Cascading nutrient limitation of the cyanobacterium Cylindrospermopsis raciborskii in a Sahelian lake (North Senegal)

Bioassays in natural water-based batch cultures were performed to identify factors that could control the development of the toxic heterocystous cyanobacterium Cylindrospermopsis raciborskii in Lake Guiers (North Senegal). Without dissolved inorganic nitrogen, C. raciborskii was unable to grow, unless EDTA was supplied. The addition of P, S, Fe, B, Ca, Co, Cu, Mg, Mn, Mo and Zn did not restore its growth. Variations in the percentages of heterocysts found in bioassays, in the concentrations of major and trace elements in Lake Guiers, and in the computed chemical speciation of all dissolved species using MINEQL+ software, led us to deduce that, after N, Fe was the second greatest growth-limiting nutrient. Assuming that the 'Free Ion Model' is valid for the indigenous species of phytoplankton, the concentrations of bioavailable Fe3+ were within the range of 10(-19) to 10(-22) M and were limited by the very low solubility of Fe-hydroxides. At such negligible concentrations, C. raciborskii is unable to take up the Fe necessary to ensure efficient nitrogenase functioning. The addition of EDTA led to the production of (III) Fe-EDTA complexes, which acted as an iron buffer that, in turn, increased the bioavailability of Fe3+ and the growth of C. raciborskii. These bioassays suggest that, in Lake Guiers, the primary limiting factor for cyanobacterial growth is nitrogen. They also demonstrate the lack of a sufficient concentration of complexing agents, which limits the bioavailability of Fe, and then the nitrogenase activity and diazotrophy of C. raciborskii. This cascading limitation could account for the seasonal fluctuations of this cyanobacterial population in Lake Guiers.