Quantifying the combined effects of substrate microstructure and environmental factors on microalgal biofilm growth:...

Microalgal biofilm cultivation is a promising strategy for achieving efficient carbon sequestration and biomass production. However, existing growth kinetics models predominantly focus on environmental stimuli, neglecting the decisive roles of substrate microstructure in mass transfer and cell attachment. In this study, a multi-factor growth kinetics model was developed for non-submerged microalgal biofilm systems. A substrate structural term of cotton fabric substrate (porosity, tortuosity, thickness, and average pore diameter) was formulated and multiplicatively coupled with light intensity, CO₂, and nitrate concentrations. The model showed good agreement with experimental μ under the tested conditions (R² > 0.9389) for Chlorella vulgaris biofilms cultivated on cotton fabrics, forecasting a maximum μ of 1.33 d^-1 in the integrated model. Sobol global sensitivity analysis showed that nitrogen supply and light irradiance were the dominant contributors to the specific growth rate (μ). Notably, within the investigated parameter range, the model suggested an attachment response associated with pore size, with a fitted pore-suitability diameter of approximately 2.04 μm. This may indicate a potential trade-off between cell attachment stability and substrate-mediated nitrate diffusion in the tested non-submerged cotton-based biofilm system. This framework describes the combined effects of substrate structural factors and environmental parameters on microalgal biofilm growth and may be useful for substrate design under similar non-submerged biofilm cultivation configurations.