Hypoxia (environmental)

Hypoxia refers to low oxygen conditions. Normally, 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure.[1] In water, oxygen levels are much lower, approximately 1%, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface (if there is more oxygen in the air, it will diffuse across the partial pressure gradient).[2]

Atmospheric hypoxia

Atmospheric hypoxia occurs naturally at high altitudes. Total atmospheric pressure decreases as altitude increases, causing a lower partial pressure of oxygen which is defined as hypobaric hypoxia. Oxygen remains at 20.9% of the total gas mixture, differing from hypoxic hypoxia, where the percentage of oxygen in the air (or blood) is decreased. This is common in the sealed burrows of some subterranean animals, such as blesmols.[3] Atmospheric hypoxia is also the basis of altitude training which is a standard part of training for elite athletes. Several companies mimic hypoxia using normobaric artificial atmosphere.

Aquatic hypoxia

Oxygen depletion is a phenomenon that occurs in aquatic environments as dissolved oxygen (DO; molecular oxygen dissolved in the water) becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system. Dissolved oxygen is typically expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity (both of which affect the solubility of oxygen in water; see oxygen saturation and underwater). An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic; a system with low concentration—in the range between 1 and 30% saturation—is called hypoxic or dysoxic. Most fish cannot live below 30% saturation. Hypoxia leads to impaired reproduction of remaining fish via endocrine disruption.[4] A "healthy" aquatic environment should seldom experience less than 80%. The exaerobic zone is found at the boundary of anoxic and hypoxic zones.

Hypoxia can occur throughout the water column and also at high altitudes as well as near sediments on the bottom. It usually extends throughout 20-50% of the water column, but depending on the water depth and location of pycnoclines (rapid changes in water density with depth). It can occur in 10-80% of the water column. For example, in a 10-meter water column, it can reach up to 2 meters below the surface. In a 20-meter water column, it can extend up to 8 meters below the surface.[5]

Causes of hypoxia

Decline of oxygen saturation to anoxia at night Kiel Fjord Germany
Decline of oxygen saturation to anoxia, measured during the night in Kiel Fjord, Germany. Depth = 5 m

Oxygen depletion can result from a number of natural factors, but is most often a concern as a consequence of pollution and eutrophication in which plant nutrients enter a river, lake, or ocean, and phytoplankton blooms are encouraged. While phytoplankton, through photosynthesis, will raise DO saturation during daylight hours, the dense population of a bloom reduces DO saturation during the night by respiration. When phytoplankton cells die, they sink towards the bottom and are decomposed by bacteria, a process that further reduces DO in the water column. If oxygen depletion progresses to hypoxia, fish kills can occur and invertebrates like worms and clams on the bottom may be killed as well.

Still frame from an underwater video of the sea floor. The floor is covered with crabs, fish, and clams apparently dead or dying from oxygen depletion.

Hypoxia may also occur in the absence of pollutants. In estuaries, for example, because freshwater flowing from a river into the sea is less dense than salt water, stratification in the water column can result. Vertical mixing between the water bodies is therefore reduced, restricting the supply of oxygen from the surface waters to the more saline bottom waters. The oxygen concentration in the bottom layer may then become low enough for hypoxia to occur. Areas particularly prone to this include shallow waters of semi-enclosed water bodies such as the Waddenzee or the Gulf of Mexico, where land run-off is substantial. In these areas a so-called "dead zone" can be created. Low dissolved oxygen conditions are often seasonal, as is the case in Hood Canal and areas of Puget Sound, in Washington State.[6] The World Resources Institute has identified 375 hypoxic coastal zones around the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, and East Asia, particularly in Japan.[7]

Jubilee photo from Mobile Bay

Hypoxia may also be the explanation for periodic phenomena such as the Mobile Bay jubilee, where aquatic life suddenly rushes to the shallows, perhaps trying to escape oxygen-depleted water. Recent widespread shellfish kills near the coasts of Oregon and Washington are also blamed on cyclic dead zone ecology.[8]

