Effects of phosphorus starvation versus limitation on the marine cyanobacterium P rochlorococcus MED4 II : gene expression

Abstract

Summary Phosphorus ( P ) availability drives niche differentiation in the most abundant phytoplankter in the oceans, the marine cyanobacterium P rochlorococcus . We compared the molecular response of P rochlorococcus 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 P rochlorococcus 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 P rochlorococcus in the oligotrophic oceans.

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