F-ratio

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Empirically derived effect of temperature and Net Primary Productivity on the f-ratio, and approximate values for some large ocean regions. F-ratio.png
Empirically derived effect of temperature and Net Primary Productivity on the f-ratio, and approximate values for some large ocean regions.

In oceanic biogeochemistry, the f-ratio is the fraction of total primary production fuelled by nitrate (as opposed to that fuelled by other nitrogen compounds such as ammonium). The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production. [1] This fraction was originally believed significant because it appeared to directly relate to the sinking (export) flux of organic marine snow from the surface ocean by the biological pump. However, this interpretation relied on the assumption of a strong depth-partitioning of a parallel process, nitrification, that more recent measurements has questioned. [2]

Contents

Overview

Gravitational sinking of organisms (or the remains of organisms) transfers carbon from the surface waters of the ocean to its deep interior. This process is known as the biological pump, and quantifying it is of interest to scientists because it is an important aspect of the Earth's carbon cycle. Essentially, this is because carbon transported to the deep ocean is isolated from the atmosphere, allowing the ocean to act as a reservoir of carbon. This biological mechanism is accompanied by a physico-chemical mechanism known as the solubility pump which also acts to transfer carbon to the ocean's deep interior.

Measuring the flux of sinking material (so-called marine snow) is usually done by deploying sediment traps which intercept and store material as it sinks down the water column. However, this is a relatively difficult process, since traps can be awkward to deploy or recover, and they must be left in situ over a long period to integrate the sinking flux. Furthermore, they are known to experience biases and to integrate horizontal as well as vertical fluxes because of water currents. [3] [4] For this reason, scientists are interested in ocean properties that can be more easily measured, and that act as a proxy for the sinking flux. The f-ratio is one such proxy.

"New" and "regenerated" production

Diagram of new and regenerated production F ratio diagram.gif
Diagram of new and regenerated production

Bio-available nitrogen occurs in the ocean in several forms, including simple ionic forms such as nitrate (NO3−), nitrite (NO2−) and ammonium (NH4+), and more complex organic forms such as urea ((NH2)2CO). These forms are used by autotrophic phytoplankton to synthesise organic molecules such as amino acids (the building blocks of proteins). Grazing of phytoplankton by zooplankton and larger organisms transfers this organic nitrogen up the food chain and throughout the marine food-web.

When nitrogenous organic molecules are ultimately metabolised by organisms, they are returned to the water column as ammonium (or more complex molecules that are then metabolised to ammonium). This is known as regeneration, since the ammonium can be used by phytoplankton, and again enter the food-web. Primary production fuelled by ammonium in this way is thus referred to as regenerated production. [5]

However, ammonium can also be oxidised to nitrate (via nitrite), by the process of nitrification. This is performed by different bacteria in two stages :

NH3 + O2 → NO2− + 3H+ + 2e−
NO2− + H2O → NO3− + 2H+ + 2e−

Crucially, this process is believed to only occur in the absence of light (or as some other function of depth). In the ocean, this leads to a vertical separation of nitrification from primary production, and confines it to the aphotic zone. This leads to the situation whereby any nitrate in the water column must be from the aphotic zone, and must have originated from organic material transported there by sinking. Primary production fuelled by nitrate is, therefore, making use of a "fresh" nutrient source rather than a regenerated one. Production by nitrate is thus referred to as new production. [5]

The figure at the head of this section illustrates this. Nitrate and ammonium are taken up by primary producers, processed through the food-web, and then regenerated as ammonium. Some of this return flux is released into the surface ocean (where it is available again for uptake), while some is returned at depth. The ammonium returned at depth is nitrified to nitrate, and ultimately mixed or upwelled into the surface ocean to repeat the cycle.

Consequently, the significance of new production lies in its connection to sinking material. At equilibrium, the export flux of organic material sinking into the aphotic zone is balanced by the upward flux of nitrate. By measuring how much nitrate is consumed by primary production, relative to that of regenerated ammonium, one should be able to estimate the export flux indirectly.

