Dissolved inorganic and organic phosphorus uptake in Trichodesmium and the microbial community: The importance of phosphorus ester in the Sargasso Sea

The dynamics and relative importance of inorganic phosphate (Pi) and dissolved organic phosphorus (DOP) uptake were examined in Trichodesmium and the microbial community. Trichodesmium DOP and Pi uptake rates were also compared to literature values from several other taxonomically important groups in the Sargasso Sea. Uptake rates and uptake kinetics of (33)Pi and DOP (using alpha-labeled adenosine-5'-triphosphate P-33-ATP as a model P ester compound) were assayed during two cruises. The in situ uptake rates suggest that the contribution of P ester to total P uptake can be greater than 25% for Trichodesmium, the microbial community, and key phytoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes, and nanoeukaryotes), depending on the location. Based on the kinetics of Pi uptake and P ester uptake, Trichodesmium is a poor competitor for Pi but a much better competitor for P esters as compared to the microbial community. Trichodesmium growth rates calculated from the Pi and P ester uptake parameters suggest that only a small fraction of the P ester pool needs to be bioavailable to allow reasonable growth rates. These data underscore the importance of P esters in supporting production by Trichodesmium in the Sargasso Sea, and provide the first in situ measurements of P ester uptake and uptake kinetics in this important N-2-fixing genus.