Dissolved organic carbon

[6] When water originates from land areas with a high proportion of organic soils, these components can drain into rivers and lakes as DOC.

[7] DOC is a basic nutrient, supporting growth of microorganisms and plays an important role in the global carbon cycle through the microbial loop.

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.

Occasionally, high concentrations of organic carbon indicate anthropogenic influences, but most DOC originates naturally.

The recommended procedure is the HTCO technique, which calls for filtration through pre-combusted glass fiber filters, typically the GF/F classification.

In the coastal ocean, organic matter from terrestrial plant litter or soils appears to be more refractory[19] and thus often behaves conservatively.

This wide range in turnover or degradation times has been linked with the chemical composition, structure and molecular size,[25][26] 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.

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.

[43] It also depends on SOM fractions (e.g., stabilized organic molecules and microbial biomass) by mineralization and immobilization processes.

[45] In well-drained soils, leached DOC can reach the water table and release nutrients and pollutants that can contaminate groundwater,[45][46] whereas runoff transports DOM and xenobiotics to other areas, rivers, and lakes.

[35] Precipitation and surface water leaches dissolved organic carbon (DOC) from vegetation and plant litter and percolates through the soil column to the saturated zone.

The concentration, composition, and bioavailability of DOC are altered during transport through the soil column by various physicochemical and biological processes, including sorption, desorption, biodegradation and biosynthesis.

The hydrophobicity and retention time of colloids and dissolved molecules in soils are controlled by their size, polarity, charge, and bioavailability.

[55][56] 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.

Prokaryotes (bacteria and archaea) contribute to the DOC pool via release of capsular material, exopolymers, and hydrolytic enzymes,[59] as well as via mortality (e.g. viral shunt).

[63] 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.

[66][7] Zooplankton-mediated release of DOC occurs through sloppy feeding, excretion and defecation which can be important energy sources for microbes.

[69][70][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).

[82][7] 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.

Also, some studies have shown that geothermal systems and petroleum seepage contribute with pre-aged DOC to the deep ocean basins,[88][89] but consistent global estimates of the overall input are currently lacking.

[93] Abiotic DOC flocculation is often observed during rapid (minutes) shifts in salinity when fresh and marine waters mix.

[97] 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 (NH4−, HPO2−4).

[111][96][112] Therefore, it generally means that photodegradation transforms recalcitrant into labile DOC molecules that can be rapidly used by prokaryotes for biomass production and respiration.

[127][61] 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.

These compounds differ not only in composition and concentration (from pM to μM), but also originate from various organisms (phytoplankton, zooplankton, and bacteria) and environments (terrestrial vegetation and soils, coastal fringe ecosystems) and may have been produced recently or thousands of years ago.

Moreover, even organic compounds deriving from the same source and of the same age may have been subjected to different processing histories prior to accumulating within the same pool of DOM.

[78] Interior ocean DOM is a highly modified fraction that remains after years of exposure to sunlight, utilization by heterotrophs, flocculation and coagulation, and interaction with particles.

[133][134][135] Microbes and other consumers are selective in the type of DOM they utilize and typically prefer certain organic compounds over others.

[137] The prevalent notion is that the recalcitrant fraction of DOC has certain chemical properties, which prevent decomposition by microbes ("intrinsic stability hypothesis").

An alternative or additional explanation is given by the "dilution hypothesis", that all compounds are labile, but exist in concentrations individually too low to sustain microbial populations but collectively form a large pool.