Dissolved organic carbon

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Dissolved organic carbon (DOC) is the fraction of total organic carbon operationally defined as that which can pass through a filter size that typically ranges in size from 0.22 and 0.7 micrometers.[1] The fraction remaining on the filter is called particulate organic carbon (POC).[2]

DOC is abundant in marine and freshwater systems and is one of the greatest cycled reservoirs of organic matter on Earth, accounting for the same amount of carbon as the atmosphere and up to 20% of all organic carbon.[3] In general, organic carbon compounds are the result of decomposition processes from dead organic matter including plants and animals. DOC can originate from within or external to the body of water. DOC originating from within the body of water is known as autochthonous DOC and typically comes from aquatic plants or algae, while DOC originating external to the body of water is known as allochthonous DOC and typically comes from soils or terrestrial plants.[4] When water originates from land areas with a high proportion of organic soils, these components can drain into rivers and lakes as DOC.

The marine DOC pool is important in the functioning of marine ecosystems, because it is at the interface between the chemical and the biological worlds, it fuels marine food webs, and is a major component of the Earth’s carbon cycling.[5]

Measurement[edit]

The dissolved fraction of total organic carbon (TOC) is an operational classification. Many researchers use the term "dissolved" for compounds that pass through a 0.45 μm filter, but 0.22 μm filters have also been used to remove higher colloidal concentrations.

A practical definition of dissolved typically used in marine chemistry is all substances that pass through a GF/F filter, which has a nominal pore size of approximately 0.7 μm (Whatman glass microfiber filter, 0.6–0.8 μm particle retention[6]). The recommended procedure is the HTCO technique, which calls for filtration through pre-combusted glass fiber filters, typically the GF/F classification.[7]

Significance[edit]

DOC is a food supplement, supporting growth of microorganisms and plays an important role in the global carbon cycle through the microbial loop.[8] In some organisms (stages) that do not feed in the traditional sense, dissolved matter may be the only external food source.[9] Moreover, DOC is an indicator of organic loadings in streams, as well as supporting terrestrial processing (e.g., within soil, forests, and wetlands) of organic matter. Dissolved organic carbon has a high proportion of biodegradable dissolved organic carbon (BDOC) in first order streams compared to higher order streams. In the absence of extensive wetlands, bogs, or swamps, baseflow concentrations of DOC in undisturbed watersheds generally range from approximately 1 to 20 mg/L carbon.[10] Carbon concentrations considerably vary across ecosystems. For example, the Everglades may be near the top of the range and the middle of oceans may be near the bottom. Occasionally, high concentrations of organic carbon indicate anthropogenic influences, but most DOC originates naturally.[11]

The BDOC fraction consists of organic molecules that heterotrophic bacteria can use as a source of energy and carbon. [12] Some subset of DOC constitutes the precursors of disinfection byproducts for drinking water.[13] BDOC can contribute to undesirable biological regrowth within water distribution systems.[14]

Distribution[edit]

A diagram showing the basic composition of dissolved organic matter in the ocean

More precise measurement techniques developed in the late 1990s have allowed for a good understanding of how dissolved organic carbon is distributed in marine environments both vertically and across the surface.[15] It is now understood that dissolved organic carbon in the ocean spans a range from very labile to very refractory. The labile dissolved organic carbon is mainly produced by marine organisms and is consumed in the surface ocean, and consists of sugars, proteins, and other compounds that are easily used by marine bacteria.[16] The refractory dissolved organic carbon is evenly spread throughout the water column and consists of high molecular weight and structurally complex compounds that are difficult for marine organisms to use such as the lignin, pollen, or humic acids.[17] Therefore, the observed vertical distribution consists of high concentrations in the upper water column and low concentrations at depth.

In addition to vertical distributions, horizontal distributions have been modeled and sampled as well.[18] In the surface ocean at a depth of 30 meters, the higher dissolved organic carbon concentrations are found in the South Pacific Gyre, the South Atlantic Gyre, and the Indian Ocean. At a depth of 3,000 meters, highest concentrations are in the North Atlantic Deep Water where dissolved organic carbon from the high concentration surface ocean is removed to depth. While in the northern Indian Ocean high DOC is observed due to high fresh water flux and sediments. Since the time scales of horizontal motion along the ocean bottom are in the thousands of years, the refractory dissolved organic carbon is slowly consumed on its way from the North Atlantic and reaches a minimum in the North Pacific.