As an aside, the f-ratio can also reveal important aspects of local ecosystem function. [6] High f-ratio values are typically associated with productive ecosystems dominated by large, eukaryotic phytoplankton (such as diatoms) that are grazed by large zooplankton (and, in turn, by larger organisms such as fish). By contrast, low f-ratio values are generally associated with low biomass, oligotrophic food webs consisting of small, prokaryotic phytoplankton (such as Prochlorococcus ) which are kept in check by microzooplankton. [7] [8]

Assumptions

Chart indicating the question of nitrification and the aphotic zone F ratio diagram 2.gif
Chart indicating the question of nitrification and the aphotic zone

A fundamental assumption in this interpretation of the f-ratio is the spatial separation of primary production and nitrification. Indeed, in their original paper, Eppley & Peterson noted that: "To relate new production to export requires that nitrification in the euphotic zone be negligible." [1] However, subsequent observational work on the distribution of nitrification has found that nitrification can occur at shallower depths, and even within the photic zone. [2] [9] [10]

As the adjacent diagram shows, if ammonium is indeed nitrified to nitrate in the ocean's surface waters it essentially "short circuits" the deep pathway of nitrate. In practice, this would lead to an overestimation of new production and a higher f-ratio, since some of the ostensibly new production would actually be fuelled by recently nitrified nitrate that had never left the surface ocean. After including nitrification measurements in its parameterisation, an ecosystem model of the oligotrophic subtropical gyre region (specifically the BATS site) found that, on an annual basis, around 40% of surface nitrate was recently nitrified (rising to almost 90% during summer). [11] A further study synthesising geographically diverse nitrification measurements found high variability but no relationship with depth, and applied this in a global-scale model to estimate that up to a half of surface nitrate is supplied by surface nitrification rather than upwelling. [12]

Although measurements of the rate of nitrification are still relatively rare, they do suggest that the f-ratio is not as straightforward a proxy for the biological pump as was once thought. For this reason, some workers have proposed distinguishing between the f-ratio and the ratio of particulate export to primary production, which they term the pe-ratio. [8] While quantitatively different from the f-ratio, the pe-ratio shows similar qualitative variation between high productivity/high biomass/high export regimes and low productivity/low biomass/low export regimes.

In addition, a further process that potentially complicates the use of the f-ratio to estimate "new" and "regenerated" production is dissimilatory nitrate reduction to ammonium (DNRA). In low oxygen environments, such as oxygen minimum zones and seafloor sediments, chemoorganoheterotrophic microbes use nitrate as an electron acceptor for respiration, [13] reducing it to nitrite, then to ammonium. Since, like nitrification, DNRA alters the balance in the availability of nitrate and ammonium, it has the potential to introduce inaccuracy to the calculated f-ratio. However, as DNRA's occurrence is limited to anaerobic situations, [14] its importance is less widespread than nitrification, although it can occur in association with primary producers. [15] [16]

See also

Related Research Articles

Phytoplankton Autotrophic members of the plankton ecosystem

Phytoplankton are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words φυτόν, meaning 'plant', and πλαγκτός, meaning 'wanderer' or 'drifter'.

Nitrogen cycle Biogeochemical cycle by which nitrogen is converted into various chemical forms

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

Biological pump Oceans biologically driven sequestration of carbon from the atmosphere to the ocean interior and seafloor

The biological pump, also known as the marine carbon pump, is, in its simplest form, the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments. It is the part of the oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO3) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).

Nitrification biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate

Nitrification is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil. Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea.

Denitrification

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.

Iron cycle

The iron cycle (Fe) is the biogeochemical cycle of iron through the atmosphere, hydrosphere, biosphere and lithosphere. While Fe is highly abundant in the Earth's crust, it is less common in oxygenated surface waters. Iron is a key micronutrient in primary productivity, and a limiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to as High-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.

High-nutrient, low-chlorophyll (HNLC) regions are regions of the ocean where the abundance of phytoplankton is low and fairly constant despite the availability of macronutrients. Phytoplankton rely on a suite of nutrients for cellular function. Macronutrients are generally available in higher quantities in surface ocean waters, and are the typical components of common garden fertilizers. Micronutrients are generally available in lower quantities and include trace metals. Macronutrients are typically available in millimolar concentrations, while micronutrients are generally available in micro- to nanomolar concentrations. In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport.

Redfield ratio

Redfield ratio or Redfield stoichiometry is the consistent atomic ratio of carbon, nitrogen and phosphorus found in marine phytoplankton and throughout the deep oceans.

Nitrobacter is a genus comprising rod-shaped, gram-negative, and chemoautotrophic bacteria. The name Nitrobacter derives from the Latin neuter gender noun nitrum, nitri, alkalis; the Ancient Greek noun βακτηρία, βακτηρίᾱς, rod. They are non-motile and reproduce via budding or binary fission. Nitrobacter cells are obligate aerobes and have a doubling time of about 13 hours.