Phytoplankton breakdown

Scientists have determined that high concentrations of minerals dumped into bodies of water causes significant growth of phytoplankton blooms. As these blooms are broken down by bacteria, such as Phanerochaete chrysosprium, oxygen is depleted by the enzymes of these organisms.[9]

Tetrapyrrol ring
Tetrapyrrol ring, the active site of Ligninperoxidase enzyme

Phytoplankton are mostly made up of lignin and cellulose, which are broken down by enzymes present in organisms such as P. chrysosprium, known as white-rot. The breakdown of cellulose does not deplete oxygen concentration in water, but the breakdown of lignin does. This breakdown of lignin includes an oxidative mechanism, and requires the presence of dissolved oxygen to take place by enzymes like ligninperoxidase. Other fungi such as brown-rot, soft-rot, and blue stain fungi also are necessary in lignin transformation. As this oxidation takes place, CO2 is formed in its place[9]

Active site of tetrapyrrol ring binding oxygen
Active site of tetrapyrrol ring binding oxygen
Oxyferroheme is converted to Ferri-LiP with the addition of veratric alcohol, and gives off diatomic oxygen radical.
Oxyferroheme is converted to Ferri-LiP with the addition of veratric alcohol, and gives off diatomic oxygen radical.
Breakdown of coniferyl alcohol
This is the breakdown of a confieryl alcohol by a hydrogen ion to make propanol and ortho-methoxyphenol.

Ligninperoxidase (LiP) serves as the most import enzyme because it is best at breaking down lignin in these organisms. LiP disrupts C-C bonds and C-O bonds within Lignin’s three-dimensional structure, causing it to break down. LiP consists of ten alpha helices, two Ca2+ structural ions, as well as a heme group called a tetrapyrrol ring. Oxygen serves an important role in the catalytic cycle of LiP to form a double bond on the Fe2+ ion in the tetrapyrrol ring. Without the presence of diatomic oxygen in the water, this breakdown cannot take place because Ferrin-LiP will not be reduced into Oxyferroheme. Oxygen gas is used to reduce Ferrin-LiP into Oxyferroheme-LiP. Oxyferroheme and veratric alcohol combine to create oxygen radical and Ferri-LiP, which can now be used to degrade lignin.[9] Oxygen radicals cannot be used in the environment, and are harmful in high presence in the environment.[10]

Once Ferri-LiP is present in the ligninperoxidase, it can be used to break down lignin molecules by removing one phenylpropane group at a time through either the LRET mechanism or the mediator mechanism. The LRET mechanism (long range electron transfer mechanism) transfers an electron from the tetrapyrrol ring onto a molecule of phenylpropane in a lignin. This electron moves onto a C-C or C-O bond to break one phenylpropane molecule from the lignin, breaking it down by removing one phenylpropane at a time.[9]

In the mediator mechanism, LiP enzyme is activated by the addition of hydrogen peroxide to make LiP radical, and a mediator such as veratric alcohol is added and activated creating veratric alcohol radical. Veratric alcohol radical transfers one electron to activate the phenylpropane on lignin, and the electron dismantles a C-C or C-O bond to release one phenylpropane from the lignin. As the size of a lignin molecule increases, the more difficult it is to break these C-C or C-O bonds. Three types of phenyl propane rings include coniferyl alcohol, sinapyl alcohol, and-coumaryl alcohol.[9]

LiP has a very low MolDock score, meaning there is little energy required to form this enzyme and stabilize it to carry out reactions. LiP has a MolDock score of -156.03 kcal/mol. This is energetically favorable due to its negative free energy requirements, and therefore this reaction catalyzed by LiP is likely to take place spontaneously.[11] Breakdown of propanol and phenols occur naturally in the environment because they are both water soluble.

The breakdown of phytoplankton in the environment depends on the presence of oxygen, and once oxygen is no longer in the bodies of water, ligninperoxidases cannot continue to break down the lignin. When oxygen is not present in the water, the breakdown of phytoplankton changes from 10.7 days to a total of 160 days for this to take place.

