Effects of phosphorus starvation versus limitation on the marine cyanobacterium ProchlorococcusMED4 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 (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 (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.