Soil ecosystems[edit]

Soil DOC sources and sinks [19]
Sources and sinks of dissolved organic carbon in the soil system

Dissolved organic matter (DOM) is one of the most active and mobile carbon pools and has an important role in global carbon cycling.[20] In addition, dissolved organic carbon (DOC) affects the soil negative electrical charges denitrification process, acid-base reactions in the soil solution, retention and translocation of nutrients (cations), and immobilization of heavy metals and xenobiotics.[21] Soil DOM can be derived from different sources (inputs), such as atmospheric carbon dissolved in rainfall, litter and crop residues, manure, root exudates, and decomposition of soil organic matter (SOM). In the soil, DOM availability depends on its interactions with mineral components (e.g., clays, Fe and Al oxides) modulated by adsorption and desorption processes.[22] It also depends on SOM fractions (e.g., stabilized organic molecules and microbial biomass) by mineralization and immobilization processes. In addition, the intensity of these interactions changes according to soil inherent properties,[23] land use, and crop management.[22][19]

During the decomposition of organic material, most carbon is lost as CO2 to the atmosphere by microbial oxidation. Soil type and landscape slope, leaching, and runoff are also important processes associated to DOM losses in the soil.[24] In well-drained soils, leached DOC can reach the water table and release nutrients and pollutants that can contaminate groundwater,[25][26] whereas runoff transports DOM and xenobiotics to other areas, rivers, and lakes.[19]

Freshwater ecosystems[edit]

Freshwater DOC sources and sinks [27]
DOC and POC — DIC and PIC
Inland waters primarily receive carbon from terrestrial ecosystems.[28] This carbon (1.9 Pg C y-1) is transported to the oceans (0.9 Pg C y-1), buried in the sediments (0.2 Pg C y-1) or emitted as CO2 (0.8 Pg C y-1).[29] More recent estimations are different: In 2013, Raymond et al. claimed CO2 emission from inland waters can be as high as 2.1 Pg C y-1.[30]
                   P = photosynthesis                    R = respiration

Aquatic carbon occurs in different forms. Firstly, a division is made between organic and inorganic carbon. Organic carbon is a mixture of organic compounds originating from detritus or primary producers. It can be divided into POC (particulate organic carbon; particles > 0.45 μm) and DOC (dissolved organic carbon; particles < 0.45 μm). DOC usually makes up 90% of the total amount of aquatic organic carbon. Its concentration ranges from 0.1 to >300 mg L-1.[31]

Likewise, inorganic carbon also consists of a particulate (PIC) and a dissolved phase (DIC). PIC mainly consists of carbonates (e.g., CaCO3), DIC consists of carbonate (CO32-), bicarbonate (HCO3-), CO2 and a negligibly small fraction of carbonic acid (H2CO3). The inorganic carbon compounds exist in equilibrium that depends on the pH of the water.[32] DIC concentrations in freshwater range from about zero in acidic waters to 60 mg C L-1 in areas with carbonate-rich sediments.[33]

POC can be degraded to form DOC; DOC can become POC by flocculation. Inorganic and organic carbon are linked through aquatic organisms. CO2 is used in photosynthesis (P) by for instance macrophytes, produced by respiration (R), and exchanged with the atmosphere. Organic carbon is produced by organisms and is released during and after their life; e.g., in rivers, 1–20% of the total amount of DOC is produced by macrophytes.[28] Carbon can enter the system from the catchment and is transported to the oceans by rivers and streams. There is also exchange with carbon in the sediments, e.g., burial of organic carbon, which is important for carbon sequestration in aquatic habitats.[34]

Aquatic systems are very important in global carbon sequestration; e.g., when different European ecosystems are compared, inland aquatic systems form the second largest carbon sink (19–41 Tg C y-1); only forests take up more carbon (125–223 Tg C y-1).[35][27]

Marine ecosystems[edit]

Sources[edit]

Ocean DOC sources and sinks [5]
Simplified view of the main sources (black text; underlined are the allochthonous sources) and sinks (yellow text) of the oceanic dissolved organic carbon (DOC) pool.
Main sources
Most commonly referred sources of DOC are: atmospheric (e.g., rain and dust), terrestrial (e.g., rivers), primary producers (e.g., microalgae, cyanobacteria, macrophytes), groundwater, food chain processes (e.g., zooplankton grazing), and benthic fluxes (exchange of DOC across the sediment-water interface but also from hydrothermal vents).[5]
Main sinks
The four main processes removing DOC from the water column are: photodegradation (particularly UV-radiation – though sometimes photodegradation "transforms" DOC rather than removing it, ending up with higher molecular weight complex molecules), microbial (mainly by prokaryotes), aggregation (primarily when river and seawater mixes) and thermal degradation (in e.g., hydrothermal systems).[5]

