BIOLOGICAL AVAILABILITY OF IRON TO THE FRESHWATER CYANOBACTERIUM ANABAENA FLOS‐AQUAE1

Iron acquisition from various ferric chelates and colloids was studied using iron‐limited cells of Anabaena flos‐aquae (Lyng.) Brèb UTEX 1444, a cyanobacterial strain that produces high levels of siderophores under iron limitation. Various chelators of greatly varying affinity for Fe³⁺ (HEDTA, EDDHA, desferrioxamine mesylate, HBED, 8‐hydroxyquinoline) were assayed for the degree of iron acquisition by iron‐limited cyanobacterial cells. Iron uptake rates (measured by graphite furnace atomic absorption spectrometry) varied approximately inversely with calculated Fe³⁺] (calculated as pFe) and decreased with increasing chelator‐to‐iron ratio. No iron uptake was observed when Fe³⁺ was chelated with HBED, the strongest of the tested chelators. Iron‐limited Anabaena cells were able to take up iron from 8‐hydroxyquinoline (oxine or 8HQ), a compound sometimes used to quantify aquatic iron bioavailability. Iron bound to purified humic acid was poorly available but did support some growth at high humic acid concentrations. These results suggest that for cyanobacteria, even tightly bound iron is biologically available, including to a limited extent iron bound to humic acids. However, iron bound to some extremely strong chelators (e.g. HBED) is likely to be biologically unavailable.