The influence of iron-complexed ehylenediaminetetraacetic acid (EDTA) was studied on nitric oxide (NO) removal using photoautotropic cultivation of green alga Scenedesmus. Fe(II)EDTA is an active solubilization agent of NO in water, while the oxidized Fe(III)EDTA is not. When a gas mixture containing 300 ppm NO was treated through the Scenedesmus culture containing 5 mM Fe(II)EDTA, a constant level of 80-85% NO removal was achieved for a prolonged period. A certain fraction of Fe(II)EDTA remained without being oxidized to Fe(III)EDTA because of the existence of reversible oxidation-reduction balance between Fe(II)EDTA and Fe(III)EDTA. When Fe(III)EDTA was added to the culture instead of Fe(II)EDTA, Fe(II) was generated via reduction of Fe(III), resulting in the increase of NO removal and cell density. This was possible because of the generated Fe(II)EDTA which contributed to the dissolution of NO. Therefore, a long-term NO removal was possible with Fe(III)EDTA, as well as with Fe(II)EDTA, in the present microalgal system. The supplementation of free EDTA was necessary to extend the period of NO removal because EDTA is consumed by biodegradation while the decrease of total iron content was not significant. (C) 2010 Elsevier Ltd. All rights reserved.