Acid rain

 

Acid rain is one of the world’s major environmental problems since 19th century. Fossil fuels such as coal burning is the major cause of SO₂ production and also vehicle emission and various fossil fuel based power generation emits nitric oxides. Both SO₂ and NO₂ produces sulphuric and nitric acid respectively by reacting with atmospheric water vapour and precipitate as wet deposition such as rain, snow and fog and dry deposition including hazardous particles of PM 2.5 ( Fine (smaller) particles PM_2.5, are more dangerous because they can get into the deep parts of lungs or even into blood.). In dry deposition sulphate and nitrate ions fall as small particles without dissolving in water, about 20-60 % of the total deposition is dry deposition.

Acid rain affects forest trees causes yellowing and leaf fall, acidified rivers and lakes causes fish death, loss of calcareous shell forming species (mollusks). It also affects soil microorganisms causes increased nitrification which also leads to eutrophication in water bodies and changes in the biodiversity. Acid rain destroys the coral reefs. It causes leaching of metal ions including toxic Aluminum and heavy metals such as chromium, cadmium and nickel, which adversely affects the soil micro flora and aquatic biota. Acid rain deteriorates the marble, stone monuments and architectures, corrode metal structures and fading paints.

The effect of acidification has been sighted all over the world such as deleterious ecological effects such as reduced reproduction of aquatic fish species, dieback and stunted growth in plants, accumulation of toxic aluminum and heavy metals in soil and water bodies, biodiversity loss including corals and shellfish, degrade to the manmade structures made up of marble and stone and corrosion of metal structures.

Causes

Acid rain as discussed is caused by emission of SO₂ and NO₂ from various sources to the atmosphere which dissolve in atmospheric water and produce acids in the rain water. SO₂ does not react much in the atmospheric chemicals but it can travel quicker to long distances and when get contact with ozone or hydrogen peroxide it produces SO₃, which is highly soluble in water and form sulphuric acid. Sulphur dioxide is naturally produced by volcanic eruptions, sea spray, planktons, rotting vegetation and forest fires.

Anthropogenic sources - 69.4 % of Sulphur dioxide released from industrial combustion (point sources), house hold heating of fire wood and coal (area or non-point sources) and 3.7% from transportation (mobile sources). Coal burning sources such as coal power plants, coal powered engines in vehicles, smelting of metal ore, production of iron and steel, oil refinery, domestic and industrial boilers, during the manufacture of sulphuric acid using in the production of disinfectants, bleaching agents and fumigants. NO₂ is naturally produced by lightening, bacterial action, forest fire and volcanoes, manmade emission are by automobiles (43%) and fertilizer industries, and other industrial combustion (32%).

Effects of acid rain can either chronic or episodic

Chronic acidification is a longterm effect due to years of acid rain, Episodic acidification is due to heavy rain storms or when snow melts. Acid rain increases nitrate levels in soil, leading to nitrogen saturation in soils. Nitrate ions remove additional calcium and magnesium from soil, excess nitrogen  also leads to eutrophication in water bodies. Trees starve for aluminum and other minerals as aluminum of soil get converted to aluminum nitrate or sulphate when get absorbed by trees cause harmful effects.

Effects on surface waters

Acid rain releases aluminum from the soil into lakes and streams which is toxic to many aquatic organisms. Acidification increases the release of aluminum from granite rocks. Acidic condition together with toxicity of heavy metals such as ions of copper, cadmium, nickel, chromium, cobalt, lead and zinc in the water body reduces the development and growth of the fish. This make the fish less immune, thus become more susceptible to diseases, kills the eggs and larval stages, reduces reproductive success.

Acidification effects shell forming molluscs, shell fish, coral reefs, and juvenile stages of aquatic organisms. In case of shell fish and corals their calcareous shell or skeleton get dissolved in acidic environment. Reduced pH encourages the growth of acid tolerant forms such as some bacteria and protozoa.

Acid rain is not the sole cause of acidification, some swamps, bogs and marshes naturally have low level of pH.

