As a finite and non-renewable resource, phosphorus (P) is essential to all life and crucial for crop growth and food production. The boosted agricultural use and associated loss of P to the aquatic environment are increasing environmental pollution, harming ecosystems, and threatening future global food security. Thus, recovering and reusing P from water bodies is urgently needed to close the P cycle. As a natural, eco-friendly, and sustainable reclamation strategy, microalgae-based biological P recovery is considered a promising solution. However, the low P accumulation capacity and P-removal efficiency of algal bioreactors restrict its application. Herein, it is demonstrated that manipulating genes involved in cellular P accumulation and signaling could triple the Chlamydomonas P-storage capacity to ~7% of dry biomass, which is the highest P concentration in plants to date. Furthermore, the engineered algae could recover P from wastewater almost three times faster than the unengineered one, which could be directly used as a P fertilizer. Thus, engineering genes involved in cellular P accumulation and signaling in microalgae could be a promising strategy to enhance P uptake and accumulation, which have the potential to accelerate the application of algae for P recovery from the water body and closing the P cycle.
The proposed model for SPAO design and closing the P cycle
The proposed model for SPAO design is shown on the left. Compared with conventional PAO (wildtype microalgae), SPAO presents greater polyP accumulation and higher P-removal capacity. Three approaches for genetic engineering of SPAO to enhance the P-removal capacity are suggested: 1) genetic operation of genes controlling the vacuolar P accumulation. Down-regulation (or loss-of-function) of SPX-SLC proteins to enhance P accumulation in vacuoles and further increase the P-removal capacity in SPAO; 2) increase the expression of PSR1, which enhances Pi acquisition through directly up-regulating the expression of P-starvation-induced genes (PSIGs); 3) combining the above two approaches – enhancing P-starvation signaling and accumulating P in vacuoles. After being applied to recover P from the water system, SAPO algae with greater polyP accumulation can be used as a slow-release P fertilizer to close the P cycle.
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