Phosphate overplus response in Chlamydomonas reinhardtii: Polyphosphate dynamics to monitor phosphate uptake and turnover

There is widespread interest in using microalgae to sustainably control and recover nutrients from wastewater to meet environmental discharge consents and recycle nutrients into agriculture or other applications. Like bacteria and yeasts, microalgae exhibit a phosphate (Pi) overplus response when Pi-deprived cells are resupplied with this nutrient. Quantitative and qualitative methods were used to follow the dynamics of polyP synthesis and turnover in four strains of Chlamydomonas reinhardtii during Pi deprivation followed by nutrient resupply (either as Pi or complete media). The lowest level of in-cell polyP during Pi deprivation, which correlates with the cessation of growth, is the key parameter for timing Pi resupply to maximise the Pi overplus response. Pi deprivation beyond this point leads to a reduced Pi uptake and reduced overplus response. Additional nutrients do not affect the magnitude of the overplus response but are important for continued growth and maximal Pi removal from the media. One strain tested shows enhanced Pi uptake and increased polyP and total in-cell P, suggesting that strain selection is also important. Although polyP levels are maintained after Pi resupply, the polymer is dynamically remodelled. Inositol hexakisphosphate (IP6) increases during this time but does not precede polyP synthesis as predicted by a model where inositol phosphates switch on polyP synthesis. Tracking polyP allows the correct time for nutrient resupply to be determined and therefore a reproducible Pi overplus response to be achieved. Depending on whether maximum cellular P content or maximum Pi removal is desired different strategies may be required. Pi deprivation until growth cessation then resupplying complete nutrients gives the best trade-off between high in-cell P accumulation, high Pi removal and algal biomass growth. This work provides robust measurements of quantitative physiological parameters, which allows reproducibility in laboratory studies and provides design parameters for algal-based nutrient recovery systems from waste waters.