Ocean fertilization Type of climate engineering

Ocean fertilization or ocean nourishment is a type of climate engineering based on the purposeful introduction of nutrients to the upper ocean to increase marine food production and to remove carbon dioxide from the atmosphere. A number of techniques, including fertilization by iron, urea and phosphorus have been proposed. But research in the early 2020s suggested that it could only permanently sequester a small amount of carbon.

Human impact on the nitrogen cycle

Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation. As a consequence of anthropogenic inputs, the global nitrogen cycle (Fig. 1) has been significantly altered over the past century. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005. Human activities account for over one-third of N2O emissions, most of which are due to the agricultural sector. This article is intended to give a brief review of the history of anthropogenic N inputs, and reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems.

Marine snow Shower of mostly organic detritus falling from the upper layers of the water column

In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below, which is referred to as the biological pump. Export production is the amount of organic matter produced in the ocean by primary production that is not recycled (remineralised) before it sinks into the aphotic zone. Because of the role of export production in the ocean's biological pump, it is typically measured in units of carbon. The term was first coined by the explorer William Beebe as he observed it from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic activity and ocean currents. Marine snow can be an important food source for organisms living in the aphotic zone, particularly for organisms which live very deep in the water column.

Bacterioplankton bacterial component of the plankton that drifts in the water column

Bacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word πλανκτος, meaning "wanderer" or "drifter", and bacterium, a Latin term coined in the 19th century by Christian Gottfried Ehrenberg. They are found in both seawater and freshwater.

In biological oceanography, new production is supported by nutrient inputs from outside the euphotic zone, especially upwelling of nutrients from deep water, but also from terrestrial and atmosphere sources. New production depends on mixing and vertical advective processes associated with the circulation.

Oceanic carbon cycle Processes that exchange carbon between various pools within the ocean and the atmosphere, Earth interior, and the seafloor.

The oceanic carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon and organic carbon. Part of the marine carbon cycle transforms carbon between non-living and living matter.

Particulate organic matter

Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 and 2 milimeters.

Dissimilatory nitrate reduction to ammonium (DNRA), also known as nitrate/nitrite ammonification, is the result of anaerobic respiration by chemoorganoheterotrophic microbes using nitrate (NO3−) as an electron acceptor for respiration. In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate (rather than oxygen) as an electron acceptor, reducing it to nitrite, then ammonium (NO3−→NO2−→NH4+).

Marine biogeochemical cycles

Marine biogeochemical cycles are biogeochemical cycles that occur within marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substances and elements move through within the marine environment. In addition, substances and elements can be imported into or exported from the marine environment. These imports and exports can occur as exchanges with the atmosphere above, the ocean floor below, or as runoff from the land.

An oxygen minimum zone (OMZ) is characterized as an oxygen-deficient layer in the world 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 Proteobacteria, Bacteroidetes, Actinobacteria, and Planctomycetes.

Viral shunt

The viral shunt is a mechanism that prevents marine microbial particulate organic matter (POM) from migrating up trophic levels by recycling them into dissolved organic matter (DOM), which can be readily taken up by microorganisms. The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.

