The bioavailability and durability of Fe released from decarburization steelmaking slag was examined for two marine diatom species. The bioavailability of Fe released from the slag was compared with that from the reagent FeCl₃·6H₂O in the presence or absence of the synthetic chelator ethylenediaminetetraacetic acid, which affects Fe speciation in seawater. The duration of bioavailability was determined by the recovery of the growth rate on intermittent additions of macro-nutrients other than Fe. Abiotic reduction of bioavailable Fe from the slag in seawater was also investigated for 5 or 15 d. Thus, the bioavailability of Fe released from the slag was observed to be sufficiently high to promote the maximum growth rate; this was similar to that observed with the reagent inorganic Fe. This implies that the iron released from the slag is a dissolved ferric and/or ferrous ion/hydroxide species. In the culture media, to which the slag was added at the concentration of 20 mg L⁻¹, the slag supplied bioavailable Fe to two diatoms for 50 d. The probable duration for which the slag was available as an Fe source was approximately 10 times longer than the reported duration in in situ iron fertilization experiments. These results indicate that continuous Fe fertilization can be achieved by a single addition of the slag, and hence, we can reduce the energy and cost of ocean fertilization and also create a resource of microalgae biofuels.