Microscopic plants and other photosynthetic organisms that drift with ocean currents lie at the heart of the marine carbon cycle. Sunlit surface waters teem with phytoplankton that convert inorganic carbon dissolved in surface waters to organic carbon, which forms the basis of the marine food web, and account for about half of all primary production on Earth (Field et al. 1998; Falkowski, Barber, and Smetacek 1998). In contrast to terrestrial carbon-turnover times that may take months to years, carbon cycles rapidly in oceans, with the entire phytoplankton population in some environments replacing itself weekly (Falkowski 2002). Phytoplankton are grazed upon by marine heterotrophs known as zooplankton. These grazer species range from microscopic protozoa and copepods to worms, krill, crabs, jellyfish, and the larvae of fish and other organisms. Comprising most of the animal mass in the ocean, zooplankton serve as the crucial link between primary producers and the rest of the marine food web. Viruses, which act as predators in oceanic food chains by infecting and lysing marine bacteria, also play an important but still poorly understood role in marine carbon turnover. The overall efficiency with which organic carbon is exported to the deep ocean depends on the type of photoautotrophic cells that create the organic material and the efficiency with which heterotrophic organisms respire it.
Carbon Flow and Fate
Carbon fixed in phytoplankton eventually enters the water column as either particulate or dissolved organic carbon through direct exudation, consumption by grazing zooplankton, viral lysis, or cell death. Subsequently, most of this carbon material is degraded by heterotrophic bacteria, resulting in particulate solubilization and conversion of organic carbon back to CO2. Some of the organic matter, however, sinks intact to the underlying twilight zone (the ocean's barely lit middle layer) and beyond, where lower temperatures, lack of oxygen, and other factors significantly slow degradation. CO2 fixed during photosynthesis by phytoplankton in the upper ocean can be transferred to the depths via three major processes: passive sinking of particles, physical mixing of particulates and dissolved organic matter through currents, and active transport by zooplankton migrating to deeper waters. Detrital particles and organic matter associated with mineral structures from phytoplankton, for example, may resist rapid microbial degradation and sift down as flakes, also called marine snow, becoming platforms for microbes to live on. As this particulate organic matter falls deeper, it can cluster with other small particles, such as zooplankton fecal pellets, molts, and larvacean houses, to form larger, heavier aggregates held together by a polysaccharide matrix. The carbon in these particles can be isolated from exchange with the atmosphere for centuries to millennia before upwelling currents return it and other nutrients from the deep ocean to warm surface waters. Some carbon is lost at each step of the way, however, as the organisms involved consume or degrade the organic carbon and remineralize it to CO2 through respiration. However, if climate change and ocean acidification significantly alter marine ecosystems' functions, the efficiency of this biologically mediated ocean carbon export may change, leading to an indirect effect on the net annual uptake of carbon.
Falkowski, P. G. 2002. "The Ocean's Invisible Forest: Marine Phytoplankton Play a Critical Role in Regulating the Earth's Climate. Could They Also be Used to Combat Global Warming?" Scientific American 287(2), 54-61.
Falkowski, P. G., R. T. Barber, and V. Smetacek. 1998. "Biogeochemical Controls and Feedbacks on Ocean Primary Production," Science 281(5374), 200-06.
Field, C. B., et al. 1998. "Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components," Science 281(5374), 237-40.
Credit or Source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber/
U.S. DOE. 2008. Carbon Cycling and Biosequestration: Report from the March 2008 Workshop, DOE/SC-108, U.S. Department of Energy Office of Science. (p. 81) (website)