Manipulating the Chlamydomonas reinhardtii Phosphate overplus response for Biological Phosphorus Recovery from Wastewater

Global dependency on phosphate (P) rich rock mining to produce inorganic phosphorus fertilisers has broken the balance of the phosphorus cycle and is threatening our ability to achieve food and water security. The predicted phosphorus scarcity crisis due to the imminent depletion of P-rock reserves represents one of the biggest challenges for humanity, as phosphorus is irreplaceable to sustain life on Earth. At the same time, nutrient-rich agricultural runoff and sewage discharges are generating an excessive load of nutrients into water bodies. This is triggering the eutrophication of aquatic ecosystems at a scale never seen before. To protect water resources, strict nutrient discharge limits have been set by local environmental agencies, particularly in Europe, the UK and North America, where limits are revised on a regular basis. For that reason, current P control systems struggle to comply with the P discharge limits; in addition, they do not facilitate P recovery and reuse. Microalgae may offer a solution as they grow naturally in wastewater, taking up nutrients in the process, but their vulnerability to changes in environmental conditions limits their use to reliably control P at wastewater treatment plants.
Chlamydomonas reinhardtii and microalgae in general, can exhibit a P overplus response when transferred from P-deprived to P-replete conditions. This is the overaccumulation of P in the form of polyphosphate (polyP) granules. Understanding the factors that influence microalgal P overplus could be key to enhancing the robustness of P recovery in wastewater treatment systems using algae-based technologies. In this thesis, a quantitative method to analyse polyP in Chlamydomonas was developed and used along with more qualitative methods to follow changes in polyP and other physiological parameters during P-depletion and repletion. The results show that polyP content in algal cells is the key physiological parameter to monitor during P-deprivation, as the lowest in-cell polyP content triggers a bigger P overplus response upon P resupply. During P resupply, the successful P recovery depends on both P uptake and biomass growth. While supplying all nutrients rather than P alone, did not affect polyP accumulation, it promoted biomass growth which led to a complete removal of P from the media in 12 h. These results contribute to better future design of P recovery systems from wastewaters using P overplus response in microalgae.
This work provides further evidence of the PSR1 transcription factor of C. reinhardtii as the most important regulator of P homeostasis. The experimental comparison between PSR1 overexpression and PSR1 knockout strains led to the determination that PSR1 is a key component of polyP synthesis and hence of P overplus responses.
Real-life application of the research findings about the P overplus phenomenon requires a deeper understanding of microalgal P metabolism in terms of P sensing, P uptake, polyP synthesis and turnover.