Differential Molecular Transformation Mechanisms of Phytohormones in Activated Sludge and Anaerobic Digestion Sludge: The Precursor Roles of Tryptophan and Linolenic Acid

Converting sludge to high-value resources is a sustainable strategy. Recent advances in alkaline thermal hydrolysis (ATH) technology have revealed its potential for phytohormone synthesis by leveraging the inherent complexity of sludge, offering an innovative approach to resource recovery. While previous studies focused on optimizing yields and agronomic applications of sludge-derived phytohormones, mechanistic investigations into their molecular transformation pathways are lacking. This study systematically compares residual activated sludge (RAS) and anaerobic digestion sludge (ADS) to unravel phytohormone generation mechanisms during ATH. Using metabolomic analysis and molecular energy calculations, we clarified the phytohormone distribution and differential characteristics between the two sludges. Key findings reveal that ATH amplifies the inherent precursor advantages of specific sludge types: RAS shows superior auxin production (1.58 × 10⁴ to 7.93 × 10⁴ ng/mL), whereas ADS preferentially synthesizes jasmonic acids (6.61 to 2.56 × 10³ ng/mL). Indole-3-acetic acid synthesis in RAS is driven by tryptophan-mediated pathways, where decarboxylation and aromatic substitution dominate precursor conversion; jasmonic acid in ADS forms via cyclization and oxidation of anaerobic-stress-induced linolenic acid derivatives. The favorable thermodynamics of both pathways was confirmed by Gibbs free energy calculations. Establishing relationships between sludge composition and phytohormone characteristics, this work provides molecular evidence to guide the agricultural application of sludge-derived phytohormones.