Siderophore production and nitrogenfixation are mutually exclusive strategies in Anabaena 712

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Abstract
The relationship between siderophore production and N fixation was investigated in cultures of the freshwater cyanobacterium Anabaena 7120 grown in chemically defined medium. Cultures were grown with and without 300 µM NO3− and 1 µM Fe in all four possible combinations. Chl a and protein concentrations were used to monitor growth response. Siderophore production and N fixation rates were measured throughout the experiments. Acid‐leachable cellular Fe was measured with the chromogen FerroZine. Siderophores were produced only by cultures with added NO3− but no added Fe. N‐fixing cultures of Anabaena 7120 did not make siderophores even at a saturating light irradiance. These results do not agree with previous theories that siderophore‐mediated Fe uptake facilitates N fixation, enabling cyanobacteria to become dominant. The economics of N and Fe acquisition systems are examined in terms of elemental investments, energy requirements, and the kinetics of siderophore‐Fe uptake. Our data are used to support a model that suggests that dilution of excreted siderophores or turbulent dispersion of the siderophore away from the cell surface would severely reduce the efficiency of siderophore‐mediated Fe uptake in aquatic systems. This model supports the idea that siderophores are most useful as an Fe recycling mechanism in quiescent waters with high cyanobacterial biomass.

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