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

Abstract

Many micro-organisms store inorganic phosphate (Pi) in the form of polyphosphate (polyP) and exhibit in-cell polyP accumulation, a phenomenon known as phosphate overplus response, when resupplied with Pi after a period of deprivation. 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. The lowest level of in-cell polyP during Pi deprivation, which also correlates with the cessation of growth, is the key parameter for the timing of Pi resupply to maximise the Pi overplus response Additional nutrients do not affect the size of the overplus response, but they are important for continued growth and maximal Pi removal from the media. 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 phosphorus (P) content or maximum Pi removal is desired different strategies may be required - e,g., Pi deprivation until growth cessation then resupplying complete nutrients gives the best trade-off between high in-cell P accumulation, high Pi uptake and algal biomass growth. Although polyP levels are maintained after Pi resupply, the polymer is dynamically remodelled. IP6 increases during this time. This increase does not precede polyP synthesis as predicted by a model where inositol phosphates switch on polyP synthesis. One strain tested, CC-5325, shows enhanced Pi uptake and levels of polyP and total in-cell P, suggesting that strain selection is also important. ImportanceThere is strong interest in using microalgae to sustainably control and recover nutrients, especially P, from wastewater. This would help to meet environmental discharge consents and recycle nutrients into agriculture or other applications. Like bacteria and yeasts, microalgae exhibit a Pi overplus phenomenon when Pi-deprived cells are resupplied with P, but microalgae do not require an additional carbon source and can simultaneously uptake nitrogen as well. Use of microalgae in wastewater treatment is limited by the unpredictability of their response and sensitivity to environmental factors, but engineered systems can greatly benefit from better understanding Pi dynamics and polyP accumulation. In the literature there is a lack of consensus regarding protocols to maximise the Pi overplus. In this work we provide robust measurements of quantitative physiological parameters, which should allow reproducibility in laboratory studies and provide design parameters for algal-based nutrient recovery systems from waste waters.