Effects of phosphorus starvation versus limitation on the marine cyanobacterium Prochlorococcus†MED4 II: gene expression.

Phosphorus (P) availability drives niche differentiation in the most abundant phytoplankter in the oceans, the marine cyanobacterium Prochlorococcus. We compared the molecular response of Prochlorococcus strain MED4 to P starvation in batch culture to P-limited growth in chemostat culture. We also identified an outer membrane porin, PMM0709, which may allow transport of organic phosphorous compounds, rather than phosphate as previously suggested. The expression of three P uptake genes, pstS, the high-affinity phosphate-binding component of the phosphate transporter, phoA, an alkaline phosphatase, and porin PMM0709, were strongly upregulated (between 10- and 700-fold) under both P starvation and limitation. pstS exhibits high basal expression under P-replete conditions and is likely necessary for P uptake regardless of P availability. A P-stress regulatory gene, ptrA, was upregulated in response to both P starvation and limitation although a second regulatory gene, phoB, was not. Elevated expression levels (> 10-fold) of phoR, a P-sensing histidine kinase, were only observed under conditions of P limitation. We suggest Prochlorococcus in P-limited systems are physiologically distinct from cells subjected to abrupt P depletion. Detection of expression of both pstS and phoR in field populations will enable discernment of the present P status of Prochlorococcus in the oligotrophic oceans.

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 Prochlorococcus MED4 grown in P-limited chemostats and batch cultures. Our results indicate that MED4 has a small cell-specific Vmax but a high specific affinity (αP ) 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 Vmax and αP for both inorganic and organic P (PO4 and ATP). The Michaelis-Menten constant, KM , for PO4 remained constant under different P stress conditions, whereas the KM 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.

Phytoplankton in the ocean use non-phosphorus lipids in response to phosphorus scarcity.

Phosphorus is an obligate requirement for the growth of all organisms; major biochemical reservoirs of phosphorus in marine plankton include nucleic acids and phospholipids. However, eukaryotic phytoplankton and cyanobacteria (that is, 'phytoplankton' collectively) have the ability to decrease their cellular phosphorus content when phosphorus in their environment is scarce. The biochemical mechanisms that allow phytoplankton to limit their phosphorus demand and still maintain growth are largely unknown. Here we show that phytoplankton, in regions of oligotrophic ocean where phosphate is scarce, reduce their cellular phosphorus requirements by substituting non-phosphorus membrane lipids for phospholipids. In the Sargasso Sea, where phosphate concentrations were less than 10 nmol l-1, we found that only 1.3 +/- 0.6% of phosphate uptake was used for phospholipid synthesis; in contrast, in the South Pacific subtropical gyre, where phosphate was greater than 100 nmol l-1, plankton used 17 6% (ref. 6). Examination of the planktonic membrane lipids at these two locations showed that classes of sulphur- and nitrogen-containing membrane lipids, which are devoid of phosphorus, were more abundant in the Sargasso Sea than in the South Pacific. Furthermore, these non-phosphorus, 'substitute lipids' were dominant in phosphorus-limited cultures of all of the phytoplankton species we examined. In contrast, the marine heterotrophic bacteria we examined contained no substitute lipids and only phospholipids. Thus heterotrophic bacteria, which compete with phytoplankton for nutrients in oligotrophic regions like the Sargasso Sea, appear to have a biochemical phosphorus requirement that phytoplankton avoid by using substitute lipids. Our results suggest that phospholipid substitutions are fundamental biochemical mechanisms that allow phytoplankton to maintain growth in the face of phosphorus limitation.