References

  1. 1 2 Eppley, R.W.; Peterson, B.J. (1979). "Particulate organic matter flux and planktonic new production in the deep ocean". Nature . 282 (5740): 677–680. Bibcode:1979Natur.282..677E. doi:10.1038/282677a0. S2CID   42385900.
  2. 1 2 Dore, J.E.; Karl, D.M. (1996). "Nitrification in the euphotic zone as a source for nitrite, nitrate, and nitrous oxide at Station ALOHA". Limnol. Oceanogr. 41 (8): 1619–1628. Bibcode:1996LimOc..41.1619D. doi: 10.4319/lo.1996.41.8.1619 . JSTOR   00243590.
  3. ↑ Thomas, S.; Ridd, P.V. (2004). "Review of methods to measure short time scale sediment accumulation". Marine Geology. 207 (1–4): 95–114. Bibcode:2004MGeol.207...95T. doi:10.1016/j.margeo.2004.03.011.
  4. ↑ Buesseler, K.O.; et al. (2007). "An assessment of the use of sediment traps for estimating upper ocean particle fluxes" (PDF). J. Mar. Res. 65 (3): 345–416. doi:10.1357/002224007781567621. hdl:1912/1803. ISSN   0022-2402.
  5. 1 2 Dugdale, R.C.; Goering, J.J. (1967). "Uptake of new and regenerated forms of nitrogen in primary productivity". Limnol. Oceanogr. 12 (2): 196–206. Bibcode:1967LimOc..12..196D. doi: 10.4319/lo.1967.12.2.0196 .
  6. ↑ Allen, A.E.; Howard-Jones, M.H.; Booth, M.G.; Frischer, M.E.; Verity, P.G.; Bronk, D.A.; Sanderson, M.P. (2002). "Importance of heterotrophic bacterial assimilation of ammonium and nitrate in the Barents Sea during summer". Journal of Marine Systems. 38 (1–2): 93–108. Bibcode:2002JMS....38...93A. doi:10.1016/s0924-7963(02)00171-9.
  7. ↑ Laws, E.A.; Falkowski, P.G.; Smith, W.O.; Ducklow, H.; McCarthy, J.J. (2000). "Temperature effects on export production in the open ocean". Global Biogeochemical Cycles. 14 (4): 1231–1246. Bibcode:2000GBioC..14.1231L. doi: 10.1029/1999GB001229 .
  8. 1 2 Dunne, J.P.; Armstrong, R.A.; Gnanadesikan, A.; Sarmiento, J.L. (2005). "Empirical and mechanistic models for the particle export ratio". Global Biogeochemical Cycles. 19 (4): GB4026. Bibcode:2005GBioC..19.4026D. doi:10.1029/2004GB002390.
  9. ↑ Raimbault, P.; Slawyk, G.; Boudjellal, B.; Coatanoan, C.; Conan, P.; Coste, B.; Garcia, N.; Moutin, T.; Pujo-Pay, M. (1999). "Carbon and nitrogen uptake and export in the equatorial Pacific at 150°W: Evidence of an efficient regenerated production cycle". J. Geophys. Res. 104 (C2): 3341–3356. Bibcode:1999JGR...104.3341R. doi:10.1029/1998JC900004.
  10. ↑ Diaz, F.; Raimbault, P. (2000). "Nitrogen regeneration and dissolved organic nitrogen release during spring in a NW Mediterranean coastal zone (Gulf of Lions): implications for the estimation of new production". Mar. Ecol. Prog. Ser. 197: 51–65. Bibcode:2000MEPS..197...51D. doi: 10.3354/meps197051 .
  11. ↑ Martin, A.P.; Pondaven, P. (2006). "New primary production and nitrification in the western subtropical North Atlantic: a modelling study". Global Biogeochemical Cycles. 20 (4): n/a. Bibcode:2006GBioC..20.4014M. doi: 10.1029/2005GB002608 .
  12. ↑ Yool, A.; Martin, A.P.; Fernández, C.; Clark, D.R. (2007). "The significance of nitrification for oceanic new production". Nature . 447 (7147): 999–1002. Bibcode:2007Natur.447..999Y. doi:10.1038/nature05885. PMID   17581584. S2CID   4416535.
  13. ↑ Kraft, B. Strous, M. and Tegetmeyer, H. E. (2011). "Microbial nitrate respiration – Genes, enzymes and environmental distribution". Journal of Biotechnology. 155 (1): 104–117. doi:10.1016/j.jbiotec.2010.12.025. PMID   21219945.CS1 maint: multiple names: authors list (link)
  14. ↑ Lam, Phyllis and Kuypers, Marcel M. M. (2011). "Microbial Nitrogen Processes in Oxygen Minimum Zones". Annual Review of Marine Science. 3: 317–345. Bibcode:2011ARMS....3..317L. doi:10.1146/annurev-marine-120709-142814. hdl: 21.11116/0000-0001-CA25-2 . PMID   21329208.CS1 maint: multiple names: authors list (link)
  15. ↑ Kamp, Anja; Beer, Dirk de; Nitsch, Jana L.; Lavik, Gaute; Stief, Peter (2011-04-05). "Diatoms respire nitrate to survive dark and anoxic conditions". Proceedings of the National Academy of Sciences. 108 (14): 5649–5654. Bibcode:2011PNAS..108.5649K. doi: 10.1073/pnas.1015744108 . ISSN   0027-8424. PMC   3078364 . PMID   21402908.
  16. ↑ Kamp, Anja; Stief, Peter; Knappe, Jan; Beer, Dirk de (2013-12-02). "Response of the Ubiquitous Pelagic Diatom Thalassiosira weissflogii to Darkness and Anoxia". PLOS ONE. 8 (12): e82605. Bibcode:2013PLoSO...882605K. doi: 10.1371/journal.pone.0082605 . ISSN   1932-6203. PMC   3846789 . PMID   24312664.
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