In marine systems DOC originates from either autochthonous or allochthonous sources. Autochthonous DOC is produced within the system, primarily by plankton organisms [36][37] and in coastal waters additionally by benthic microalgae, benthic fluxes, and macrophytes,[38] whereas allochthonous DOC is mainly of terrestrial origin supplemented by groundwater and atmospheric inputs.[39][40] In addition to soil derived humic substances, terrestrial DOC also includes material leached from plants exported during rain events, emissions of plant materials to the atmosphere and deposition in aquatic environments (e.g., volatile organic carbon and pollens), and also thousands of synthetic human-made organic chemicals that can be measured in the ocean at trace concentrations.[41][42][5]

Phytoplankton[edit]

Phytoplankton produces DOC by extracellular release commonly accounting between 5 and 30% of their total primary production,[43] although this varies from species to species.[44] Nonetheless, this release of extracellular DOC is enhanced under high light and low nutrient levels, and thus should increase relatively from eutrophic to oligotrophic areas, probably as a mechanism for dissipating cellular energy.[45] Phytoplankton can also produce DOC by autolysis during physiological stress situations e.g., nutrient limitation.[46] Other studies have demonstrated DOC production in association with meso- and macro-zooplankton feeding on phytoplankton and bacteria.[47][5]

Zooplankton[edit]

Zooplankton-mediated release of DOC occurs through sloppy feeding, excretion and defecation which can be important energy sources for microbes.[48][47] Such DOC production is largest during periods with high food concentration and dominance of large zooplankton species.[49][5]

Bacteria and viruses[edit]

Bacteria are often viewed as the main consumers of DOC, but they can also produce DOC during cell division and viral lysis.[50][51][52] The biochemical components of bacteria are largely the same as other organisms, but some compounds from the cell wall are unique and are used to trace bacterial derived DOC (e.g., peptidoglycan). These compounds are widely distributed in the ocean, suggesting that bacterial DOC production could be important in marine systems.[53] Viruses are the most abundant life forms in the oceans infecting all life forms including algae, bacteria and zooplankton.[54] After infection, the virus either enters a dormant (lysogenic) or productive (lytic) state.[55] The lytic cycle causes disruption of the cell(s) and release of DOC.[56][5]

Macrophytes[edit]

Marine macrophytes (i.e., macroalgae and seagrass) are highly productive and extend over large areas in coastal waters but their production of DOC has not received much attention. Macrophytes release DOC during growth with a conservative estimate (excluding release from decaying tissues) suggesting that macroalgae release between 1-39% of their gross primary production,[57][58] while seagrasses release less than 5% as DOC of their gross primary production.[59] The released DOC has been shown to be rich in carbohydrates, with rates depending on temperature and light availability.[60][61] Globally the macrophyte communities have been suggested to produce ∼160 Tg C yr–1 of DOC, which is approximately half the annual global river DOC input (250 Tg C yr–1).[60][5]

Marine sediments[edit]

Marine sediments represent the main sites of OM degradation and burial in the ocean, hosting microbes in densities up to 1000 times higher than found in the water column.[62] The DOC concentrations in sediments are often an order of magnitude higher than in the overlying water column.[63] This concentration difference results in a continued diffusive flux and suggests that sediments are a major DOC source releasing 350 Tg C yr–1, which is comparable to the input of DOC from rivers.[64] This estimate is based on calculated diffusive fluxes and does not include resuspension events which also releases DOC [65] and therefore the estimate could be conservative. Also, some studies have shown that geothermal systems and petroleum seepage contribute with pre-aged DOC to the deep ocean basins,[66][67] but consistent global estimates of the overall input are currently lacking. Globally, groundwaters account for an unknown part of the freshwater DOC flux to the oceans.[68] The DOC in groundwater is a mixture of terrestrial, infiltrated marine, and in situ microbially produced material.[69] This flux of DOC to coastal waters could be important, as concentrations in groundwater are generally higher than in coastal seawater,[70] but reliable global estimates are also currently lacking.[5]

Sinks[edit]

The main processes that remove DOC from the ocean water column are: (1) Thermal degradation in e.g., submarine hydrothermal systems;[71] (2) bubble coagulation and abiotic flocculation into microparticles [72] or sorption to particles;[73] (3) abiotic degradation via photochemical reactions;[74][75] and (4) biotic degradation by heterotrophic marine prokaryotes.[76] It has been suggested that the combined effects of photochemical and microbial degradation represent the major sinks of DOC.[77][5]

Thermal degradation[edit]