Nitrogen dioxide deposition in water bodies is another major reason for episodic acidification, about 10- 45 % of the nitrogen dioxide reaching water bodies are airborne and they are released to atmosphere mainly from anthropogenic sources.

 

Effects on forest

Acid precipitation on vegetation reduces the photosynthesis and growth also increase the susceptibility to draught and disease, process called ‘dieback’ it causes browning of leaf and fall off. It can cause thinning of annual growth ring and reduction in biomass (due to reduced growth), damage the fine root system, affect root mycorrhiza (due to increase in Aluminium and acidity) and decrease the lichens, reduction of soil fertility and causes loss of chlorophyll. Young seed lings are more susceptible than older plants

Effects to manmade structures

Nitric acid, sulphurus and sulphuric acid concentrated in dew or rain deposited on automotive coating causes fading of the paint, thus the modern vehicle manufactures are coating with acid resistant top paint and modern buildings are painted with acid resistant exterior wall paints. Metal such as bronze and alloy structures get corroded, acid also degrade marble (limestone) architectures.

Visibility impairment

Acid fog particularly particles of suphur dioxide and sulphur trioxide reduces the visibility. SO₂ & SO₃ increase the incidence of asthma and emphysema. Particulate deposition of particles less than PM 2.5 can even reach the blood stream via lungs and cause harmful effects such as lung cancer.

To overcome the effects of acid rain

Liming Lime stone is added to neutralize the acid in the water body; it also facilitates the release of locked nutrients of the acidified mud bottoms by neutralizing the ions. Essential nutrients such as phosphorus and other limiting minerals get released and thereby planktons and plant productivity get increased. In addition, it also reduces the toxic effect of heavy metals which are normally high in acidified waters. The calcium in lime supports the mollusks population in developing their calcareous exoskeleton. As calcium and phosphorus are essential plant nutrients, liming enhances the primary production and subsequently the entire community of the water system.

Soil acidity can be overcome by addition of lime, whereas alkalinity of limestone neutralizes the negative ions in acid

 

Reduce acid rain 

This can be done either fuel switching or scrubbing. Fuel switching includes limiting the use of Sulphur containing fuels such as coal or switching to low sulphur containing coal or oil, switching to alternative energy sources such as using gas boilers instead of coal or oil boilers, nuclear power generation, using renewable energy sources such as wind, air, wave and geothermal energy. Use solar batteries, fuel cells, natural gas and electric motor vehicles. Using public transportation, maintaining the vehicle for low NO₂ emission. Use energy efficient boilers and using filters or scrubbers to catch the oxides of sulphur and Nitrogen in industrial effluents and vehicles.

 Scrubbing includes use of electrostatic precipitators where positively charged sulphur particles are get attracted by negatively charged plate or chemicals

 

Microbial Nutrition -requirements and modes of nutrition

 

Microbial cells are structurally complex and carry out numerous functions. Nutrients are substances used in biosynthesis and energy release and therefore are required for microbial growth.In order to construct new cellular components and do cellular work, organisms must have a supply of raw materials or nutrients and a source of energy.

COMMON NUTRIENT REQUIREMENTS

Over 95% of microbial cell dry weight is made up of a few major elements: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and iron. These are called macroelements or macronutrients because they are required by microorganisms in relatively large amounts. 

The first six (C, O, H, N, S, and P) are components of  carbohydrates, lipids, proteins, and nucleic acids.

The remaining four macroelements exist in the cell as cations and play a variety of roles. For example, potassium (K⁺) is required for activity by a number of enzymes, including some of those involved in protein synthesis. Calcium (Ca2⁺), among other functions, contributes to the heat resistance of bacterial endospores. Magnesium (Mg2⁺) serves as a cofactor for many enzymes, complexes with ATP, and stabilizes ribosomes and cell membranes. Iron (Fe2⁺and Fe3⁺) is a part of cytochromes and a cofactor for enzymes and electron-carrying proteins.

