Diatoms and their response to phosphate limitation

Phosphorus is an essential element for all living forms. It is an integral part of several biomolecules that play a crucial role in the cellular structures and processes. Phosphate, which is the most common form of phosphorus in the biological systems, is present in a plethora of important biomolecules (e.g. DNA, phospholipids) and it is involved in fundamental cellular activities such as the modulation of proteins activity via phospho/dephosphorylation. Cellular activity is often influenced by environmental P availability and in the case of marine protists such as microalgae, this can potentially impact on the global primary production. Diatoms, which are believed to largely contribute to the global carbon fixation, are able to adapt to fluctuations in nutrient concentrations such as phosphorus. Recent transcriptomic and proteomic studies indicated possible strategies that diatoms adopt to cope with P scarcity showing a significant impact on cell metabolism and physiology. In the model diatom Phaeodactylum tricornutum specific traits of P-stress response can be recognized in the induction of genes encoding for alkaline phosphatases and P transporters. Several important players that might be essential for the cellular acclimation to P deficiency were identified by the “omics” studies, providing a first general overall. However, the understanding of P-homeostasis requires more detailed knowledge on P-responsive specific proteins. Further studies on these proteins (e.g. on function and/or subcellular localization) are needed to clarify and characterize more aspects of the response. The work presented here aims to integrate the existing omics data with subcellular localization, transcriptional, and posttranslational regulation studies on several P-regulated/non-regulated proteins that are supposed to play major roles in maintaining P-homeostasis in P. tricornutum. The in vivo localization and expression studies showed that P. tricornutum in response to P-starvation expresses extracellular alkaline phosphatases, one phytase-like, and 5´ nucleotidase and one intracellular alkaline phosphatase in the endomembrane system. Pi- transporters are localized at cell borders, endomembrane systems and vacuolar membranes. These results highlight the ability of the diatom to mobilize Pi from alternative intra/extracellular P source, uptake and distribute it intracellularly. An early investigation on candidates related to a possible polyP metabolism and P-storage was also conducted. Some of the investigated proteins were studied also with respect to their transcriptional regulation, showing interesting regulation patterns under diverse extracellular P-conditions. The results shown here integrate the knowledge about P-starvation response in diatoms, providing additional informations that are necessary to sketch a P-homeostasis atlas in P. tricornutum. As a side aspect of this part, strongly Pi-dependent promoter/terminator modules were identified, providing new molecular tools for the expression of transgenes in the P. tricornutum model organism.