The rate of phytoplankton breakdown can be represented using this equation:

In this equation, G(t) is the amount of particulate organic carbon (POC) overall at a given time, t. G(0) is the concentration of POC before breakdown takes place. k is a rate constant in year-1, and t is time in years. For most POC of phytoplankton, the k is around 12.8 years-1, or about 28 days for nearly 96% of carbon to be broken down in these systems. Whereas for anoxic systems, POC breakdown takes 125 days, over four times longer.[12] It takes approximately 1 mg of Oxygen to break down 1 mg of POC in the environment, and therefore, hypoxia takes place quickly as oxygen is used up quickly to digest POC. About 9% of POC in phytoplankton can be broken down in a single day at 18 °C, therefore it takes about eleven days to completely break down a full phytoplankton.[13]

After POC is broken down, this particulate matter can be turned into other dissolved organic carbon, such as carbon dioxide, bicarbonate ions, and carbonate. As much as 30% of phytoplankton can be broken down into dissolved organic carbon. When this particulate organic carbon interacts with 350 nm ultraviolet light, dissolved organic carbon is formed, removing even more oxygen from the environment in the forms of carbon dioxide, bicarbonate ions, and carbonate. Dissolved inorganic carbon is made at a rate of 2.3-6.5 mg/(m^3)day.[14]

As phytoplankton breakdown, free phosphorus and nitrogen become available in the environment, which also fosters hypoxic conditions. As the breakdown of these phytoplankton takes place, the more phosphorus turns into phosphates, and nitrogens turn into nitrates. This depletes the oxygen even more so in the environment, further creating hypoxic zones in higher quantities. As more minerals such as phosphorus and nitrogen are displaced into these aquatic systems, the growth of phytoplankton greatly increases, and after their death, hypoxic zones are formed.[15]


To combat hypoxia, it is essential to reduce the amount of land-derived nutrients reaching rivers in runoff. This can be done by improving sewage treatment and by reducing the amount of fertilizers leaching into the rivers. Alternately, this can be done by restoring natural environments along a river; marshes are particularly effective in reducing the amount of phosphorus and nitrogen (nutrients) in water. Other natural habitat-based solutions include restoration of shellfish populations, such as oysters. Oyster reefs remove nitrogen from the water column and filter out suspended solids, subsequently reducing the likelihood or extent of harmful algal blooms or anoxic conditions.[16] Foundational work toward the idea of improving marine water quality through shellfish cultivation was conducted by Odd Lindahl et al., using mussels in Sweden.[17] More involved than single-species shellfish cultivation, integrated multi-trophic aquaculture mimics natural marine ecosystems, relying on polyculture to improve marine water quality.

Anoxie allemagne2
Graphs of oxygen and salinity levels at Kiel Fjord in September 1998.

Technological solutions are also possible, such as that used in the redeveloped Salford Docks area of the Manchester Ship Canal in England, where years of runoff from sewers and roads had accumulated in the slow running waters. In 2001 a compressed air injection system was introduced, which raised the oxygen levels in the water by up to 300%. The resulting improvement in water quality led to an increase in the number of invertebrate species, such as freshwater shrimp, to more than 30. Spawning and growth rates of fish species such as roach and perch also increased to such an extent that they are now amongst the highest in England.[18]

In a very short time the oxygen saturation can drop to zero when offshore blowing winds drive surface water out and anoxic depth water rises up. At the same time a decline in temperature and a rise in salinity is observed (from the longterm ecological observatory in the seas at Kiel Fjord, Germany). New approaches of long-term monitoring of oxygen regime in the ocean observe online the behavior of fish and zooplankton, which changes drastically under reduced oxygen saturations (ecoSCOPE) and already at very low levels of water pollution.

