Dissolved organic carbon

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

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

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
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.

Ocean sources
Ocean DOC sources and sinks
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 (most notably UV-radiation; it should be noted that sometimes photodegradation "transforms" rather than "removes" DOC, ending up in 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 [19][20] and in coastal waters additionally by benthic microalgae, benthic fluxes, and macrophytes,[21] whereas allochthonous DOC is mainly of terrestrial origin supplemented by groundwater and atmospheric inputs.[22][23] In addition to soil derived humic material, 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.[24][25][5]

Phytoplankton produces DOC by extracellular release commonly accounting between 5 and 30% of their total primary production,[26] although this varies from species to species.[27] 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.[28] Phytoplankton can also produce DOC by autolysis during physiological stress situations e.g., nutrient limitation.[29] Other studies have demonstrated DOC production in association with meso- and macro-zooplankton feeding on phytoplankton and bacteria.[30][5]

Zooplankton-mediated release of DOC occurs through sloppy feeding, excretion and defecation which can be important energy sources for microbes.[31][30] Such DOC production is largest during periods with high food concentration and dominance of large zooplankton species.[32][5]

Bacteria are often viewed as the main consumers of DOC, but they can also produce DOC during cell division and viral lysis.[33][34][35] 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.[36] Viruses are the most abundant life forms in the oceans infecting all life forms including algae, bacteria and zooplankton.[37] After infection, the virus either enters a dormant (lysogenic) or productive (lytic) state.[38] The lytic cycle causes disruption of the cell(s) and release of DOC.[39][5]

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,[40][41] while seagrasses release less than 5% as DOC of their gross primary production.[42] The released DOC has been shown to be rich in carbohydrates, with rates depending on temperature and light availability.[43][44] 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).[43][5]

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.[45] The DOC concentrations in sediments are often an order of magnitude higher than in the overlying water column.[46] 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.[47] This estimate is based on calculated diffusive fluxes and does not include resuspension events which also releases DOC [48] 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,[49][50] 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.[51] The DOC in groundwater is a mixture of terrestrial, infiltrated marine, and in situ microbially produced material.[52] This flux of DOC to coastal waters could be important, as concentrations in groundwater are generally higher than in coastal seawater,[53] but reliable global estimates are also currently lacking.[5]

Ocean sinks

The main processes that remove DOC from the ocean water column are: (1) Thermal degradation in e.g., submarine hydrothermal systems;[54] (2) bubble coagulation and abiotic flocculation into microparticles [55] or sorption to particles;[56] (3) abiotic degradation via photochemical reactions;[57][58] and (4) biotic degradation by heterotrophic marine prokaryotes.[59] It has been suggested that the combined effects of photochemical and microbial degradation represent the major sinks of DOC.[60][5]

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.[61] Abiotic DOC flocculation is often observed during rapid (minutes) shifts in salinity when fresh and marine waters mix.[62] 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.[63] 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,[64][65] while others find much lower values (3–6%;[66]). Such differences could be explained by seasonal and system differences in the DOC chemical composition, pH, metallic cation concentration, microbial reactivity, and ionic strength.[62][67][5]

Interaction with metals

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

See also

References
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