Intestinal iron uptake in the European flounder

and oxygen transport. In higher vertebrates, there is no known regulated excretory mechanism for iron, and iron homeostasis is tightly controlled via its uptake. In the case of fish, iron is acquired predominantly from the diet, with the contribution from iron uptake from the water via the gills probably being negligible (Roeder and Roeder, 1966; Andersen, 1997). The estimated daily dietary requirement for iron of teleost fish ranges between 30 and 170 mg kg−1 (Watanabe et al., 1997), and aquacultural practice is to add iron to the feed. Deviations from this supplementation can compromise fish health, and iron-deficient diets result in a reduction in hepatic iron stores and haematocrit (Andersen et al., 1997), whereas iron-rich diets are toxic, causing reduced growth (Desjardins et al., 1987) as well as being linked to an increase in pathogen virulence (Fouz et al., 1994). The mechanisms by which the teleost fish intestine absorbs iron are poorly understood, but bioavailability is influenced by the form in which iron is found in the diet (Andersen et al., 1997). In Atlantic salmon, Fe(III)2O3 is poorly absorbed (Maage and Sreier, 1998), whereas haem iron is more bioavailable than iron(II) sulphate, which in turn is more bioavailable than elemental iron (Andersen et al., 1997). In contrast, mammalian dietary iron uptake processes have been characterised, and there are two distinct forms of iron, haem and non-haem (Fe3+). In the case of non-haem iron, the acidic environment of the stomach solubilises iron from its ingested matrix (Powell et al., 1999), probably in the ferric form (Fe3+), and gastric and small intestine mucins bind this iron and maintain iron solubility at pH values approaching neutrality in the intestine (Whitehead et al., 1996). Ferric iron is reduced either via dietary compounds, such as ascorbate (Raja et al., 1992), or via a membrane-bound ferric reductase (McKie et al., 2001). The microclimate close to the tissue is maintained acidic and protected from the lumen via the mucus layer covering the mucosa. This environment helps solubilise ferrous iron and it is this form that enters the enterocyte via a pH-dependent Fe2+/H+ cotransporter, termed 3779 The Journal of Experimental Biology 204, 3779–3787 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3677