In addition to macroelements, all microorganisms require several nutrients in small amounts. These are called micronutrients or trace elements. The micronutrients—manganese, zinc, cobalt, molybdenum, nickel, and copper—are needed by most cells. However, cells require such small amounts that contaminants from water, glassware, and regular media components often are adequate for growth. In nature, micronutrients are ubiquitous and probably do not usually limit growth. 

Micronutrients are normally a part of enzymes and cofactors, and they aid in the catalysis of reactions and maintenance of protein structure. For example, zinc (Zn2⁺) is present at the active site of some enzymes but can also be involved in the association of regulatory and catalytic subunits.  (e.g., E. coli aspartate carbamoyl transferase). Manganese (Mn2⁺) aids many enzymes that catalyze the transfer of phosphate groups. Molybdenum (Mo2⁺) is required for nitrogen fixation, and cobalt (Co2⁺) is a component of vitamin B12.

Besides the common macroelements and trace elements, microorganisms may have particular requirements that reflect their specific morphology or environment. 

Diatoms need silicic acid (H₄SiO₄) to construct their beautiful cell walls of silica [(SiO₂)n].

Although most procaryotes do not require large amounts of sodium, many archaea growing in saline lakes and oceans depend on the presence of high concentrations of sodium ion (Na⁺).

Finally, it must be emphasized that microorganisms require a balanced mixture of nutrients. If an essential nutrient is in short supply, microbial growth will be limited regardless of the concentrations of other nutrients.

  

REQUIREMENTS FOR CARBON, HYDROGEN, OXYGEN, AND ELECTRONS

All organisms need carbon, hydrogen, oxygen, and a source of electrons. Carbon is needed for the skeletons or backbones of all the organic molecules from which organisms are built. Hydrogen and oxygen are also important elements found in organic molecules. Electrons are needed for electron transport chain and for other oxidation-reduction reactions which can provide energy for use in cellular work. Electrons are also needed to reduce molecules during biosynthesis (e.g., the reduction of CO₂ to form organic molecules).

The requirements for carbon, hydrogen, and oxygen often are satisfied together because molecules serving as carbon sources provide hydrogen and oxygen also. Heterotrophs—organisms that use reduced, preformed organic molecules as their carbon source—can also obtain hydrogen, oxygen, and electrons from the same molecules. These organic carbon sources also provide electrons to be used in electron transport as well as in other oxidation-reduction reactions, so heterotrophs use their carbon source as an energy source also. The more reduced the organic carbon source (i.e., the more electrons it carries), the higher its energy content. Thus, lipids have a higher energy content than carbohydrates.

However, one carbon source, carbon dioxide (CO₂), supplies only carbon and oxygen, so it cannot be used as a source of hydrogen, electrons, or energy. This is because CO₂ is the most oxidized form of carbon, lacks hydrogen, and is unable to donate electrons during oxidation-reduction reactions. Organisms that use CO₂ as their sole or principal source of carbon are called autotrophs. Because CO₂ cannot supply their energy needs, they must obtain energy from other sources, such as light or reduced inorganic molecules.

Heterotrophic microorganisms have great flexibility with respect to carbon sources. There is no naturally occurring organic molecule that cannot be used by some microorganism. Actinomycetes, common soil bacteria, will degrade amyl alcohol, paraffin, and even rubber. Some bacteria can use almost anything as a carbon source; for example, Burkholderia cepacia can use over 100 different carbon compounds. Microbes can degrade even relatively indigestible human-made substances such as pesticides. This is usually carried out by complex microbial communities (microbial consortia). These molecules sometimes are degraded in the presence of a growth-promoting nutrient that is metabolized at the same time—a process called cometabolism. Other microorganisms can use the products of this breakdown process as nutrients. But some microbes are extremely fastidious and catabolize only a few carbon compounds. Cultures of methylotrophic bacteria metabolize methane, methanol, carbon monoxide, formic acid, and related one-carbon molecules. Parasitic members of the genus Leptospira use only long-chain fatty acids as their major source of carbon and energy.

