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The interaction between cadmium and iron in marine phytoplankton Lane, Erin Susan


This study examined the intracellular Cd content (Cd:C) of seven species of cultured phytoplankton under varying degrees of Fe-limitation. A significant increase in Cd:C ratios was observed for all species grown under Fe-limitation, with an average 2- fold increase in Cd quotas. The mechanism behind this increase in Cd quotas was further investigated in a model diatom, Thalassiosira oceanica. A significant interaction was found between Cd and inorganic Fe(II). Inorganic Fe(II) uptake rates by Fe-limited T. oceanica were reduced by 50% when Cd was present, while Cd uptake was inhibited 36% in the presence of Fe(II). Inorganic Fe(II) uptake rates were also 15 times faster in Fe-limited cultures than in Fe-replete cultures. Cadmium and Fe(II) appear to enter the cell through common non-specific divalent metal transporters that are up-regulated under Fe-limitation, thereby leading to enhanced Cd accumulation. In addition to the effect of Fe-limitation on Cd:C ratios of cultured phytoplankton, a species/phyla effect was observed. A greater than 65-fold difference in Cd:C ratios was also observed between species grown under identical Fe concentrations. Oceanic diatoms had the highest Cd quotas and naked pryrnesiophytes the lowest. The intracellular Cd requirements of cultured phytoplankton were compared to the Cd content of natural phytoplankton assemblages from Fe-replete and Fe-limited waters in the subarctic Pacific, as well as to a global dataset of particulate Cd:P0₄3- ratios in surface waters. The phytoplankton from the Fe-limited station had the highest Cd:C ratios, which was attributed to a combination of species composition and Fe limitation. The same trend was found in our global data with HNLC stations having higher particulate Cd: P0₄ 3- ratios than Fe-replete stations. The combined laboratory, field, and global dataset results suggest that deviations in surface water Cd:PO₄3- ratios can be explained by both changes in species composition well as Fe-limitation. We propose that the "kink" in the global dissolved Cd:PO₄3- relationship is a result of the export of Fe-limited diatoms with high intristic Cd:P0₄3- ratios in HNLC regions.

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