The phenomenon of macroalgae depletion is a serious worldwide problem in terms of the destruction of ecosystems and reducing carbon fixation capacity in coastal areas. A lack of dissolved Fe is thought to be one of reasons for this phenomenon, and attempts have been made to develop Fe fertilization techniques. This study reports on the bioavailability of Fe species that are complexed with seawater extractable organic matter (SWEOM) derived from a compost and various chelators on oogenesis in a brown macroalga (Saccharina japonica). Oogenesis is induced by Fe uptake. Oogenesis assays indicate that Fe uptake can be explained by a ligand exchange reaction, suggesting that dissolved Fe in the form of a labile complex species could be used as a fertilizer. Fifty to eighty percent of gametophytes induced oogenesis in the presence of 0.16–0.27 μM Fe species complexed with SWEOM. However, no oogenesis was observed in the presence of 1 μM Fe species complexed with N,N'-bis(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid, an inert Fe-containing complex. These results show that SWEOM can serve as a chelator for Fe fertilization. Moreover, the concentration of SWEOM extracted from a bark compost was more than ten times higher than the corresponding values obtained from dredged soil and peat, suggesting this type of compost represents a suitable source of SWEOM. These findings extend our current knowledge of the use of Fe fertilization for the restoration of seaweed beds.
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