REQUIREMENTS FOR NITROGEN, PHOSPHORUS, AND SULFUR

Microorganisms require large quantities of nitrogen, phosphorus, and sulfur for growth. These elements may be acquired from the same nutrients/organic sources that supply carbon, or from other inorganic sources.

Nitrogen is needed for the synthesis of amino acids, purines, pyrimidines, some carbohydrates and lipids, enzyme cofactors, and other substances. Many microorganisms can use the nitrogen obtained in amino acids. Others incorporate ammonia directly through the action of enzymes such as glutamate dehydrogenase or glutamine synthetase and glutamate synthase. Most phototrophs and many chemotrophic microorganisms reduce nitrate to ammonia and incorporate the ammonia in a process known as assimilatory nitrate reduction. A variety of bacteria (e.g., many cyanobacteria and the symbiotic bacterium Rhizobium) can assimilate atmospheric nitrogen (N₂) by reducing it to ammonium (NH₄). This is called nitrogen fixation.

Phosphorus is present in nucleic acids, phospholipids, nucleotides like ATP, several cofactors, some proteins, and other cell components. Almost all microorganisms use inorganic phosphate as their phosphorus source and incorporate it directly. Low phosphate levels can limit microbial growth in many aquatic environments. Some microbes, such as Escherichia coli, can use both organic and inorganic phosphate. Some organophosphates such as hexose 6-phosphates can be taken up directly by the cell. Other organophosphates are hydrolyzed in the periplasm by the enzyme alkaline phosphatase to produce inorganic phosphate, which then is transported across the plasma membrane.

Sulfur is needed for the synthesis of substances like the amino acids cysteine and methionine, and some carbohydrates, biotin, and thiamine. Most microorganisms use sulfate as a source of sulfur and reduce it by assimilatory sulfate reduction. A few microorganisms require a reduced form of sulfur such as cysteine.

 GROWTH FACTORS

Some microorganisms have the enzymes and biochemical pathways needed to synthesize all cell components using minerals and sources of energy, carbon, nitrogen, phosphorus, and sulfur. Some lack one or more of the enzymes needed to manufacture some indispensable constituents. So, they must obtain these constituents or their precursors from the environment. Organic compounds that are essential cell components or precursors of such components but cannot be synthesized by the organism are called growth factors. There are three major classes of growth factors:

(1) amino acids, (2) purines and pyrimidines, and (3) vitamins.

Amino acids are needed for protein synthesis; purines and pyrimidines for nucleic acid synthesis. Vitamins are small organic molecules that usually make up all or part of enzyme cofactors and are needed in only very small amounts to sustain growth.

 

Functions of Some Common Vitamins in Microorganisms
Vitamin	Functions	Microorganisms Requiring Vitamin
Biotin	Carboxylation (CO2 fixation) One-carbon metabolism	Leuconostoc mesenteroides Saccharomyces cerevisiae
Cyanocobalamin (B12)	Molecular rearrangements	Lactobacillus spp.
Folic acid	One-carbon metabolism	Enterococcus faecalis
Pyridoxine (B6)	Amino acid metabolism (e.g., transamination)	Lactobacillus spp.
Niacin (nicotinic acid)	Precursor of NAD and NADP	Haemophilus influenzae
Riboflavin (B2)	Precursor of FAD and FMN	Dictyostelium spp

 

Some microorganisms require many vitamins; for example, Enterococcus faecalis needs eight different vitamins for growth.

Other growth factors required are; heme (from hemoglobin or cytochromes) is required by Haemophilus influenzae, and some mycoplasmas need cholesterol.

Understanding the growth factor requirements of microbes has significant practical applications. Microbes with known, specific requirements and those that produce large quantities of a substance (e.g., vitamins) are useful.

