Polyphosphate is a polymer of orthophosphate groups identified in all kingdoms of life. In the last two decades, this polymer has been ascribed to a wide variety of functions and recognized as integral to many cellular stress responses. Recently, it has been described that polyphosphate synthesis helps microalgal cells acclimate to nutrient deprivation by maintaining cellular ATP homeostasis. Under sulfur and nitrogen starvation, cell division and growth are arrested, and cells undergo a drastic reduction in their ATP demands. Thus, the synthesis of polyphosphate, which requires the hydrolysis of one ATP molecule for each orthophosphate group attached to the growing chain, is essential to cope with excess ATP. However, how the synthesis of this polymer is triggered in microalgae remains poorly understood. In this project, we use the unicellular green alga Chlamydomonas reinhardtii to study the molecular mechanisms underlying polyphosphate synthesis and its links with other cellular processes relevant to ATP homeostasis, such as inositol metabolism and the TOR pathway.

 

   Polyphosphate is not the only energy-rich polymer accumulated upon nutrient deprivation. In most microalgae, the synthesis of starch and neutral lipids is also induced under these conditions. In this research project, we study the relevance of polyphosphate, starch, and lipid synthesis as ATP-consuming processes and how they are integrated to sustain intracellular ATP balance in cells facing nutrient (nitrogen and sulfur) limitations. Additionally, we are interested in rewiring cellular energetics in order to reroute the energy towards the synthesis of a specific polymer, generating new strains in which the synthesis of the selected polymer may be enhanced. Starch and lipids are high-value products in the biotechnological industry as they are used as raw materials for the production of biofuels, bioplastics, and biolubricants. On the other hand, the isolation of algal strains with the capacity to accumulate high polyphosphate levels may help us develop strategies to recover phosphate from wastewater and use the P-overloaded cells as biofertilizers in agriculture.