Partitioning of iron and plutonium to exopolymeric substances and intracellular biopolymers: A comparison study between the coccolithophore Emiliania huxleyi and the diatom Skeletonema costatum

Abstract

Iron (Fe), a micronutrient for algal growth, and plutonium (Pu), an anthropogenic radionuclide, share some common features. This includes similar oceanic distributions when different input modes are taken into account, as well as their chemical behavior, such as a high affinity to natural organic matter (NOM). The NOM produced by various phytoplankton communities can potentially influence Fe cycling in the ocean, and likely also influence the transport behavior of Pu. We conducted laboratory incubation experiments using the coccolithophore Emiliania huxleyi and the diatom Skeletonema costatum, in the presence of 59Fe and 238Pu as radiotracers, in order to differentiate Fe and Pu uptake by extracellular exopolymeric substances (EPS) and intracellular biopolymers. The Fe and Pu distributions in select organic compound classes produced by these two types of phytoplankton, including proteins, total carbohydrates (TCHO) and uronic acids (URA), were compared. Our results indicated that most of the Fe and Pu (>95%) were found concentrated in E. huxleyi-derived non-attached EPS, while much less (<2%) was present in the intracellular fraction of E. huxleyi. In contrast, in the diatom S. costatum, Fe and Pu were both distributed with EPS > intracellular biopolymers > outer cell covering (i.e., frustule). In fact, over 50% of the Fe was concentrated in S. costatum-derived attached EPS and intracellular biopolymers. The diatom derived Fe-EPS complexes were more hydrophobic, with stronger tendency to aggregate in seawater. Fe binding to biopolymers in both E. huxleyi and S. costatum cultures was related to URA concentrations, but the overall distribution of URA between these two phytoplankton species was different (e.g., high intracellular abundance of URA in S. costatum but low intracellular URA abundance in E. huxleyi). Our findings suggest that the presence of URA on cellular surfaces of S. costatum (i.e., attached EPS) and its high intracellular fraction could be an indicator for Fe transport from the surrounding seawater to the diatom cells. However, for the coccolithophore E. huxleyi, Fe was not efficiently taken up during its growth. Instead, the more hydrophilic non-attached EPS (i.e., low protein/TCHO ratio) produced by E. huxleyi could have stabilized Fe in the colloidal form as Fe-EPS complexes. Similar partitioning behavior of Fe and Pu suggests that Pu isotopes can potentially serve as a tracer for Fe biogeochemistry in the ocean.