ИСТИНА

Large amounts of phosphorus (P), a major non-renewable plant nutrient, are lost annually with wastewater. Microalgae are a promising vehicle for recycling P from wastewater into biofertilizers, since they are naturally equipped to take up much more P than is necessary to support their growth, expressing a behavior known as ‘luxury uptake’ of P. Advantages of microalgal capture of P include simultaneous removal of different nitrogen species, destruction of organic pollutants, suppression of pathogenic microflora, capture of the greenhouse gasses CO2 and NOx, as well as co-generation of P-enriched biomass suitable for conversion into slow-release P biofertilizer. However, the use of microalgae is limited by insufficient knowledge of P uptake mechanisms under eutrophic conditions. We followed the relationships between cell division rate, inorganic phosphate (Pi) uptake rate from a P-rich medium and polyphosphate (PolyP) formation 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 bi-phasic kinetics of Pi uptake. 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. At this phase, the cells accumulated up to 5–7 %DW of P. 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. Little, if any, PolyP accumulation was detected during this phase. 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 and a decline in Pi uptake rate. The P content of the microalgal biomass did not exceed 2% DW at this stage. It is likely that the rate of PolyP accumulation in microalgal cells is defined predominantly by the rates of Pi uptake (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.). These results suggest that the most efficient biocapture of P and generation of polyphosphate-enriched cells is observed in the periods characterized with slow cell division. On the contrary, vigorously dividing cultures characterized by a rapid accumulation of biomass are not likely to accumulate large amounts of PolyP. To develop an economically viable process ensuring efficient P biocapture together with a high degree of PolyP enrichment, one will need a multi-stage ‘feast and famine’ approach including (i) the P removal phase (by pre-starved cultures) and (ii) regeneration of P-hungry culture.