The effects of Cu and Fe availability on the growth and Cu : C ratios of marine diatoms

We investigated the effects of copper (Cu) and iron (Fe) availability on the growth rates, cellular Cu content, and steady-state Cu uptake rates of eight species of centric diatoms (coastal and oceanic strains). Whereas Fe and Cu availability had a significant effect on the growth rates of both costal and oceanic diatoms, an interaction between Fe and Cu availability and growth rates was only observed for the oceanic diatoms. Determination of cellular Cu : carbon (C) quotas using the radiotracers 67Cu and 14C revealed that under Cu-sufficient conditions oceanic diatoms had elevated Cu : C ratios relative to coastal strains, regardless of Fe availability. Two species (one oceanic and one coastal) significantly increased their Cu demands in response to Fe limitation, indicating upregulation of the Cu-dependent high-affinity Fe uptake system in these organisms. The changes in cellular Cu : C ratios were accompanied by variations in steady-state Cu uptake rates. Thus, in some cases Cu uptake rates appear to be regulated by the cell in response to Fe availability. Rates of Cu acquisition also responded significantly to Cu variability. The variation in Cu uptake was more closely correlated with changes in total Cu concentration in the medium than in inorganic, free Cu concentrations, implying that organic Cu complexes may be bioavailable to diatoms. These findings indicate a greater biological role for Cu than was previously thought in open ocean regions. Iron (Fe) is an essential micronutrient for phytoplankton growth and has been shown to control primary productivity in large oceanic regions (Martin 1990; Martin et al. 1994; Boyd 2004). In these open ocean waters, dissolved Fe is present in extremely low concentrations of ,0.07 nmol L21 (Johnson et al. 1997) and is predominantly complexed by strong organic ligands (Rue and Bruland 1995). Although eukaryotic phytoplankton are not believed to access organically bound Fe directly (Hudson and Morel 1993), lab and field data have shown that these organisms upregulate an inducible high-affinity Fe uptake system in response to Fe limitation that allows them to acquire Fe bound to strong organic complexes, such as the siderophore ferrioxamine B (Maldonado and Price 1999, 2001). The high-affinity Fe uptake system is composed of putative Fe(III) reductases, multi–copper (Cu)-containing Fe(II) oxidases, and Fe(III) permeases (Maldonado and Price 2001; Shaked et al. 2005; Maldonado et al. 2006) and is similar to those previously identified in the yeast Saccharomyces cerevisiae (Askwith et al. 1994) and in the green alga Chlamydomonas reinhardtii (Herbik et al. 2002; La Fontaine et al. 2002). The role of Cu in the high-affinity Fe uptake system of Fe-limited Thalassiosira oceanica is such that cells grown with low Cu show significantly slower rates of Fe(II) oxidation and Fe acquisition from the siderophore ferrioxamine B (Maldonado et al. 2006). In addition, when Thalassiosira pseudonana is subjected to Fe limitation, the transcription levels of the putative gene encoding for the multi–Cu-containing oxidase increase 60-fold (Maldonado et al. 2006). These physiological and genomic data imply that Cu availability may affect the ability of Fe-limited diatoms to acquire organically bound Fe and that the intracellular Cu quotas of Fe-limited diatoms may be significantly higher than those of Fe-sufficient cells. In addition to the Cu demand of the high-affinity Fe transport system in Fe-limited coastal and oceanic diatoms, oceanic species may have additional Cu requirements associated with the substitution of Fe-containing enzymes by Cu-containing ones, as recently shown for plastocyanin in T. oceanica (Peers and Price 2006). The hypothetically higher Cu requirements of the oceanic isolates may help explain how these phytoplankton are able to survive in open ocean waters with intracellular Fe levels that are 70% 1 Current address: Grant Institute, School of Geosciences, University of Edinburgh, Edinburgh, EH9 3JW, United Kingdom ([email protected]). Acknowledgments We thank J. Granger, D. Semeniuk, and L. Moccia for providing helpful insights throughout the completion of this study. We also thank Philippe D. Tortell and two anonymous reviewers for thoughtful suggestions on the manuscript. This work was supported by funding from the National Sciences and Research Council of Canada (NSERC) and from the Tri-University Meson Facility Life Science Program. Limnol. Oceanogr., 53(6), 2008, 2451–2461 E 2008, by the American Society of Limnology and Oceanography, Inc.