A Revised Estimate of the Iron Use Efficiency of Nitrogen Fixation, With Special Reference to the Marine Cyanobacterium Trichodesmium Spp. (Cyanophyta)¹

Estimates of the iron use efficiency (IUE) for diazotrophic plant growth have been used to suggest iron limitation of marine N 2 fixation. However, in the course of these inferences, neither the physiological complexity of these estimates nor the specific physiological parameters of marine diazotrophs were evaluated. Here, a semiempirical prediction of the IUE of diazotrophic growth for Trichodesmium was computed from considerations of the Fe content and reaction rates of the nitrogenase complex and PSI:PSII ratios, as well as field measurements of Mehler activity, cellular Fe‐superoxide dismutase activity, and diel variability in C and N 2 fixation. With a PSI:PSII ratio of 1 and 48% Mehler activity, the instantaneous IUE (0.33 mol C fixed·mol cellular Fe − 1 ·s − 1 ) was only 4‐fold lower than that calculated for a phytoplankter growing on reduced N. We computed a range of daily integrated IUE values from 2900 to 7700 mol C·mol Fe − 1 ·d − 1 , accounting for the diel variability in C and N 2 fixation as well as the uncertainties in cyanobacterial nitrogenase biochemistry and PSI:II ratios of field‐collected Trichodesmium. The lowest observed Fe‐superoxide dismutase:C quota of 2.9 (μmol:mol) suggests a maintenance requirement for this enzyme. The maintenance Fe:C requirement of 13.5 μmol:mol (derived from cultures of Trichodesmium IMS 101) and values of the IUE yielded an Fe requirement ranging from 27 to 48 Fe:C (μmol:mol) to achieve a diazotrophic growth rate of 0.1 d − 1 . Based on these predicted requirements, the Fe:C contents of Caribbean Sea and most North Atlantic Ocean populations sampled thus far exceed that required to support the observed rates of N 2 fixation.