammonium decreases its rate of N2-fixation, which decreases the amount of nitrogen that is exported into the adjacent hyphae. N2-fixation is energy dependent and very costly for lichens. The lichens at the region with high nitrogen deposition, have a lower uptake of nitrogen in comparison to the Antarctic green algal lichen, which takes up 90% of nitrogen deposition in both nitrate and ammonium 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
N2-fixing lichens actively fixes atmospheric nitrogen using the Nostoc, 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 (essential nutrient).

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 can not live due to harsh environments as well as lack of nutrients such as nitrogen. This means that in some regions, the biogeochemical cycle (including nitrogen cycle and carbon cycle) is unlikely to run smoothly. Lichen is able to absorb nitrogen in multiple forms from soil, rock, and air, and taking a part in carbon cycle at the same time. Even though, the only 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 amount of food with better quality, but it has a negative impact on ecology in the long run. Deposition of nitrogen causes the soil acidification and the nitrogen in the fertilizer to often leached through soil and water, running off to different areas. Soil acidification result an increased 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 however, 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 rocks, 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 Plants/Vegetation
Photobiont 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 stimulate 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 a certain point.

Depending on the environmental nitrogen availability, the addition of nitrogen can either increase and/or 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 it's host plant as the other nutrients are too limiting.

Lichen’s response to nutrient enrichment is both species-specific and depends on its environmental factors: 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 the significant change in chitin:chlorophyll ratios, but ergosterol concentration showed significantly increased indicating a higher load of respiratory.

The Haber-Bosch process is the main industrial procedure for nitrogen fertilizer (ammonia) production today which is used for the agricultural purpose and it is stated that more than 5 billion people owe their existence to it. It has increased the food production and the farmers no longer have to sacrifice crop yield for nitrogen application not have to use rotating sequence. However, nitrogen deposition cause soil acidification as well as nitrogen leaching. In the long term, use of nitrogen fertilizer causes the soil to acidify which reduce the crop performance due heavy metal toxicity and reduction of other nutrients. Leached nitrogen can also travel to other regions via water which will contaminate the water and affect plant biodiversity in the nearby areas such as forest or empty fields where lichens are likely to be present.

According to the study, the ammonium toxicity due to nitrogen deposition reduced the vitality of lichen greatly at different regions such as inland dunes, boreal conditions, and Antarctic heaths.