Minding Their Ps and Ns

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Science  04 Mar 2011:
Vol. 331, Issue 6021, pp. 1114-1115
DOI: 10.1126/science.331.6021.1114-c

In the mid-20th century, Alfred Redfield posited that the bulk ratio of nitrogen to phosphorus atoms (N:P) in marine microorganisms should maintain a relatively constant value of ∼16. Work since then has shown that the ratio indeed remains relatively constant across many environments and time scales, including deep oceans and coastal waters, but questions remain about whether innate biochemical or environmental factors are responsible. Loladze and Elser compiled literature values of nutrient ratios in prokaryotic and eukaryotic microorganisms, which, combined with a theoretical model, suggest that the N:P ratio is determined by a balance of maximum macromolecule biosynthesis rates—specifically for nitrogen-rich proteins and phosphorus-rich ribosomal RNA. Although the analysis considered cases in which growth rates were optimal, an N:P ratio of ∼16 isn't necessarily always desirable for efficient growth; communities in environments where the paucity of nitrogen or phosphorus limits growth may have optimal N:P ratios that are shifted away from 16. Because one of these two nutrients is often the main limiting growth factor in aquatic and terrestrial systems, observable deviations in N:P ratios would therefore provide insight into the biogeochemical processes that shape microbial community structure.

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