[ShaunW]

MARINE BIOGEOCHEMISTRY: ON REDFIELD RATIOS

The following points are made by P.G. Falkowski and C.S. Davis (Nature 2004 431:131):

1) An interesting empirical observation in biology is the relationship between the elemental composition of organisms and ecosystems. All organisms are composed primarily of a mixture of six major elements: hydrogen, carbon, nitrogen, oxygen, phosphorus and sulphur. But the proportion of these basic ingredients varies between organisms -- and such variations can lead to interesting properties within ecosystems.

2) For example, in the oceans most of the biomass comprises small drifting organisms (plankton) that are rich in nitrogen. These organisms are essentially functionally similar ensembles of metabolites, often encased in a shell formed from the most readily available ingredients. Much plankton is consumed by other plankton with similar chemical compositions. The result is that on average, the nitrogen: phosphorus (N:P) ratios of plankton in the oceans are remarkably similar throughout the world, averaging approximately 16:1 by atoms. When these organisms or their body parts sink into the ocean interior, their energy-rich bodies are consumed by bacteria which, in aerobic conditions, oxidize the organic matter to form dissolved inorganic nutrients, especially CO2, NO3(-) and PO4(3-).

3) In 1934, Alfred Redfield (1890-1983) wrote a now classic paper in which he proposed that the N:P ratio of plankton (16:1) causes the ocean to have a remarkably similar ratio of dissolved NO3(-) and PO4(3-). This hypothesis suggested that, devoid of life, the chemical composition of the oceans would be markedly different. The concept of Redfield ratios has been fundamental to our understanding of the biogeochemistry of the oceans ever since.

4) The basic problem with Redfield ratios is that they are empirical. The ratios were originally derived from measurements of the elemental composition of plankton, and the NO3(-) and PO4(3-) content of seawater from a few stations in the Atlantic, but were subsequently supported by hundreds of independent measurements. Yet there is no known reason why the average N:P ratio of plankton should be 16:1. Why not 6:1? Or 60:1? If one looks at the elemental composition of individual species of phytoplankton grown under nitrogen or phosphorus limitation, the N:P ratio can vary from around 6:1 to 60:1. Redfield understood this problem, but did not try explain it, except to note that the N:P ratio of inorganic nutrients in the ocean interior was an average, and that small-scale variability around the mean was to be expected.

5) Despite many reports that the elemental composition of organisms in a region of the ocean does not conform to Redfield ratios, or that the elemental composition of marine phytoplankton grown in cultures is not 16:1, Redfield's fundamental concept remains valid. It cannot be rationalized by reductionist arguments, nor refuted by anecdotal observations. The fact that the NO(3-):PO4(3-) ratio in the interior of all major ocean basins is remarkably similar to the N:P ratio of plankton is due to the residence times of these two elements in the ocean (roughly 10^(4) years), relative to the ocean's circulation time (roughly 10^(3) years). As the residence times exceed the mixing times by an order of magnitude, it should not be surprising that the NO(3-):PO4(3-) ratios in the ocean interior are remarkably constant.

6) The specific elemental composition that is the Redfield ratio is truly an "emergent" property that reflects the interaction of multiple processes, including the acquisition of the elements by plankton, the formation of new biomass and the remineralization of the biomass by bacteria in the ocean interior, as well as losses of nutrients from the ocean because of burial in the sediments (for example, phosphorus in apatite), or outgassing to the atmosphere (for example, production and loss of N2, due to denitrification).(1-4)

References:

1. Falkowski, P. G. et al. Science 305, 354-360 (2004)

2. Hedin, L. Proc. Natl Acad. Sci. USA 30, 10849-10850 (2004)

3. Redfield, A. C. in James Johnstone Memorial Volume (ed. Daniel, R. J.) 176-192. (Liverpool Univ. Press, 1934)

4. Sterner, R. W. & Elser, J. J. Ecological Stoichiometry: The Biology of the Elements from Molecules to the Biosphere (Princeton Univ. Press, 2002)

Nature http://www.nature.com/nature