Anoxic waters

Last updated

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. [1] Anoxic waters can be contrasted with hypoxic waters, which are low (but not lacking) in dissolved oxygen. This condition is generally found in areas that have restricted water exchange.

Contents

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier, [2] 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, [3] and have occurred throughout geological history. The Permian–Triassic extinction event, a mass extinction of species from the world's oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of carbon dioxide into Earth's atmosphere. [4] Many lakes have a permanent or temporary anoxic layer created by respiration depleting oxygen at depth and thermal stratification preventing its resupply. [5]

Anoxic basins exist in the Baltic Sea, [6] the Black Sea, the Cariaco Trench, various fjord valleys, and elsewhere. [7] Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, the Gulf of Mexico, [8] and Hood Canal in Washington State. [9]

Causes and effects

Anoxic conditions result from a combination of environmental conditions including density stratification, [10] inputs of organic material or other reducing agents, and physical barriers to water circulation. In fjords, shallow sills at the entrance may prevent circulation, while at continental boundaries, circulation may be especially low while organic material input from production at upper levels is exceptionally high. [11] In wastewater treatment, the absence of oxygen alone is indicated anoxic while the term anaerobic is used to indicate the absence of any common electron acceptor such as nitrate, sulfate or oxygen.

When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water, is nitrate. Denitrification occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing sulfate. This results in the byproduct of hydrogen sulfide (H2S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color: [12] [13]

2 CH2O + SO2−
4 → 2 HCO−
3 + H2S + chemical energy

These sulfides will mostly be oxidized to either sulfates (~90%) in more oxygen-rich water or precipitated and converted into pyrite (~10%), according to the following chemical equations: [13]

  1. H2S ⇌ HS− + H+
    HS− + 2 O2 → HSO−
    4
  2. H2S ⇌ HS− + H+
    Fe2+ + HS− → FeS + H+
    FeS + H2S → FeS2 + H2

Some chemolithotrophs can also facilitate the oxidation of hydrogen sulfide into elemental sulfur, according to the following chemical equation: [14]

H2S + O2 → S + H2O2

Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and low levels of inflow of oxygenated water through the sediment. Below a few centimeters from the surface the interstitial water (water between sediment) is oxygen free.

Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount of oxygen used by marine organisms in the process of breaking down organic matter. BOD is influenced by the type of organisms present, the pH of the water, temperature, and the type of organic matter present in the area. BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams. As BOD increases, available oxygen decreases. This causes stress on larger organisms. BOD comes from natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff. [15]

Human caused anoxic conditions

Eutrophication, an influx of nutrients (phosphate/nitrate), often a byproduct of agricultural run-off and sewage discharge, can result in large but short-lived algae blooms. Upon a bloom's conclusion, the dead algae sink to the bottom and are broken down until all oxygen is expended. Such a case is the Gulf of Mexico where a seasonal dead zone occurs, which can be disturbed by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient concentrated "sludge", can be especially damaging to ecosystem diversity. Species sensitive to anoxic conditions are replaced by fewer hardier species, reducing the overall variability of the affected area. [12]

Gradual environmental changes through eutrophication or global warming can cause major oxic-anoxic regime shifts. Based on model studies this can occur abruptly, with a transition between an oxic state dominated by cyanobacteria, and an anoxic state with sulfate-reducing bacteria and phototrophic sulfur bacteria. [16]

Daily and seasonal cycles

The temperature of a body of water directly affects the amount of dissolved oxygen it can hold. Following Henry's law, as water becomes warmer, oxygen becomes less soluble in it. This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on larger scales. Thus, bodies of water are more vulnerable to anoxic conditions during the warmest period of the day and during summer months. This problem can be further exacerbated in the vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released.

Daily cycles are also influenced by the activity of photosynthetic organisms. The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise. [17]

Biological adaptation

Individual species’ reactions to eutrophication can vary widely. For example, some organisms, such as primary producers, can adapt very quickly and even thrive under anoxic conditions. However, most organisms are extremely susceptible to slight changes in aquatic oxygen levels. Simply put - if a respirating organism is presented with little to no oxygen, its chances of survival will decrease. Therefore, eutrophication and anoxic conditions in water leads to a decrease in biodiversity.

For example, the soft coral Xenia umbellata can resist some anoxic conditions for short periods of time, but after about 3 weeks, mean survival decreases to about 81% and about 40% of surviving species experience size reductions, lessening in coloration, and compromised pinnate structures (Simancas-Giraldo et al., 2021). Another example of a susceptible organism is observed with The Sydney Cockle, Anadara trapezia. Enriched sediments have lethal and sublethal effects on this Cockle and, as stated in [Vadillo Gonzalez et al., 2021], “movement of cockles was reduced in enriched sediments compared to natural treatments.” These are just a few examples of the hundreds of thousands of aquatic species that exist, but these and other examples show important results.

