Redfield revisited:: variability of C:N:P in marine microalgae and its biochemical basis

A compilation of data on the elemental composition of marine phytoplankton from published studies was used to determine the range of C: N: P. The N: P ratio of algae and cyanobacteria is very plastic in nutrient-limited cells, ranging from < 5 mol N:mol P when phosphate is available greatly in excess of nitrate or ammonium to > 100 mol N:mol P when inorganic N is present greatly in excess of P. Under optimal nutrient-replete growth conditions. the cellular N: P ratio is somewhat more constrained, ranging from 5 to 19 mol N: mol P, with most observations below the Redfield ratio of 16. Limited data indicate that the critical N: P that marks the transition between N- and P-limitation of phytoplankton growth lies in the range 20-50 mol N: mol P, considerably in excess of the Redfield ratio. Biochemical composition can be used to constrain the critical N: P. Although the biochemical data do not preclude the critical N: P from being as high as 50, the typical biochemical composition of nutrient-replete algae and cyanobacteria suggests that the critical N:P is more likely to lie in the range between 15 and 30. Despite the observation that the overall average N: P composition of marine particulate matter closely approximates the Redfield ratio of 16, there are significant local variations with a range from 5 to 34. Consistent with the culture studies, lowest values of N: P are associated with nitrate- and phosphate-replete conditions. The highest values of N: P are observed in oligotrophic waters and are within the range of critical N: P observed in cultures, but are not so high as to necessarily invoke P-limitation. The C: N ratio is also plastic. The average C: N ratios of nutrient-replete phytoplankton cultures, oceanic particulate matter and inorganic N and C draw-down are slightly greater than the Redfield ratio of 6.6. Neither the analysis of laboratory C: N: P data nor a more theoretical approach based on the relative abundance of the major biochemical molecules in the phytoplankton can support the contention that the Redfield N: P reflects a physiological or biochemical constraint on the elemental composition of primary production.