Agar production and potential application in CO₂ capture and sequestration

Agar, a polysaccharide abundant in red macroalgae, has been extensively used in the fields of food, chemical and biomedical industries due to its unique physicochemical properties. However, current understanding of its production potential and application in carbon capture and sequestration remains fragmented. This review comprehensively compiles and analyzes the agar content in red macroalgae (43 species from 29 studies), revealing a wide interspecific variation ranging from as low as 3.5% to up to 62.3% of dry weight, which is also significantly influenced by different extraction methods. Notably, alkali treatment commonly reduces agar recovery compared with traditional water extraction. Global biomass trends of Gracilaria and Gelidium, two key agar-production genera, are further analyzed noting a distinct shift in their supply mode from wild harvest to aquaculture, particularly for Gracilaria. Based on suitable area for seaweed cultivation worldwide, the agar production could reach 61.25 × 10⁹ t yr^-1. Due to its structural complexity and bacterial metabolic constraints, agar exhibits the potential to sequester CO₂ in the oceans, with a theoretical carbon sequestration of 19.29× 10⁹ t yr^-1. The latest agar-based green technology in CO₂ capture and current research challenges is also reviewed. Finally, a model of agar's sustainable utilization in carbon capture and sequestration is proposed, integrating high-value biomedicine and large-volume carbon sequestration. By synthesizing production dynamics and green technologies, this study provides novel insights into unlocking agar's potential as a sustainable biopolymer for blue bioeconomy and carbon-neutral initiatives.