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.