Physiological responses of the freshwater N2-fixing cyanobacterium Raphidiopsis raciborskii to Fe and N availabilities |
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| Authors: | Qing-Long Fu Anna C. Y. Yeung Manabu Fujii Brett A. Neilan T. David Waite |
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| Affiliation: | 1. Department of Civil and Environmental Engineering, Tokyo Institute of Technology, Ookayama, Tokyo, Japan;2. School of Environmental and Life Sciences, The University of Newcastle, Newcastle, Australia;3. School of Civil and Environmental Engineering, The University of New South Wales, Sydney, Australia |
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| Abstract: | The cyanobacterium Raphidiopsis raciborskii is of environmental and social concern in view of its toxicity, bloom-forming characteristics and increasingly widespread occurrence. However, while availability of macronutrients and micronutrients such as N and Fe are critically important for the growth and metabolism of this organism, the physiological response of toxic and non-toxic strains of R. raciborskii to varying Fe and N availabilities remains unclear. By determining physiological parameters as a function of Fe and N availability, we demonstrate that R. raciborskii growth and N2-fixing activity are facilitated at higher Fe availability under N2-limited conditions with faster growth of the CS-506 (cylindrospermopsin-producing) strain compared with that of CS-509 (the non-toxic) strain. Radiolabelled Fe uptake assays indicated that R. raciborskii acclimated under Fe-limited conditions acquires Fe at significantly higher rates than under Fe replete conditions, principally via unchelated Fe(II) generated as a result of photoreduction of complexed Fe(III). While N2-fixation of both strains occurred during both day and night, the CS-506 strain overall exhibited higher N2-fixing and Fe uptake rates than the CS-509 strain under N-deficient and Fe-limited conditions. The findings of this study highlight that Fe availability is of significance for the ecological advantage of CS-506 over CS-509 in N-deficient freshwaters. |
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