See also


  1. ^ Brandon, John. "The Atmosphere, Pressure and Forces". Meteorology. Pilot Friend. Retrieved 21 December 2012.
  2. ^ "Dissolved Oxygen". Water Quality. Water on the Web. Archived from the original on 13 December 2012. Retrieved 21 December 2012.
  3. ^ Roper, T.J.; et al. (2001). "Environmental conditions in burrows of two species of African mole-rat, Georychus capensis and Cryptomys damarensis". Journal of Zoology. 254 (1): 101–107. doi:10.1017/S0952836901000590.
  4. ^ Wu, R. et al. 2003. Aquatic Hypoxia Is an Endocrine Disruptor and Impairs Fish Reproduction
  5. ^ Rabalais, Nancy; Turner, R. Eugene; Justic´, Dubravko; Dortch, Quay; Wiseman, William J. Jr. Characterization of Hypoxia: Topic 1 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. Ch. 3. NOAA Coastal Ocean Program, Decision Analysis Series No. 15. May 1999. < http://oceanservice.noaa.gov/products/hypox_t1final.pdf >. Retrieved February 11, 2009.
  6. ^ Encyclopedia of Puget Sound: Hypoxia http://www.eopugetsound.org/science-review/section-4-dissolved-oxygen-hypoxia
  7. ^ Selman, Mindy (2007) Eutrophication: An Overview of Status, Trends, Policies, and Strategies. World Resources Institute.
  8. ^ oregonstate.edu Archived 2006-09-01 at the Wayback Machine – Dead Zone Causing a Wave of Death Off Oregon Coast (8/9/2006)
  9. ^ a b c d e Gubernatorova, T. N.; Dolgonosov, B. M. (2010-05-01). "Modeling the biodegradation of multicomponent organic matter in an aquatic environment: 3. Analysis of lignin degradation mechanisms". Water Resources. 37 (3): 332–346. doi:10.1134/S0097807810030085. ISSN 0097-8078.
  10. ^ Betteridge, D. John (2000). "What is oxidative stress?". Metabolism. 49 (2): 3–8. doi:10.1016/s0026-0495(00)80077-3.
  11. ^ Chen, Ming; Zeng, Guangming; Tan, Zhongyang; Jiang, Min; Li, Hui; Liu, Lifeng; Zhu, Yi; Yu, Zhen; Wei, Zhen (2011-09-29). "Understanding Lignin-Degrading Reactions of Ligninolytic Enzymes: Binding Affinity and Interactional Profile". PLOS ONE. 6 (9): e25647. doi:10.1371/journal.pone.0025647. ISSN 1932-6203. PMC 3183068. PMID 21980516.
  12. ^ Harvey, H. Rodger (1995). "Kinetics of phytoplankton decay during simulated sedimentation: Changes in biochemical composition and microbial activity under oxic and anoxic conditions". Geochimica et Cosmochimica Acta. 59 (16): 3367–3377. doi:10.1016/0016-7037(95)00217-n.
  13. ^ Jewell, William J. "Aquatic Weed Decay: Dissolved Oxygen Utilization and Nitrogen and Phosphorus Regeneration". Journal - Water Pollution Control Federation. 43: 1457–1467.
  14. ^ Johannessen, Sophia C.; Peña, M. Angelica; Quenneville, Melanie L. (2007). "Photochemical production of carbon dioxide during a coastal phytoplankton bloom". Estuarine, Coastal and Shelf Science. 73 (1–2): 236–242. doi:10.1016/j.ecss.2007.01.006.
  15. ^ Conley, Daniel J.; Paerl, Hans W.; Howarth, Robert W.; Boesch, Donald F.; Seitzinger, Sybil P.; Havens, Karl E.; Lancelot, Christiane; Likens, Gene E. (2009-02-20). "Controlling Eutrophication: Nitrogen and Phosphorus". Science. 323 (5917): 1014–1015. doi:10.1126/science.1167755. ISSN 0036-8075. PMID 19229022.
  16. ^ Kroeger, Timm (2012) Dollars and Sense: Economic Benefits and Impacts from two Oyster Reef Restoration Projects in the Northern Gulf of Mexico. TNC Report.
  17. ^ Lindahl, O.; Hart, R.; Hernroth, B.; Kollberg, S.; Loo, L. O.; Olrog, L.; Rehnstam-Holm, A. S.; Svensson, J.; Svensson, S.; Syversen, U. (2005). "Improving marine water quality by mussel farming: A profitable solution for Swedish society". Ambio. 34 (2): 131–138. CiteSeerX doi:10.1579/0044-7447-34.2.131. PMID 15865310.
  18. ^ Hindle, P.(1998) (2003-08-21). "Exploring Greater Manchester — a fieldwork guide: The fluvioglacial gravel ridges of Salford and flooding on the River Irwell" (PDF). Manchester Geographical Society. Retrieved 2007-12-11. p.13


