Marine dissolved organic matter is a complex mixture of molecules of diverse origins found in seawater from throughout the world ocean. The concentrations of individual molecules are low, but there are tens of thousands of different molecules in seawater and the volume of the world ocean is large making this a major global reservoir of non-living organic matter. The total amount of marine dissolved organic matter is most commonly measured as the organic carbon in seawater that has passed through a filter with a pore size of about 0.5 microns to remove large particles and organisms.
Marine dissolved organic matter (DOM) is important in the global carbon cycle and marine food webs, and it affects the penetration of light, the exchange of gases at the sea surface and the availability of trace metals and other nutrients to biota. The world ocean is one of the largest reservoirs of actively cycling organic carbon on Earth, and most of the actively cycling organic carbon in seawater resides in the operationally-defined dissolved phase. The amount of dissolved organic carbon in seawater (700 Pg) is similar to the amount of carbon in atmospheric carbon dioxide, so understanding the factors influencing the production and remineralization of marine DOM is of great significance to the global carbon cycle and climate change.
Phytoplankton, including photosynthetic algae and bacteria, are the primary source of marine dissolved organic matter. Marine DOM is released from phytoplankton by at least three different mechanisms: direct extracellular release during growth; release during predation by grazing organisms; and release during viral lysis of cells. The major mechanism for the removal of marine DOM is consumption by heterotrophic bacteria. The transfer of carbon and associated elements from marine DOM to bacteria forms the base of the microbial food web in the ocean. It is estimated that about half of the carbon fixed during photosynthesis by marine phytoplankton passes through DOM and the microbial food web. This component of marine DOM is very reactive and cycles rapidly, and includes many common biochemicals, such as amino acids and carbohydrates. These biological processes and cycling of DOM are most rapid in the upper hundred meters of the water column where there is sufficient sunlight for photosynthesis. Photochemical oxidation of DOM to carbon dioxide and carbon monoxide also occurs in the surface ocean, and recent estimates indicate this process is a quantitatively significant component of the ocean carbon cycle.
Most marine DOM is produced and consumed in the upper ocean (above 1000 m depth). The concentration of dissolved organic carbon in the deep ocean is about half of the concentration found in the surface ocean. The average depth of the world ocean is about 4000 m, so most marine DOM resides in the deep ocean (below 1000 m depth). Relatively little is known about biological processes and marine DOM cycling in the deep ocean, but several different types of data indicate deep marine DOM is resistant to biological utilization and degradation. A relatively small fraction of deep marine DOM is recognizable biochemicals, and its chemical structure and nature are largely unknown. Mixing of the world ocean occurs on a millennial time scale, and there is evidence indicating photochemical and biological processes occurring in surface waters enhance the oxidation of DOM from the deep ocean.
- Hansell, D. A., and C. A. Carlson (eds). 2002. Biogeochemistry of Marine Dissolved Organic Matter, 774 pages, Academic Press.
- Kirchman, D. L. (ed). 2000. Microbial Ecology of the Oceans, 542 pages, Wiley-Liss.