Microbes with a specific growth factor requirement can be used in bioassays for the factor they need. A typical assay is a growth-response assay, which allows the amount of growth factor in a solution to be determined. The amount of growth in a culture is related to the amount of growth factor present. Ideally, the amount of growth is directly proportional to the amount of growth factor; if the growth factor concentration doubles the amount of microbial growth doubles. 

For example, Lactobacillus and Streptococcus can be used in microbiological assays of most vitamins and amino acids. Microbiological assays are specific, sensitive, and simple. They still are used in the assay of substances like vitamin B₁₂ and biotin, despite advances in chemical assay techniques.

Microorganisms which are able to synthesize large quantities of vitamins can be used to manufacture these compounds for human use. Several water-soluble and fat-soluble vitamins are produced partly or completely using industrial fermentations. Good examples of such vitamins and the microorganisms that synthesize them are

·       riboflavin (Clostridium, Candida, Ashbya, Eremothecium)

·       coenzyme A (Brevibacterium)

·       vitamin B12 (Streptomyces, Propionibacterium, Pseudomonas)

·       vitamin C (Gluconobacter, Erwinia, Corynebacterium)

·       carotene (Dunaliella)

·       vitamin D (Saccharomyces).

Research focuses on improving yields and finding microorganisms that can produce large quantities of other vitamins.

Nutritional Types of Microorganisms

Depending on how the need for carbon, energy, and electrons is fulfilled microorganisms can be classified. With respect to their preferred source of carbon, there are heterotrophs or autotrophs. There are only two sources of energy available to organisms: Phototrophs use light as their energy source; chemotrophs obtain energy from the oxidation of chemical compounds (either organic or inorganic). Microorganisms also have only two sources of electrons. There are Lithotrophs (i.e., “rock-eaters”) which use reduced inorganic substances as their electron source, and organotrophs which obtain electrons from reduced organic compounds.

Sources of Carbon, Energy, and Electrons
Carbon Sources
Autotrophs	CO2 sole or principal biosynthetic carbon source
Heterotrophs	Reduced, preformed, organic molecules from other organisms
Energy Sources
Phototrophs	Light
Chemotrophs	Oxidation of organic or inorganic compounds
Electron Sources
Lithotrophs		Reduced inorganic molecules
Organotrophs		Organic molecules

 

In spite of the great metabolic diversity seen in microorganisms, most can be placed in one of five nutritional classes based on their primary sources of carbon, energy, and electrons.

Major Nutritional Types of Microorganisms
Nutritional Type	Carbon Source	Energy Source	Electron Source	Representative Microorganisms
Photolithoautotrophy (photolithotrophic autotrophy)	CO2	Light	Inorganic e- donor	Purple and green sulfur bacteria, cyanobacteria
Photoorganoheterotrophy (photoorganotrophic heterotrophy)	Organic carbon but CO2 may also be used	Light	Organic e- donor	Purple nonsulfur bacteria, green nonsulfur bacteria
Chemolithoautotrophy (chemolithotrophic autotrophy)	CO2	Inorganic chemicals	Inorganic e- donor	Sulfur-oxidizing bacteria, hydrogen-oxidizing bacteria, methanogens, nitrifying bacteria, iron-oxidizing bacteria
Chemolithoheterotrophy or mixotrophy (chemolithotrophic heterotrophy)	Organic carbon, but CO2 may also be used	Inorganic chemicals	Inorganic e-donor	Some sulfur-oxidizing bacteria (e.g., Beggiatoa)
Chemoorganoheterotrophy (chemoorganotrophic heterotrophy)	Organic carbon	Organic chemicals often same as C source	Organic e- donor often same as C source	Most non photosynthetic microbes, including most pathogens, fungi, many protists, and many archaea

 

The majority of microorganisms are either photolithotrophic autotrophs or chemoorganotrophic heterotrophs.

Photolithotrophic autotrophs (often called photoautotrophs or photolithoautotrophs) use light energy and have CO₂ as their carbon source. Photosynthetic protists and cyanobacteria employ water as the electron donor and release oxygen. Other photolithoautotrophs, such as the purple and green sulfur bacteria, cannot oxidize water but extract electrons from inorganic donors like hydrogen, hydrogen sulfide, and elemental sulphur.

