Trace element uptake by macroalgae: Organic colloids as a source of metals, including Fe and rare earth elements

We determined the concentrations of trace elements including Fe, Al, rare earth elements and Y (REY), in Ascophyllum nodosum, one of the most abundant brown macroalgae in the North Atlantic. Samples were collected in the Bay of Brest (Brittany, France) and in the estuary of its main contributing river. The Y/Ho, Al/Ga, and Zr/Hf ratios display values distinctive from seawater, but similar to the continental crust; an observation which we show cannot be explained by the incorporation of terrigenous particles, nor inorganic colloids. On the other hand, REY, Ga, Al, as well as other trace elements such as Th, Sc, Pb and Cr, correlate strongly with Fe abundances. Since all these elements are chiefly carried by organic colloids, we propose that colloidal uptake onto the surface of the algae controls the bioaccumulation of these metals. Their assimilation or internalization by algae requires biological pathways yet to be determined. This process is vital for these organisms, as organic colloids appear to be their main source of Fe, an essential nutrient. However, it also allows the accumulation of some potentially toxic metals in algae (e.g., Pb), with implications on the overall health of coastal ecosystems.