Thermal degradation of DOC has been found at high-temperature hydrothermal ridge-flanks, where outflow DOC concentrations are lower than in the inflow. While the global impact of these processes has not been investigated, current data suggest it is a minor DOC sink.[78] Abiotic DOC flocculation is often observed during rapid (minutes) shifts in salinity when fresh and marine waters mix.[79] Flocculation changes the DOC chemical composition, by removing humic compounds and reducing molecular size, transforming DOC to particulate organic flocs which can sediment and/or be consumed by grazers and filter feeders, but it also stimulates the bacterial degradation of the flocculated DOC.[80] The impacts of flocculation on the removal of DOC from coastal waters are highly variable with some studies suggesting it can remove up to 30% of the DOC pool,[81][82] while others find much lower values (3–6%;[83]). Such differences could be explained by seasonal and system differences in the DOC chemical composition, pH, metallic cation concentration, microbial reactivity, and ionic strength.[79][84][5]

CDOM[edit]

The colored fraction of DOC (CDOM) absorbs light in the blue and UV-light range and therefore influences plankton productivity both negatively by absorbing light, that otherwise would be available for photosynthesis, and positively by protecting plankton organisms from harmful UV-light.[85][86] However, as the impact of UV damage and ability to repair is extremely variable, there is no consensus on how UV-light changes might impact overall plankton communities.[87][88] The CDOM absorption of light initiates a complex range of photochemical processes, which can impact nutrient, trace metal and DOC chemical composition, and promote DOC degradation.[89]

Photodegradation[edit]

Photodegradation involves the transformation of CDOM into smaller and less colored molecules (e.g., organic acids), or into inorganic carbon (CO, CO2), and nutrient salts (NH+4, HPO2−4).[90][91][92] Therefore, it generally means that photodegradation transforms recalcitrant into labile DOC molecules that can be rapidly used by prokaryotes for biomass production and respiration. However, it can also increase CDOM through the transformation of compounds such as triglycerides, into more complex aromatic compounds,[93][94] which are less degradable by microbes. Moreover, UV radiation can produce e.g., reactive oxygen species, which are harmful to microbes.[95] The impact of photochemical processes on the DOC pool depends also on the chemical composition,[96] with some studies suggesting that recently produced autochthonous DOC becomes less bioavailable while allochthonous DOC becomes more bioavailable to prokaryotes after sunlight exposure, albeit others have found the contrary.[97][98][99] Photochemical reactions are particularly important in coastal waters which receive high loads of terrestrial derived CDOM, with an estimated ∼20–30% of terrestrial DOC being rapidly photodegraded and consumed.[100] Global estimates also suggests that in marine systems photodegradation of DOC produces ∼180 Tg C yr–1 of inorganic carbon, with an additional 100 Tg C yr–1 of DOC made more available to microbial degradation.[101][102] Another attempt at global ocean estimates also suggest that photodegradation (210 Tg C yr–1) is approximately the same as the annual global input of riverine DOC (250 Tg C yr–1;[103]), while others suggest that direct photodegradation exceeds the riverine DOC inputs.[104][105][5]

Degradation times[edit]

Due to the continuous production and degradation in natural systems the DOC pool contains a spectrum of reactive compounds, each with their own reactivity,[106] that have been divided into the following pools, from labile to recalcitrant, depending on the turnover times: (1) from hours to days (labile pool: DOCL < 200 Tg C), (2) weeks to months (semi-labile fraction: DOCSL; ∼600 Tg C), (3) over decades (semi-recalcitrant fraction: DOCSR; ∼1400 Tg C), (4) over thousands of years (recalcitrant fraction: DOCR; ∼63000 Tg C), and (5) a pool resistant to removal for tens of thousands of years.[107] This wide range in degradation times has been linked with the chemical composition, structure and molecular size,[108][109] but degradation also depends on the environmental conditions (e.g., nutrients), prokaryote diversity, redox state, iron availability, mineral-particle associations, temperature, sun-light exposure, biological production of recalcitrant compounds, and the effect of priming or dilution of individual molecules.[108][110][111][112][113][114] For example, lignin can be degraded in aerobic soils but is relatively recalcitrant in anoxic marine sediments.[115] This example shows bioavailability varies as a function of the ecosystem’s properties. Accordingly, even normally ancient and recalcitrant compounds (e.g., petroleum, carboxyl-rich alicyclic molecules) can be degraded in the appropriate environmental setting.[116][117]

Interaction with metals[edit]

DOC also facilitates the transport of metals in aquatic systems. Metals form complexes with DOC, enhancing metal solubility while also reducing metal bioavailability.

See also[edit]

References[edit]

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