ИСТИНА

Taking up nutrients such as phosphorus (P) in amounts exceeding the demand for supporting cell division in the absence of limitation is termed luxury uptake (LU). Although LU is long known in microalgae and regarded as a promising vehicle for recycling of P from waste streams [1], it remain poorly elucidated. We scrutinized the relationships between cell division rate, inorganic phosphate (Pi) uptake, accumulation of polyphosphate (PolyP), formation of P-rich cell inclusions and expression of related genes in the P-starving and P-sufficient cells of three green microalgae (Chlorella vulgaris strains IPPAS C-1 and CCALA 256 and Parachlorella kessleri CCALA 251) as a function of external Pi concentration and P nutrition status. Phosphorus-starved cells of all organisms exhibited a polyphasic kinetics of Pi uptake which was confirmed by Pi uptake measurements, in vivo 31P NMR, confocal Raman microspectrometry [2] and analytical electron microscopy (EDX) [3]. Within the first 2–4 h after Pi replenishment in the medium, the highest rate of Pi uptake was recorded; this rate depended on the external Pi concentration. A transient accumulation of PolyP was detected during this period peaking approximately at 2 h. The second phase coincided with the beginning of exponential growth (starting from ca. 1 d after Pi replenishment) when the cultures displayed a sustained rate of Pi uptake which was 5–10 times lower than that recorded at the first phase. The Pi uptake at the second phase was proportional to the cell division rate with a little PolyP accumulation. The onset of stationary phase (slowdown of cell division rate) because of light limitation on the background of excess of Pi in the medium brought about the appearance of PolyP in the cells (apparent as P-rich vacuolar inclusions of distinct morphology [3]) and a decline in Pi uptake rate. The NGS transcriptomics-based evaluation of differential gene expression revealed that the transition of the microalgal cells from P-shortage to LU was accompanied by (i) disengagement of P-sparing mechanisms, (ii) replacement of the high-affinity Pi transporter system by low-affinity one, and (iii) up-regulation of putative PolyP polymerase(s)—vacuolar transport chaperone (VTC) complexes. It is likely that the rate of PolyP accumulation in and P storage of microalgal cell is a tradeoff between Pi uptake rate (which, in turn, depends on the external Pi concentration and the P nutrition history of the cell) and Pi expenditure for biosynthesis of cell building blocks (nucleic acids, phospholipids etc.).