Toward Climate-Responsive Ocean–Terrestrial Biorefineries: Integrating Algae and Agricultural Residues for Circular Bioeconomies

This paper evaluates climate-responsive biorefineries that integrate pelagic Sargassum with agricultural residues to advance circular bioeconomies. Building on scientific priorities emphasized at the 2025 One Ocean Science Congress and emerging policy directions from the draft resolution of the United Nations Environment Agency (UNEA-7) aimed at strengthening global responses to the large influxes of Sargassum, we explore various technological pathways—biochemical, thermochemical, and hybrid—as well as the deployment barriers for coupled ocean-terrestrial biomass systems. A comparative framework was applied to biochemical (enzymatic hydrolysis and anaerobic digestion) and thermochemical (pyrolysis and hydrothermal liquefaction) pathways, incorporating heat transfer modeling to account for the high moisture content of marine biomass. Life cycle assessment demonstrated that hybrid systems achieved up to 35% lower greenhouse gas emissions compared to single-feedstock facilities, while techno-economic analysis estimated processing costs of USD 120–150 per ton of biomass under technology readiness level (TRL) 4 conditions. Results highlight operational challenges, including slagging and fouling in thermochemical reactors due to elevated ash and salt fractions, and variability in syngas composition between agricultural residues and algae blends. The findings underscore the need for modular, adaptive system designs and targeted engineering strategies to transition toward TRL 5 prototypes. We concluded that coupling algae and agricultural residues can support SDG 14 and circular bioeconomies, provided that adaptive preprocessing, heat transfer engineering, and contaminant management are addressed. This work positions algae–agricultural residue integration as a viable pathway linking ocean health, climate mitigation, and resource recovery.