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. 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).
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. 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. 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.
Causes of hypoxia
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. 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.
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.
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.
Breakdown of lignin
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.
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.
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. Oxygen radicals cannot be used in the environment, and are harmful in high presence in the environment.
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.
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.
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. Breakdown of propanol and phenols occur naturally in the environment because they are both water-soluble.
Environmental factors
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.
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.
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.
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.
Solutions
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.
Foundational work toward the idea of improving marine water quality
through shellfish cultivation was conducted by Odd Lindahl et al., using
mussels in Sweden. More involved than single-species shellfish cultivation, integrated multi-trophic aquaculture mimics natural marine ecosystems, relying on polyculture to improve marine water quality.
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.
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.