  • Kils, U., U. Waller, and P. Fischer (1989). "The Fish Kill of the Autumn 1988 in Kiel Bay". International Council for the Exploration of the Sea. C M 1989/L:14.CS1 maint: Multiple names: authors list (link)
  • Fischer P.; U. Kils (1990). "In situ Investigations on Respiration and Behaviour of Stickleback Gasterosteus aculeatus and the Eelpout Zoaraes viviparus During Low Oxygen Stress". International Council for the Exploration of the Sea. C M 1990/F:23.
  • Fischer P.; K. Rademacher; U. Kils (1992). "In situ investigations on the respiration and behaviour of the eelpout Zoarces viviparus under short term hypoxia". Mar Ecol Prog Ser. 88: 181–184. doi:10.3354/meps088181.

External links


Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:

Anaerobic adhesive, a bonding agent that does not cure in the presence of air

Anaerobic clarigester, an anaerobic digester that treats dilute biodegradable feedstocks and allows different retention times for solids and liquids

Anaerobic contact process, an anaerobic digester with a set of reactors in series

Anaerobic digestion, the use of anaerobic bacteria to break down waste, with biogas as a valuable byproduct

Hypoxia (environmental) (anaerobic environment), an environment with little or no available oxygen

Anaerobic exercise, exercise intense enough to cause lactate to form, used in non-endurance sports

Anaerobic filter, an anaerobic digester with a tank containing a filter medium where anaerobic microbes can establish themselves

Anaerobic lagoon, used to dispose of animal waste, particularly that of cows and pigs

Anaerobic organism, any organism whose redox metabolism does not depend on free oxygen

Anaerobic respiration, respiration in the absence of oxygen, using some other molecule as the final electron acceptor

Anammox, anaerobic ammonium oxidation, a globally important microbial process of the nitrogen cycle

Anaerobic digestion

Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material without oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.

Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments, where it is usually referred to as "anaerobic activity". This is the source of marsh gas methane as discovered by Alessandro Volta in 1776.The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide. The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide, and traces of other ‘contaminant’ gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

With the re-use of waste as a resource and new technological approaches that have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom (2011), Germany and Denmark (2011).


The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:

Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved oxygen

Anoxic event, when the Earth's oceans become completely depleted of oxygen below the surface levels

Euxinic, anoxic conditions in the presence of hydrogen sulfide

Hypoxia (environmental), low oxygen conditions

Hypoxia (medical), when the body or a region of the body is deprived of adequate oxygen supply

Cerebral anoxia, when the brain is completely deprived of oxygen, an extreme form of cerebral hypoxia

Anoxic event

Oceanic anoxic events or anoxic events (anoxia conditions) were intervals in the Earth's past where portions of oceans became depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia, waters that contained hydrogen sulfide, H2S, developed. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".

Anoxic waters

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen and are a more severe condition of hypoxia. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. This condition is generally found in areas that have restricted water exchange.

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.

Anoxic waters are a natural phenomenon, and have occurred throughout geological history. In fact, some postulate that the Permian–Triassic extinction event, a mass extinction of species from world's oceans, resulted from widespread anoxic conditions. At present anoxic basins exist, for example, in the Baltic Sea, and elsewhere (see below). Recently, there have been some indications that eutrophication has increased the extent of the anoxic zones in areas including the Baltic Sea, the Gulf of Mexico, and Hood Canal in Washington State.

Chemical oceanography

Chemical oceanography is the study of ocean chemistry: the behavior of the chemical elements within the Earth's oceans. The ocean is unique in that it contains - in greater or lesser quantities - nearly every naturally occurring element in the periodic table.

Much of chemical oceanography describes the cycling of these elements both within the ocean and with the other spheres of the Earth system (see biogeochemical cycle). These cycles are usually characterized as quantitative fluxes between constituent reservoirs defined within the ocean system and as residence times within the ocean. Of particular global and climatic significance are the cycles of the biologically active elements such as carbon, nitrogen, and phosphorus as well as those of some important trace elements such as iron.

