Phosphorus starvation and luxury uptake in green microalgae revisited

Phosphorus (P) is central to storing and transferring energy and information in living cells, including those of microalgae. Many microalgal species dwelling in low P environments are naturally equipped to take up and store P whenever it becomes available through a complex phenomenon known as "luxury P uptake." Its research is required for better understanding of the nutrient geochemical cycles in aquatic environments but also for biotechnological applications such as sequestration of nutrients from wastewater and production of algal fertilizers. Here, we report on our recent insights into luxury P uptake and polyphosphate formation originating from physiological, ultrastructural, and transcriptomic evidence. The cultures pre-starved of P and re-fed with inorganic phosphate (P-i) exhibited a bi-phasic kinetics of P-i uptake comprising fast (1-2 h after re-feeding) and slow (1-3 d after re-feeding) phases. The rate of P-i uptake in the fast phase was ca. 10 times higher than in the slow phase with an opposite trend shown for the cell division rate. The transient peak of polyphosphate accumulation was determined 2-4 h after re-feeding and coincided with the period of slow cell division and fast P-i uptake. In this phase, the microalgal cells reached the highest P content (up to 5% of dry cell weight). The P re-feeding also reversed the characteristic changes in cell lipids induced by P starvation, namely increase in the major membrane glycolipid (DGDG/MGDG) ratio and betaine lipids. These changes were reversed upon Pi re-feeding of the starved culture. Electron microscopy revealed the ordered organization of vacuolar polyphosphate indicative of the possible involvement of an enzyme (complex) in their synthesis. A candidate gene encoding a protein similar to the vacuolar transport chaperone (VTC) protein, featuring an expression pattern corresponding to polyphosphate accumulation, was revealed. Implications of the findings for efficient biocapture of phosphorus are discussed.