A study collecting over 850 published experiments "reporting oxygen thresholds and/or lethal times for a total of 206 species spanning the full taxonomic range of benthic metazoans." [18]

Individual species will have different adaptive responses to anoxic conditions based on their biological makeup and the condition of their habitat. While some are able to pump oxygen from higher water levels down into the sediment, other adaptations include specific hemoglobins for low oxygen environments, slow movement to reduce rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases, the prevalence of excess nutrients results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic. [12]

Anoxic basins

See also

Related Research Articles

<span class="mw-page-title-main">Late Ordovician mass extinction</span> Extinction event around 444 million years ago

The Late Ordovician mass extinction (LOME), sometimes known as the end-Ordovician mass extinction or the Ordovician-Silurian extinction, is the first of the "big five" major mass extinction events in Earth's history, occurring roughly 445 million years ago (Ma). It is often considered to be the second-largest known extinction event, in terms of the percentage of genera that became extinct. Extinction was global during this interval, eliminating 49–60% of marine genera and nearly 85% of marine species. Under most tabulations, only the Permian-Triassic mass extinction exceeds the Late Ordovician mass extinction in biodiversity loss. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. Despite its taxonomic severity, the Late Ordovician mass extinction did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over the first 5 million years of the Silurian period.

The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including hot springs, stagnant water bodies, as well as microbial mats in intertidal zones. Unlike plants, algae, and cyanobacteria, purple sulfur bacteria do not use water as their reducing agent, and therefore do not produce oxygen. Instead, they can use sulfur in the form of sulfide, or thiosulfate (as well, some species can use H2, Fe2+, or NO2−) as the electron donor in their photosynthetic pathways. The sulfur is oxidized to produce granules of elemental sulfur. This, in turn, may be oxidized to form sulfuric acid.

<span class="mw-page-title-main">Dead zone (ecology)</span> Low-oxygen areas in coastal zones and lakes caused by eutrophication

Dead zones are hypoxic (low-oxygen) areas in the world's oceans and large lakes. Hypoxia occurs when dissolved oxygen (DO) concentration falls to or below 2 mg of O2/liter. When a body of water experiences hypoxic conditions, aquatic flora and fauna begin to change behavior in order to reach sections of water with higher oxygen levels. Once DO declines below 0.5 ml O2/liter in a body of water, mass mortality occurs. With such a low concentration of DO, these bodies of water fail to support the aquatic life living there. Historically, many of these sites were naturally occurring. However, in the 1970s, oceanographers began noting increased instances and expanses of dead zones. These occur near inhabited coastlines, where aquatic life is most concentrated.

Oceanic anoxic events or anoxic events (anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not happened for millions of years, the geologic 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. On the other hand, there are widespread, various black-shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions. Many geologists believe oceanic anoxic events are strongly linked to the 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."

<span class="mw-page-title-main">Sulfur cycle</span> Biogeochemical cycle of sulfur

The sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes. Steps of the sulfur cycle are:

<i>Beggiatoa</i> Genus of bacteria

Beggiatoa is a genus of Gammaproteobacteria belonging to the order Thiotrichales, in the Pseudomonadota phylum. This genus was one of the first bacteria discovered by Ukrainian botanist Sergei Winogradsky. During his research in Anton de Bary's laboratory of botany in 1887, he found that  Beggiatoa  oxidized  hydrogen sulfide  (H2S) as an energy source, forming intracellular  sulfur  droplets, with oxygen as the terminal electron acceptor and CO2 used as a carbon source. Winogradsky named it in honor of the Italian doctor and botanist  Francesco Secondo Beggiato (1806 - 1883), from Venice. Winogradsky referred to this form of metabolism as "inorgoxidation" (oxidation of inorganic compounds), today called chemolithotrophy. These organisms live in sulfur-rich environments such as soil, both marine and freshwater, in the deep sea hydrothermal vents and in polluted marine environments. The finding represented the first discovery of lithotrophy. Two species of Beggiatoa have been formally described: the type species Beggiatoa alba and Beggiatoa leptomitoformis, the latter of which was only published in 2017. This colorless and filamentous bacterium, sometimes in association with other sulfur bacteria (for example the genus Thiothrix), can be arranged in biofilm visible to the naked eye formed by a very long white filamentous mat, the white color is due to the stored sulfur. Species of Beggiatoa have cells up to 200 µm in diameter and they are one of the largest prokaryotes on Earth.

Sapropel is a term used in marine geology to describe dark-coloured sediments that are rich in organic matter. Organic carbon concentrations in sapropels commonly exceed 2 wt.% in weight.

A chemocline is a type of cline, a layer of fluid with different properties, characterized by a strong, vertical chemistry gradient within a body of water. In bodies of water where chemoclines occur, the cline separates the upper and lower layers, resulting in different properties for those layers. The lower layer shows a change in the concentration of dissolved gases and solids compared to the upper layer.