Another important area of study in chemical oceanography is the behaviour of isotopes (see isotope geochemistry) and how they can be used as tracers of past and present oceanographic and climatic processes. For example, the incidence of 18O (the heavy isotope of oxygen) can be used as an indicator of polar ice sheet extent, and boron isotopes are key indicators of the pH and CO2 content of oceans in the geologic past.


Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitrification as a type of respiration that reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2) completing the nitrogen cycle. Denitrifying microbes require a very low oxygen concentration of less than 10%, as well as organic C for energy. Since denitrification can remove NO3−, reducing its leaching to groundwater, it can be strategically used to treat sewage or animal residues of high nitrogen content. Denitrification can leak N2O, which is an ozone-depleting substance and a greenhouse gas that can have a considerable influence on global warming.

The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g., Thiobacillus denitrificans). Denitrifiers are represented in all main phylogenetic groups. Generally several species of bacteria are involved in the complete reduction of nitrate to N2, and more than one enzymatic pathway has been identified in the reduction process.Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene. This is less common than denitrification in most ecosystems as a means of nitrate reduction. Other genes known in microorganisms which denitrify include nir (nitrite reductase) and nos (nitrous oxide reductase) among others; organisms identified as having these genes include Alcaligenes faecalis, Alcaligenes xylosoxidans, many in the genus Pseudomonas, Bradyrhizobium japonicum, and Blastobacter denitrificans.


Elrathia is a genus of ptychopariid trilobite species that lived during the Middle Cambrian of Utah, and possibly British Columbia. E. kingii is one of the most common trilobite fossils in the USA locally found in extremely high concentrations within the Wheeler Formation in the U.S. state of Utah.E. kingii has been considered the most recognizable trilobite. Commercial quarries extract E. kingii in prolific numbers, with just one commercial collector estimating 1.5 million specimens extracted in a 20-year career. 1950 specimens of Elrathia are known from the Greater Phyllopod bed, where they comprise 3.7% of the community.

...trilobite occupied the exaerobic zone, at the boundary of anoxic and dysoxic bottom waters. E. kingii consistently occur in settings below the oxygen levels required by other contemporaneous epifaunal and infaunal benthic biota and may have derived energy from a food web that existed independently of phototrophic primary productivity. Although other fossil organisms are known to have preferred such environments, E. kingii is the earliest-known inhabitant of them, extending the documented range of the exaerobic ecological strategy into the Cambrian Period.


Eutrophication (from Greek eutrophos, "well-nourished"), or hypertrophication, is when a body of water becomes overly enriched with minerals and nutrients which induce excessive growth of algae. This process may result in oxygen depletion of the water body. One example is an "algal bloom" or great increase of phytoplankton in a water body as a response to increased levels of nutrients. Eutrophication is often induced by the discharge of nitrate or phosphate-containing detergents, fertilizers, or sewage into an aquatic system.

Hypoxia in fish

Fish are exposed to large oxygen fluctuations in their aquatic environment since the inherent properties of water can result in marked spatial and temporal differences in the concentration of oxygen (see oxygenation and underwater). Fish respond to hypoxia with varied behavioral, physiological, and cellular responses in order to maintain homeostasis and organism function in an oxygen-depleted environment. The biggest challenge fish face when exposed to low oxygen conditions is maintaining metabolic energy balance, as 95% of the oxygen consumed by fish is used for ATP production through the electron transport chain. Therefore, hypoxia survival requires a coordinated response to secure more oxygen from the depleted environment and counteract the metabolic consequences of decreased ATP production at the mitochondria. This article is a review of the effects of hypoxia on all aspects of fish, ranging from behavior down to genes.

Index of environmental articles

The natural environment, commonly referred to simply as the environment, includes all living and non-living things occurring naturally on Earth.

The natural environment includes complete ecological units that function as natural systems without massive human intervention, including all vegetation, animals, microorganisms, soil, rocks, atmosphere and natural phenomena that occur within their boundaries. Also part of the natural environment is universal natural resources and physical phenomena that lack clear-cut boundaries, such as air, water, and climate.

