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

Summary Recent measurements of natural populations of the marine cyanobacterium P rochlorococcus indicate this numerically dominant phototroph assimilates phosphorus ( P ) at significant rates in P ‐limited oceanic regions. To better understand uptake capabilities of P rochlorococcus under different P stress conditions, uptake kinetic experiments were performed on P rochlorococcus   MED4 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 (α 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 α P for both inorganic and organic P ( PO 4 and ATP ). The Michaelis–Menten constant, K M , for PO 4 remained constant under different P stress conditions, whereas the K M for ATP was higher when cells were stressed for PO 4 , pointing to additional processes involved in uptake of ATP . MED4 cleaves the PO 4 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.