The biochemical and elemental compositions of marine plankton: A NMR perspective

The traditional Redfield–Ketchum–Richards (1963) equation for the production (or respiration) of “average marine plankton” 106 CO2+16 HNO3+H3PO4+122 H2O=(CH2O)106(NH3)16(H3PO4)+138 O2 has long been a useful guideline for establishing the ratios and reaction extents of the bioactive elements in ocean systems. The empirical formula on the right of the above equation for marine plankton biomass adequately represents the C/N/P of mixed marine plankton collected in towed nets, but includes an impossibly elevated hydrogen content and a questionably high level of organic oxygen. An elevated estimate of oxygen content is particularly critical because it would lead to an underestimate of the amount of O2 required for complete respiration of plankton biomass. Although direct biochemical measurements have been used previously to constrain the compositions, and hence the reaction stoichiometries, of marine plankton and their remains, such analyses can be prone to error and analytical bias. To cast a new light on the chemical composition of marine plankton, we determined the major functional group distribution of organic carbon in mixed plankton tows from five contrasting ocean sites using cross-polarization, magic-angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13C NMR). Using a mixing model that relates NMR spectral data to biochemical composition, we estimate an average major biochemical composition (weight basis) for these plankton samples of 65% protein, 19% lipid and 16% carbohydrate. This biochemical composition corresponds to an average elemental formula for plankton biomass of C106H177O37N17S0.4, whose complete oxidation requires 154 moles of O2. Although preliminary, this 13C NMR-based estimate indicates elemental compositions and respiratory oxygen demands that are widely different from those indicated by the RKR composition (C106H260O106N16 and 138 O2, respectively) and those determined in many previous field studies.