Phosphate and adenosine‐5'‐triphosphate uptake by cyanobacteria and heterotrophic bacteria in the Sargasso Sea

Abstract

We used flow cytometry sorting to quantify assimilation of phosphate and adenosine‐59‐triphosphate (ATP) by heterotrophic bacteria and the cyanobacteria Prochlorococcus and Synechococcus during the fall of 2007 and 2008 and the spring of 2009 in the western Sargasso Sea. Phosphate and ATP uptake rates per cell were 50‐fold and 80‐fold higher, respectively, for Synechococcus than for the other two microbial groups, whereas there was no significant difference between ATP and phosphate uptake per cell by Prochlorococcus and heterotrophic bacteria. Both cyanobacterial groups had higher rates of phosphate uptake per surface area than did the heterotrophic bacteria; Synechococcus had the highest rates per cellular phosphorus (P)‐quota for ATP uptake, and Prochlorococcus usually had lower rates. Heterotrophic bacteria had the lowest uptake rates of ATP and phosphate per cellular P quota. In contrast, total uptake of phosphate and ATP per liter of seawater was dominated by heterotrophic bacteria, while uptake by Prochlorococcus and Synechococcus was less than 10% of the total, as a result of their low abundance (15% and 1.5% of total prokaryotes, respectively). Uptake rates of phosphate and ATP for heterotrophic bacteria and Prochlorococcus were less tightly coupled than for Synechococcus, and phosphate appeared to be favored over ATP for all three groups. Uptake rates of both compounds by Synechococcus were much higher than by the other microbial groups. Cyanobacteria are successful competitors for phosphate and ATP in the Sargasso Sea.