Diel variation of molybdenum and iron in marine diazotrophic cyanobacteria

Tuit, Caroline, John Waterbury, Gregory Ravizza

Limnol. Oceanogr., 49(4), 2004, 978-990 | DOI: 10.4319/lo.2004.49.4.0978

ABSTRACT: Measurements of Mo:C and Fe :C ratios in cultured cells of two N2-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 N2 fixer, showed two- to eightfold increases in Mo and Fe cellular concentrations in response to nitrogen fixation activity. Mo required for N2 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 N2-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 N2 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 N2 at rates normally measured in the field, despite equivalent Fe :C ratios (66 ± 39 [field samples] and 87 ± 64 [cultures] mmol mol-1). The average Fe :C ratio measured in N2-fixing Crocosphaera (16 ± 11 mmol mol-1) was equivalent to theoretical estimates of Fe demand based on nitrogenase requirements (13 ± 5 mmol 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 N2 fixation.

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