Abstract
Nitrogen fixing organisms such as Trichodesmium spp. are abundant in the oligotrophic tropical North Atlantic Ocean, where microplankton (including other diazotrophs) are more likely to be phosphorus (P) than nitrogen (N) limited. Thus, understanding the ability of different functional groups in the plankton to compete for P in this area is important for understanding their relative success. The uptake of phosphate by Trichodesmium spp. colonies and bulk water plankton was measured using 33 PO4 3- over a range of concentrations, and kinetic parameters were determined. Nano- and pico-plankton present in bulk water samples have a Ks that is nearly 30 times lower than that of Trichodesmium spp. While chl a-normalized alkaline phosphatase activity (APA) in bulk water was an order of magnitude greater than in Trichodesmium spp., Trichodesmium spp. contributes sub- stantially to total APA in the water. Trichodesmium spp. is outcompeted for dissolved inorganic P (DIP), but colonies can satisfy their P needs by supplementing DIP uptake with P cleaved from dissolved organic P (DOP) via alkaline phosphatase.
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