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

Abstract

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.

Physiological Regulation of Isocitrate Dehydrogenase and the Role of 2-Oxoglutarate in Prochlorococcus sp. Strain PCC 9511

“…Recent studies have addressed the effects of P limitation/starvation in Prochlorococcus sp. MED4 cultures, showing significant changes, specially in the physiology of P uptake [16] and in the expression of P uptake genes and a P stress regulatory gene [64]. These studies did not report, though, specific effects for ICDH or icd expression.…”

“…Thirteen Prochlorococcus genomes [6]–[9], representative of the different ecotypes [10], [11], have been sequenced to date. However, an important fraction of the genomic information has been obtained by comparison with other organisms, while there are not many physiological studies carried out in Prochlorococcus [12]–[16], due to the problematic culturing of this microorganism [17]. Consequently, there is a clear need for in vivo studies addressing the physiology of Prochlorococcus [18], [19], in order to further understand the underpinnings of differences among ecotypes, which might illuminate the reasons explaining the tremendous ecological success of this organism.…”

More mixotrophy in the marine microbial mix

“…One might expect this strategy to be widespread in the minimal genome of Prochlorococcus (23). However, the high-affinity P-uptake system, PstSABC, the only other uptake system in Prochlorococcus that has been characterized, did not show multiphasic uptake kinetics (24), so more physiological characterization of uptake systems and transporters is needed to determine how common this feature might be in Prochlorococcus, other cyanobacteria, and oligotrophic bacteria.…”

SAR11 lipid renovation in response to phosphate starvation

“…Microbes inhabiting these low P i environments have evolved numerous strategies to maintain growth and enhance their competitiveness for trace amounts of P i . These mechanisms are commonly induced by P i starvation and include one or more of the following: (i) expression of high affinity P i transporters (6); (ii) reduction of cellular P i quotas (7,8); (iii) utilization of alternate phosphorus sources (9, 10); and (iv) polyphosphate storage and breakdown (11,12). Such strategies facilitate survival in the face of P i insufficiency.…”