The bioavailability of colloidal P to freshwater algae Stijn Baken, Sophie Nawara, Emmanuel Van Acker, Erik Smolders KU Leuven, BELGIUM PSP5 - August 28, 2014 Thanks to: Beamline I811 staff, MAX-lab lightsource, Lund, Sweden FWO – Research Foundation Flanders

Environmental context P lost from agricultural land ends up in lakes and streams P limits primary production in many waters Eutrophication risk ↑ The bioavailability to microalgae, and hence the eutrophication risk, depends on the chemical speciation of P Free orthophosphate: fully bioavailable Organic P: sparingly available Colloidal P = P bound to colloidal Fe and Al oxides: ?? Goal of this study: to address the availability of colloidal P to aquatic organisms

What is colloidal P? P adsorbed to hydroxides of Fe and Al < 0.45 µm stabilized by adsorbed organic matter In soil solution (Hens & Merckx, 2003) Alcacio et al In aquatic systems (Nawara et al. 2014) May contribute to P uptake in plants (Santner et al., 2012)

Approach Well-defined model system Single test species: R. subcapitata Synthetic test medium with P as limiting nutrient (0.5—10 µM) Synthetic ferrihydrite colloids (< 0.45 µm) produced by oxidation of Fe(II) (0.5—5 mg/L) in the presence of natural organic matter (10 mg/L) Short term (1h) availability: P uptake flux Long term (14d) availability: Algal growth response

Short term availability of colloidal P Synthetic test media with or without ferrihydrite colloids Determination of free ortho-P and colloidal P by ultrafiltration (10 kDa) Availability measured as P uptake flux (1h) using 33 P-tracer No colloids ( X ) Michaelis-Menten kinetics With colloids ( ● ) (40—96% colloidal P) P uptake flux is reduced P uptake flux coincides with Michaelis-Menten kinetics if only free orthophosphate is considered Colloidal P does not contribute to uptake Baken et al. (2014)

Long term availability of colloidal P Synthetic test media with or without ferrihydrite colloids (0.5—3 µM P; 1—5 mg Fe/L) Availability measured as algal growth response (14d) Calibration with the growth response in media without colloids With colloids (> 95% colloidal P): 25—70% of colloidal P became available after 14 days

Long term availability of colloidal P With colloids: slower growth during initial phase Mechanism = slow release of colloidal P Rate of P internalization Dialysis against ferrihydrite

Conclusions and implications Colloidal P is sparingly available to freshwater algae Mechanism: depletion of free ortho-P -> slow desorption of colloidal P Limitations of this study Artificial model system Only one test species We did not consider reductive dissolution Colloidal P is largely included in molybdate reactive P Often used in legislation Over-protective environmental limits? Thanks for your attention! Contact: stijn.baken (at) ees.kuleuven.be