Process-integrated nanozyme-assisted bioconversion and fermentation of agar-rich red macroalgae to...

Agar-rich red macroalgae such as Gelidium amansii represent an attractive non-food marine biomass for biopolymer production; however, their utilization is limited by slow polysaccharide hydrolysis and mismatches between sugar release and microbial uptake during fermentation. Nanozymes have emerged as potential process enhancers, yet their integration with microbial conversion and process control strategies remains poorly understood. In this study, a nanozyme-assisted bioconversion strategy was developed for direct polyhydroxyalkanoate (PHA) production from G. amansii using Saccharophagus degradans. Redox-active CeFe₃O₄ nanozymes were applied to modulate biomass hydrolysis, primarily influencing early-stage sugar release dynamics rather than serving as primary depolymerizing agents, while pulse-feeding strategies were implemented to regulate carbon availability over time. The effects of biomass loading, nanozyme concentration, and feeding regime on sugar profiles, microbial performance, PHA accumulation, yield, and productivity were systematically evaluated at both flask and stirred-tank scales. Moderate nanozyme concentrations (0.02-0.05 wt%) resulted in higher transient soluble sugar levels; however, increased soluble sugar alone did not necessarily translate into higher PHA production. In contrast, pulse-feeding stabilized sugar availability and increased maximum PHA productivity by more than two-fold compared to single pulse-feeding. Overall, the results indicate that PHA productivity is closely associated with the temporal coordination between substrate release and microbial metabolic capacity rather than total carbon input alone. By integrating nanozyme-assisted hydrolysis modulation with controlled feeding strategies, this study offers a process-level strategy for improving PHA production from red macroalgal biomass.