III.13 The Marine Carbon Cycle
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III.13 The Marine Carbon Cycle Paul Falkowski OUTLINE 1. The two carbon cycles 2. Chemoautotrophy 3. The evolution of photoautotrophy 4. Selective pressure in the evolution of oxygenic photosynthesis 5. Primary production 6. Who are the photoautotrophs? 7. Carbon burial 8. Carbon isotope fractionation in organic matter and carbonates 9. Concluding remarks The system of scholastic disputations encouraged in the Universities of the middle ages had unfortunately trained men to habits of indefinite argumentation , and they often preferred absurd and extravagant propositions, because greater skill was required to maintain them; the end and object of such intellectual combats being victory and not the truth. —Charles Lyell, Principles of Geology, 1830 Approximately 50% of all the primary production on Earth occurs in the oceans, virtually all by microscopic, singlecelled organisms that drift with the currents, the phytoplankton . On ecological time scales of days to years, the vast majority of the organic matter produced by phytoplankton is consumed by grazers such that the turnover time of marine organic carbon is on the order of 1 week, compared with over a decade for terrestrial plant ecosystems . On geological time scales of millions of years, however , a small fraction of the carbon fixed by phytoplankton organisms is buried in marine sediments, thereby both giving rise to oxygen in Earth’s atmosphere and providing fossil fuel in the form of petroleum and natural gas. In this chapter, we examine the factors controlling the marine carbon cycle and its role in the ecology and biogeochemistry of Earth. GLOSSARY acid–base reactions. A class of (bio)chemical reactions that involve the transfer of protons without electrons . chemoautotrophy. A mode of nutrition by which an organism can reduce inorganic carbon to organic matter in the absence of light using preformed bond energy contained in other molecules. isotopic record of carbon. The changes in the ratio of 13 C to 12 C over geological time in marine carbonates or in organic matter in sediments or sedimentary rocks. net primary production. The organic carbon that is produced by photosynthetic organisms and becomes available for other trophic levels in an ecosystem. photoautotrophy. A mode of nutrition by which an organism can reduce inorganic carbon to organic matter using light energy. phytoplankton. Microscopic, mostly single-celled photosynthetic organisms that drift with the currents. redox reactions. A class of (bio)chemical reactions that involve the transfer of electrons with or without protons (i.e., hydrogen atoms). Addition of electrons or hydrogen atoms to a molecule is called ‘‘reduction ’’; removal of electrons or hydrogen atoms from a molecule is called ‘‘oxidation.’’ ‘‘Redox’’ is a contraction of the terms reduction and oxidation. 1. THE TWO CARBON CYCLES All life on Earth is critically dependent on the fluxes of six elements: H, C, N, O, S, and P. Of these, the flux of C is unique. Not only is C used to make the substrates of key biological polymers, such as lipids, carbohydrates , proteins, and nucleic acids, the oxidation and reduction of C provide the major conduit of energy supply for life itself. The biological carbon cycle is based on electron transfer (i.e., redox) reactions in which the formation and utilization of the bond energy of C–H and, to a lesser extent, C–C molecules provide the major driving force of life. However, and perhaps paradoxically, the overwhelming majority of C on Earth is contained in a relatively immobile pool in the lithosphere in the form of carbonate rocks (table 1). This oxidized pool of carbon contains no biologically available energy. To sustain a flux of carbon (and hence, an essential biological building block and energy supply) on geological time scales, the lithospheric, oxidized carbon in carbonates must enter one of two mobile pools, either the atmosphere or the ocean, from which biological processes can access the carbon, reduce it to organic matter, and transfer the organic matter through metabolic processes. Hence, there are two parallel carbon cycles on Earth. One cycle is slow and abiotic, and its chemistry is based on acid–base reactions. The physical processes that drive this cycle play a key role in Earth’s climate. The second is fast and biologically driven; its chemistry is based on electron transfer reactions. The biological processes that drive this cycle play a key role in sustaining ecosystems. Let us briefly consider the two carbon cycles and then focus on the unique role of the ocean as the conduit where both cycles meet. The ‘‘Slow’’ Geological Carbon Cycle The slow...


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