Chemoorganotrophic heterotrophs (often called heterotrophs or chemoheterotrophs, or chemoorganoheterotrophs) use organic compounds as sources of energy, hydrogen, electrons, and carbon. Usually, the same organic nutrient will satisfy all these requirements. All pathogenic microorganisms are chemoheterotrophs.

The other nutritional classes are very important ecologically though they have only fewer known microorganisms. Some photosynthetic bacteria (purple and green bacteria) use organic matter as their electron donor and carbon source. These photoorganotrophic heterotrophs (photoorganoheterotrophs) are common inhabitants of polluted lakes and streams. Some of these bacteria also can grow as photoautotrophs with molecular hydrogen as an electron donor.

Chemolithotrophic autotrophs (chemolithoautotrophs), oxidize reduced inorganic compounds such as iron, nitrogen, or sulfur molecules to derive both energy and electrons for biosynthesis. Carbon dioxide is the carbon source.

Chemolithoheterotrophs, also known as mixotrophs, use reduced inorganic molecules as their energy and electron source, but derive their carbon from organic sources. Chemolithotrophs contribute greatly to the chemical transformations of elements (e.g., the conversion of ammonia to nitrate or sulfur to sulfate) that continually occur in ecosystems.

A particular species usually belongs in only one of the nutritional classes, but some show great metabolic flexibility and alter their metabolic patterns in response to environmental changes. For example, many purple nonsulfur bacteria act as photoorganotrophic heterotrophs in the absence of oxygen but oxidize organic molecules and function chemoorganotrophically at normal oxygen levels. When oxygen is low, photosynthesis and chemoorganotrophic metabolism may function simultaneously. This sort of flexibility seems complex and confusing, but it gives these microbes a definite advantage if environmental conditions frequently change.

 Conclusion

     Microorganisms require about 10 elements in large quantities for the synthesis of macromolecules. Several other elements are needed in very small amounts and are parts of enzymes and cofactors.

     All microorganisms can be placed in one of a few nutritional categories on the basis of their requirements for carbon, energy, and electrons.

Nitrogen Cycle - Assimilation, Nitrification, Ammmonification, Denitrification

 

Assimilation is the process by which plants and animals incorporate the NO^3- and ammonia formed through nitrogen fixation and nitrification. Plants take up these forms of nitrogen through their roots, and incorporate them into plant proteins and nucleic acids. Animals are then able to utilize nitrogen from the plant tissues.

Nitrification

Nitrification is the process that converts ammonia to nitrite and then to nitrate and is an important step in the global nitrogen cycle. It is a two-step process in which NH₃/ NH⁴⁺ is converted to NO^2- by the soil bacteria called ammonia-oxidizers such as Nitrosopumilus, Nitrosospira etc.  Ammonia-oxidizing bacteria have been found to be abundant in oceans, soils, and salt marshes, also.

                         

The oxidation of nitrite (NO^2-) to nitrate (NO^3-) is carried out by a completely separate group of prokaryotes, known as nitrite-oxidizing Bacteria. Some of the genera involved in nitrite oxidation include  Nitrobacter and Nitrococcus.

For complete nitrification, both ammonia oxidation and nitrite oxidation must occur.

 

Ammonia- and nitrite-oxidizers also play a very important role in wastewater treatment facilities by removing harmful levels of ammonium that could lead to the pollution of the receiving waters. Ammonia- and nitrite-oxidizers help to maintain healthy aquaria by facilitating the removal of potentially toxic ammonium excreted in fish urine.

