Dual-pathway modeling framework for CO₂ sequestration in algal biofilms via biomass assimilation and exopolysaccharide secretion

• Dual-pathway model separates biomass uptake from exopolysaccharide secretion. • Validated with multiple datasets, showing strong accuracy (R² > 0.97). • Carbon allocation fraction (α) is used as a model parameter for carbon partitioning scenarios. • EPS represents ∼20–50 % of fixed carbon and doubles effective retention time. • Framework quantitatively links biofilm kinetics with photobioreactor design. Algal biofilms are increasingly being explored for CO₂ capture in environmental biotechnology; however, most studies emphasize photosynthetic biomass accumulation as the primary sequestration route. This work introduces a dual-pathway modeling framework that couples intracellular assimilation with extracellular stabilization via exopolysaccharide secretion. In the conventional pathway, CO₂ is assimilated into algal biomass, forming a short- to medium-term sink. In the parallel pathway, a fraction of fixed carbon is secreted as exopolysaccharides, forming a relatively slow-turnover extracellular pool that functions as metabolic overflow and contributes to enhanced biofilm stability. Literature reports confirm that exopolysaccharide secretion is stimulated under elevated CO₂ and nutrient stress. A steady-state reaction–diffusion model, parameterized with biofilm data and validated against independent published experimental datasets (Willmott’s d = 0.992, RMSE = 0.24), shows that exopolysaccharides secretion accounts for 20–50 % of total carbon retention, extending residence times by nearly two-fold. These results reframe algal biofilms as hybrid systems that integrate biomass production with the stabilization of extracellular carbon. The framework provides predictive tools for optimizing CO₂ capture technologies and supporting the valorization of extracellular polymeric substances as a sustainable bioproduct.