Diel variation of molybdenum and iron in marine diazotrophic cyanobacteria

Measurements of Mo:C and Fe :C ratios in cultured cells of two N 2 ‐fixing cyanobacteria, Crocosphaera watsonii strain WH8501 and Trichodesmium erythraeum strain IMS101, agree with estimated metal : carbon ratios based on growth rate and the metal use efficiency of the nitrogenase enzyme. Crocosphaera , a single‐celled nocturnal N 2 fixer, showed two‐ to eightfold increases in Mo and Fe cellular concentrations in response to nitrogen fixation activity. Mo required for N 2 assimilation can account for almost the entire Mo pool measured in the cells, implying that Crocosphaera synthesizes its entire nitrogenase pool de novo each night. In contrast, cultures of Trichodesmium , a filamentous, diurnal N 2 ‐fixing cyanobacterium, did not show diel variations in Mo or Fe carbon ratios or in cellular metal concentrations. Trichodesmium appears to maintain an internal pool of Mo. In Trichodesmium cultures, Mo concentrations were up to 30% higher than needed to support measured N 2 fixation. Trichodesmium colonies collected from the field had Mo:C ratios 10‐fold larger than those measured in culture, far in excess of what is needed to fix N 2 at rates normally measured in the field, despite equivalent Fe :C ratios (66 ± 39 [field samples] and 87 ± 64 [cultures] µmol mol −1 ). The average Fe :C ratio measured in N 2 ‐fixing Crocosphaera (16 ± 11 µmol mol −1 ) was equivalent to theoretical estimates of Fe demand based on nitrogenase requirements (13 ± 5 µmol mol −1 ). These results demonstrate the extremely efficient use of Fe by these organisms and provide support for the use of theoretical estimates of Fe :C ratios to calculate biological Fe demand for N 2 fixation.