 

Anammox

Traditionally, all nitrification was thought to be carried out under aerobic conditions, but recently a new type of ammonia oxidation occurring under anoxic conditions was discovered. Anammox (anaerobic ammonia oxidation) is carried out by anammox bacterium like Brocadia anammoxidans. Anammox bacteria oxidize ammonia by using nitrite as the electron acceptor to produce gaseous nitrogen. Anammox bacteria were first discovered in anoxic bioreactors of wasterwater treatment plants but have since been found in a variety of aquatic systems, including low-oxygen zones of the ocean, coastal and estuarine sediments, mangroves, and freshwater lakes. In some areas of the ocean, the anammox process is considered to be responsible for a significant loss of nitrogen.

 

Ammmonification

When an organism excretes waste or dies, the nitrogen in its tissues is in the form of organic nitrogen (e.g. amino acids, DNA). Various fungi and prokaryotes then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia in the process known as ammonification. The ammonia then becomes available for uptake by plants and other microorganisms for growth.

 

Denitrification

Denitrification is the process that converts nitrate to nitrogen gas, thus removing bioavailable nitrogen and returning it to the atmosphere. Dinitrogen gas (N₂) is the ultimate end product of denitrification, but other intermediate gaseous forms of nitrogen exist. Some of these gases, such as nitrous oxide (N₂O), are considered greenhouse gasses, reacting with ozone and contributing to air pollution.

 

Denitrification is an anaerobic process, occurring mostly in soils and sediments and anoxic zones in lakes and oceans. Some denitrifying bacteria include species in the genera Bacillus, Paracoccus, and Pseudomonas.

Denitrification is important in that it removes fixed nitrogen (i.e., nitrate) from the ecosystem and returns it to the atmosphere in a biologically inert form (N₂). Wetlands provide a valuable place for reducing excess nitrogen levels via denitrification processes. This is particularly important in agriculture where the loss of nitrates in fertilizer is detrimental and costly. Denitrification in wastewater treatment plays a very beneficial role by removing unwanted nitrates from the wastewater effluent, thereby reducing the chances that the water discharged from the treatment plants will cause undesirable consequences (e.g., algal blooms).

  

Ecological Implications of Human Alterations to the Nitrogen Cycle

1.     Many human activities have a significant impact on the nitrogen cycle. Burning fossil fuels, application of nitrogen-based fertilizers, and other activities can dramatically increase the amount of biologically available nitrogen in an ecosystem.

2.     Nitrogen availability often limits the primary productivity of many ecosystems, hence, large changes in the availability of nitrogen can lead to severe alterations of the nitrogen cycle in both aquatic and terrestrial ecosystems. Industrial nitrogen fixation has increased exponentially since the 1940s, and human activity has doubled the amount of global nitrogen fixation

3.     In terrestrial ecosystems, the addition of nitrogen can lead to nutrient imbalance in trees, changes in forest health, and declines in biodiversity. With increased nitrogen availability there is often a change in carbon storage, thus impacting more processes than just the nitrogen cycle.

4.     In agricultural systems, fertilizers are used extensively to increase plant production, but unused nitrogen, usually in the form of nitrate, can leach out of the soil, enter streams and rivers, and ultimately make its way into our drinking water.

5.     Much of the nitrogen applied to agricultural and urban areas ultimately enters rivers and nearshore coastal systems. In nearshore marine systems, increases in nitrogen can often lead to anoxia (no oxygen) or hypoxia (low oxygen), altered biodiversity, changes in food-web structure, and general habitat degradation. One common consequence of increased nitrogen is an increase in harmful algal blooms. Toxic blooms of certain types of dinoflagellates have been associated with high fish and shellfish mortality in some areas.

6.     Alterations to the nitrogen cycle may lead to an increased risk of parasitic and infectious diseases among humans and wildlife.

7.     Increases in nitrogen in aquatic systems can lead to increased acidification in freshwater ecosystems.

 

Nitrogen is the most important nutrient in regulating primary productivity and species diversity in both aquatic and terrestrial ecosystems. Microbially-driven processes such as nitrogen fixation, nitrification, and denitrification, constitute the bulk of nitrogen transformations, and play a critical role in the fate of nitrogen in the Earth's ecosystems.