<span class="mw-page-title-main">Cambrian substrate revolution</span> Diversification of animal burrowing

The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is a sudden diversification of animal burrowing during the early Cambrian period.

<span class="mw-page-title-main">Redox gradient</span>

A redox gradient is a series of reduction-oxidation (redox) reactions sorted according to redox potential. The redox ladder displays the order in which redox reactions occur based on the free energy gained from redox pairs. These redox gradients form both spatially and temporally as a result of differences in microbial processes, chemical composition of the environment, and oxidative potential. Common environments where redox gradients exist are coastal marshes, lakes, contaminant plumes, and soils.

<span class="mw-page-title-main">Soda lake</span> Lake that is strongly alkaline

A soda lake or alkaline lake is a lake on the strongly alkaline side of neutrality, typically with a pH value between 9 and 12. They are characterized by high concentrations of carbonate salts, typically sodium carbonate, giving rise to their alkalinity. In addition, many soda lakes also contain high concentrations of sodium chloride and other dissolved salts, making them saline or hypersaline lakes as well. High pH and salinity often coincide, because of how soda lakes develop. The resulting hypersaline and highly alkalic soda lakes are considered some of the most extreme aquatic environments on Earth.

<span class="mw-page-title-main">Hypoxia (environmental)</span> Low oxygen conditions or levels

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 7 ppm or 0.0007% in good quality water, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface.

<span class="mw-page-title-main">Isorenieratene</span> Chemical compound

Isorenieratene /ˌaɪsoʊrəˈnɪərətiːn/ is a carotenoid light harvesting pigment produced exclusively by the genus Chlorobium. Chlorobium are the brown-colored strains of the family of green sulfur bacteria (Chlorobiaceae). Green sulfur bacteria are anaerobic photoautotrophic organisms meaning they perform photosynthesis in the absence of oxygen using hydrogen sulfide in the following reaction:

<span class="mw-page-title-main">Western Interior Seaway anoxia</span>

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia. During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleoenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, which delivered nutrients and oxygen-adsorbing compounds into the water column.

Okenane, the diagenetic end product of okenone, is a biomarker for Chromatiaceae, the purple sulfur bacteria. These anoxygenic phototrophs use light for energy and sulfide as their electron donor and sulfur source. Discovery of okenane in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is potentially tremendously important for reconstructing past oceanic conditions, but so far okenane has only been identified in one Paleoproterozoic rock sample from Northern Australia.

Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly productive, thin surface layer, and have anoxic, sulfidic bottom water. The word euxinia is derived from the Greek name for the Black Sea (Εὔξεινος Πόντος (Euxeinos Pontos)) which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during the Proterozoic period (known as the Boring Billion) proposed by Donald Canfield, an American geologist, in 1998. There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.

<span class="mw-page-title-main">Microbial oxidation of sulfur</span>

Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to build their structural components. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy to survive, grow and reproduce. Some inorganic forms of reduced sulfur, mainly sulfide (H2S/HS−) and elemental sulfur (S0), can be oxidized by chemolithotrophic sulfur-oxidizing prokaryotes, usually coupled to the reduction of oxygen (O2) or nitrate (NO3−). Anaerobic sulfur oxidizers include photolithoautotrophs that obtain their energy from sunlight, hydrogen from sulfide, and carbon from carbon dioxide (CO2).

The sulfate-methane transition zone (SMTZ) is a zone in oceans, lakes, and rivers typically found below the sediment surface in which sulfate and methane coexist. The formation of a SMTZ is driven by the diffusion of sulfate down the sediment column and the diffusion of methane up the sediments. At the SMTZ, their diffusion profiles meet and sulfate and methane react with one another, which allows the SMTZ to harbor a unique microbial community whose main form of metabolism is anaerobic oxidation of methane (AOM). The presence of AOM marks the transition from dissimilatory sulfate reduction to methanogenesis as the main metabolism utilized by organisms.

An oxygen minimum zone (OMZ) is characterized as an oxygen-deficient layer in the world's oceans. Typically found between 200m to 1500m deep below regions of high productivity, such as the western coasts of continents. OMZs can be seasonal following the spring-summer upwelling season. Upwelling of nutrient-rich water leads to high productivity and labile organic matter, that is respired by heterotrophs as it sinks down the water column. High respiration rates deplete the oxygen in the water column to concentrations of 2 mg/L or less forming the OMZ. OMZs are expanding, with increasing ocean deoxygenation. Under these oxygen-starved conditions, energy is diverted from higher trophic levels to microbial communities that have evolved to use other biogeochemical species instead of oxygen, these species include Nitrate, Nitrite, Sulphate etc. Several Bacteria and Archea have adapted to live in these environments by using these alternate chemical species and thrive. The most abundant phyla in OMZs are Pseudomonadota, Bacteroidota, Actinomycetota, and Planctomycetota.

