title: Iron colloids reduce the bioavailability of phosphorus to the green alga Raphidocelis subcapitata.
abstract: Phosphorus (P) is a limiting nutrient in many aquatic systems. The bioavailability of P in natural waters strongly depends on its speciation. In this study, structural properties of iron colloids were determined and related to their effect on P sorption and P bioavailability. The freshwater green alga Raphidocelis subcapitata was exposed to media spiked with radiolabelled (33)PO4, and the uptake of (33)P was monitored for 1 h. The media contained various concentrations of synthetic iron colloids with a size between 10 kDa and 0.45 μm. The iron colloids were stabilised by natural organic matter. EXAFS spectroscopy showed that these colloids predominantly consisted of ferrihydrite with small amounts of organically complexed Fe. In colloid-free treatments, the P uptake flux by the algae obeyed Michaelis-Menten kinetics. In the presence of iron colloids at 9 or 90 μM Fe, corresponding to molar P:Fe ratios between 0.02 and 0.17, the truly dissolved P (<10 kDa) was between 4 and 60% of the total dissolved P (<0.45 μm). These colloids reduced the P uptake flux by R. subcapitata compared to colloid-free treatments at the same total dissolved P concentration. However, the P uptake flux from colloid containing solutions equalled that from colloid-free ones when expressed as truly dissolved P. This demonstrates that colloidal P did not contribute to the P uptake flux. It is concluded that, on the short term, phosphate adsorbed to ferrihydrite colloids is not available to the green alga R. subcapitata.
authors.affiliation: KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 Bus 2459, 3001 Leuven, Belgium. Electronic address: stijn.baken@ees.kuleuven.be.
authors.forename: Stijn
authors.initials: S
authors.lastname: Baken
authors.affiliation: KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 Bus 2459, 3001 Leuven, Belgium.
authors.forename: Sophie
authors.initials: S
authors.lastname: Nawara
authors.affiliation: KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 Bus 2459, 3001 Leuven, Belgium.
authors.forename: Christoff
authors.initials: C
authors.lastname: Van Moorleghem
authors.affiliation: KU Leuven, Department of Earth and Environmental Sciences, Kasteelpark Arenberg 20 Bus 2459, 3001 Leuven, Belgium.
authors.forename: Erik
authors.initials: E
authors.lastname: Smolders
mesh_terms: D000460:Chlorophyta / Q000187:drug effects* / Q000378:metabolism*; D003102:Colloids / Q000737:chemistry / Q000494:pharmacology*; D058085:Iron Compounds / Q000737:chemistry / Q000494:pharmacology*; D010758:Phosphorus / Q000378:metabolism*
chemical_list: D003102:Colloids; D058085:Iron Compounds; D010758:Phosphorus
keywords: Colloidal phosphorus; Ferrihydrite; Free phosphorus; Phosphorus bioavailability; Phosphorus sorption; Phosphorus uptake