Theoretical limits of oxygen:carbon and oxygen:nitrogen ratios during photosynthesis and mineralisation of organic matter in the sea

Abstract

It is frequent to observe disagreements between the relationships R_C =-ΔO₂/ΔC and R_N = -ΔO₂/ΔN found during photosynthesis-mineralisation studies in the ocean and those expected according to the Redfield plankton composition. That is because R_C and R_N depend on the C/N and Carbohydrates/Lipids (Cbh/Lip) ratios of the organic matter. Starting from the mean biochemical composition of the main groups of phytoplankton biomolecules and from the stoichiometry of photosynthesis-mineralisation, we establish a new formulation to enable us to calculate R_C and R_N as a function of C/N and Cbh/Lip ratios independent of the variance in the N/P ratio. The approach proposed here permits the theoretical limits of R_C and R_N to be calculated. The R_C value can vary between 1 and 1.58 while R_N can vary between 5.8 and infinity with the interrelationships between the ratios being defined by R_N/R_C=C/N which, in its turn, can vary between 3.67 and infinity. For a given R_C the limits of R_N become narrower and vice versa. When C/N ratio is fixed the limits of the R_C and R_N values also become narrower. A graphic representation to check and visualise these limits and other experimental biochemical composition values is developed. According to the average value of the Cbh/Lip relationship, 0.82 in carbon, recovered from the literature and for a C/N = 106/16, the estimate mean R_C and R_N values are 1.41 and 9.33 respectively which are coincident with previous ratios given by other authors.