<span class="mw-page-title-main">Hydrothermal vent microbial communities</span> Undersea unicellular organisms

The hydrothermal vent microbial community includes all unicellular organisms that live and reproduce in a chemically distinct area around hydrothermal vents. These include organisms in the microbial mat, free floating cells, or bacteria in an endosymbiotic relationship with animals. Chemolithoautotrophic bacteria derive nutrients and energy from the geological activity at Hydrothermal vents to fix carbon into organic forms. Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.

References

  1. ↑ "Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells: Supporting Information: Glossary". US Geological Survey. Retrieved 3 December 2013.
  2. ↑ Bjork, Mats; Short, Fred; McLeod, Elizabeth; Beer, Sven (2008). Managing Sea-grasses for Resilience to Climate Change. Volume 3 of IUCN Resilience Science Group Working Papers. Gland, Switzerland: International Union for Conservation of Nature (IUCN). p.  24. ISBN   978-2-8317-1089-1.
  3. ↑ Richards, 1965; Sarmiento 1988-B
  4. ↑ McElwain, Jennifer C.; Wade-Murphy, Jessica; Hesselbo, Stephen P. (2005). "Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals". Nature. 435 (7041): 479–482. Bibcode:2005Natur.435..479M. doi:10.1038/nature03618. ISSN   0028-0836. PMID   15917805. S2CID   4339259.
  5. ↑ Wetzel, Robert G. (2001). Limnology : lake and river ecosystems (3rd ed.). San Diego: Academic Press. ISBN   0-12-744760-1. OCLC   46393244.
  6. ↑ Jerbo, 1972;Hallberg, 1974
  7. ↑ Skei, J. M. (1983). "Permanently Anoxic Marine Basins: Exchange of Substances across Boundaries". Ecological Bulletins (35): 419–429. ISSN   0346-6868. JSTOR   20112877.
  8. ↑ "Streamflow and Nutrient Delivery to the Gulf of Mexico for October 2009 to May 2010 (Preliminary)". Archived from the original on 2012-11-29. Retrieved 2011-02-09.
  9. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2011-09-27. Retrieved 2013-03-05.{{cite web}}: CS1 maint: archived copy as title (link)
  10. ↑ Gerlach, 1994
  11. ↑ Helly, John J; Levin, Lisa A (2004). "Global distribution of naturally occurring marine hypoxia on continental margins". Deep Sea Research Part I: Oceanographic Research Papers. 51 (9): 1159–1168. Bibcode:2004DSRI...51.1159H. doi:10.1016/j.dsr.2004.03.009.
  12. 1 2 3 Castro, Peter; Huber, Michael E. (2005). Marine Biology (5th ed.). McGraw Hill. ISBN   978-0-07-250934-2.
  13. 1 2 Rickard, David (2012), "Sedimentary Sulfides", Sulfidic Sediments and Sedimentary Rocks, Developments in Sedimentology, vol. 65, Elsevier, pp. 543–604, doi:10.1016/B978-0-444-52989-3.00014-3, ISBN   9780444529893 , retrieved 2021-09-18
  14. ↑ Luther, George W.; Findlay, Alyssa J.; MacDonald, Daniel J.; Owings, Shannon M.; Hanson, Thomas E.; Beinart, Roxanne A.; Girguis, Peter R. (2011). "Thermodynamics and Kinetics of Sulfide Oxidation by Oxygen: A Look at Inorganically Controlled Reactions and Biologically Mediated Processes in the Environment". Frontiers in Microbiology. 2: 62. doi: 10.3389/fmicb.2011.00062 . ISSN   1664-302X. PMC   3153037 . PMID   21833317.
  15. ↑ "5.2 Dissolved Oxygen and Biochemical Oxygen Demand". Water: Monitoring & Assessment. US Environmental Protection Agency. Retrieved 3 December 2013.
  16. ↑ Bush; et al. (2017). "Oxic-anoxic regime shifts mediated by feedbacks between biogeochemical processes and microbial community dynamics". Nature Communications. 8 (1): 789. Bibcode:2017NatCo...8..789B. doi:10.1038/s41467-017-00912-x. PMC   5630580 . PMID   28986518.
  17. ↑ "Dissolved Oxygen Depletion in Lake Erie". Great Lakes Monitoring. US Environmental Protection Agency. Retrieved 3 December 2013.
  18. ↑ Vaquer-Sunyer, Raquel; Duarte, Carlos M. (2008). "Thresholds of hypoxia for marine biodiversity". PNAS. 105 (40): 15452–15457. Bibcode:2008PNAS..10515452V. doi: 10.1073/pnas.0803833105 . PMC   2556360 . PMID   18824689.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.