Effects of phosphorus starvation versus limitation on the marine cyanobacterium Prochlorococcus MED4 I: uptake physiology

Recent measurements of natural populations of the marine cyanobacterium Prochlorococcus indicate this numerically dominant phototroph assimilates phosphorus (P) at significant rates in P-limited oceanic regions. To better understand uptake capabilities of Prochlorococcus under different P stress conditions, uptake kinetic experiments were performed on ProchlorococcusMED4 grown in P-limited chemostats and batch cultures. Our results indicate that MED4 has a small cell-specific V-max but a high specific affinity ((P)) for P, making it competitive with other marine cyanobacteria at low P concentrations. Additionally, MED4 regulates its uptake kinetics in response to P stress by significantly increasing V-max and (P) for both inorganic and organic P (PO4 and ATP). The Michaelis-Menten constant, K-M, for PO4 remained constant under different P stress conditions, whereas the K-M for ATP was higher when cells were stressed for PO4, pointing to additional processes involved in uptake of ATP. MED4 cleaves the PO4 moieties from ATP, likely with a 5-nucleotidase-like enzyme rather than alkaline phosphatase. MED4 exhibited distinct physiological differences between cells under steady-state P limitation versus those transitioning from P-replete to P-starved conditions. Thus, MED4 employs a variety of strategies to deal with changing P sources in the oceans and displays complexity in P stress acclimation and regulatory mechanisms.