An integrated biorefinery scheme towards freshwater microalgae as phycoerythrin factories for circular economy: Feedstock selection, optimized synthesis, wastewater synergy and cascading application

Phycoerythrin (PE) is a high-value fluorescent pigment-protein with broad applications in food, diagnostics, and therapeutics, yet its conventional production from marine algae faces challenges of unstable yield, high cost, and ecological concerns. This narrative review systematically evaluates the potential of freshwater microalgae as sustainable platforms for PE production within a circular economy framework. A structured literature search was conducted across Web of Science and Google Scholar using keywords related to freshwater algae, PE biosynthesis, genetic engineering, wastewater bioremediation, and biorefinery. Key findings demonstrate that specific Cyanobacteria and Cryptophyta strains (e.g., Anabaena fertilissima: 475 mg/g dry weight; Cryptomonas pyrenoidifera: 345 mg/g dry weight) exhibit yields comparable or superior to those of traditional marine sources. Genetic engineering strategies-including promoter engineering (e.g., psbA modification increasing expression 3.7-fold), heterologous expression (achieving 96.7% chromophorylation), and CRISPR-Cas9-mediated metabolic flux redirection-offer robust tools for enhancing PE yield and stability. Wastewater cultivation enables efficient removal of nutrients (>90% N/P) and emerging contaminants (e.g., >95% heavy metals, 50% pharmaceuticals, >84% microplastics) alongside biomass production. High-value PE applications-as fluorescent probes (quantum yield 0.98), photosensitizers in photodynamic therapy, and photocatalysts for pollutant degradation (>90% removal)-provide economic drivers. A SWOT-TOWS analysis identifies strategic pathways addressing cost, scalability, and regulatory barriers. We propose an integrated biorefinery model coupling wastewater treatment, multi-product cascading (PE, lipids, biochar, biofertilizers), and policy instruments (carbon credits, purity standards) to accelerate sustainable commercialization of freshwater algae-derived PE.