Lake Tarpon

Lake Tarpon is a freshwater lake located about 10 miles (16 km) west of Tampa in Palm Harbor and Tarpon Springs, Florida. Lake Tarpon is the largest lake in Pinellas County with a surface area of 2,500 acres (10 km2).

Its watershed encompasses 52 square miles (130 km2) including the two largest tributaries, South Creek and Brooker Creek. Lake Tarpon can be visited at Anderson Park, Chesnut Park, or several other public access points.

The lake is a regional recreational destination and is renowned for its largemouth bass fishing.

Although Lake Tarpon is designated as a fishing and swimming lake, it fails to meet the EPA's standards, and is therefore listed as an impaired lake because it is over polluted. Lake Tarpon is not in attainment because the waters exceed its Total Maximum Daily Load (or TMDL) for Nutrients and as a result of excessive nutrients, low Dissolved Oxygen. Part of the pollution that enters Lake Tarpon is caused by fertilizers that have caused algae and unwanted weeds to grow. Some have suggested using liquid fertilizer to stop this. The logic being that liquid fertilizers are taken up by the plants while granular fertilizers are more likely to run off in rain water. Another problem Lake Tarpon faces is having low dissolved oxygen. As the nutrient load in the water increases, so do bacteria and other micro-organisms that consume the excess nutrients. These micro-organisms are largely aerobic (require oxygen to live). The micro-organisms consume the oxygen in the water, thus lowering the Dissolved Oxygen. The resultant decreased Dissolved Oxygen lowers biodiversity in the ecosystem. Increased nutrients in the water also increase the turbidity (murkiness) that recent samples show), and as a result can cause hypoxia (low oxygen) or a dead zone where life is unsustainable, as exemplified in the Gulf of Mexico.

Outline of oceanography

The following outline is provided as an overview of and introduction to Oceanography.


Oxygen is the chemical element with the symbol O and atomic number 8, meaning its nucleus has 8 protons. The number of neutrons varies according to the isotope: the stable isotopes have 8, 9, or 10 neutrons. Oxygen is a member of the chalcogen group on the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up almost half of the Earth's crust.

Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form (allotrope) of oxygen, ozone (O3), strongly absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of smog and thus a pollutant.

Oxygen was isolated by Michael Sendivogius before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774. Priority is often given for Priestley because his work was published first. Priestley, however, called oxygen "dephlogisticated air", and did not recognize it as a chemical element. The name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and correctly characterized the role it plays in combustion.

Common uses of oxygen include production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy, and life support systems in aircraft, submarines, spaceflight and diving.


Oxygen-free may refer to the absence of oxygen in an environment or in a material.

Oxygen minimum zone

The Oxygen minimum zone (OMZ), sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,500 m (660–4,920 ft), depending on local circumstances. OMZs are found worldwide, typically along the western coast of continents, in areas where an interplay of physical and biological processes concurrently lower the oxygen concentration (biological processes) and restrict the water from mixing with surrounding waters (physical processes), creating a “pool” of water where oxygen concentrations fall from the normal range of 4–6 mg/l to below 2 mg/l.

Streeter–Phelps equation

The Streeter–Phelps equation is used in the study of water pollution as a water quality modelling tool. The model describes how dissolved oxygen (DO) decreases in a river or stream along a certain distance by degradation of biochemical oxygen demand (BOD). The equation was derived by H. W. Streeter, a sanitary engineer, and Earle B. Phelps, a consultant for the U.S. Public Health Service, in 1925, based on field data from the Ohio River. The equation is also known as the DO sag equation.

Wastewater quality indicators

Wastewater quality indicators are laboratory test methodologies to assess suitability of wastewater for disposal or re-use. Tests selected and desired test results vary with the intended use or discharge location. Tests measure physical, chemical, and biological characteristics of the waste water.

Air pollution
Water pollution
Soil contamination
Radioactive contamination
Other types of pollution
Pollution response
Inter-government treaties
Major organizations
Aquatic ecosystems

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