Screening vertical-oriented carriers for microalgal biofilms: Interfacial thermodynamics and adaptive secretion of extracellular polymeric substances

Microalgae attachment cultivation has garnered extensive attention due to its advantages of high biomass productivity and cost-effective harvesting. However, the selection of suitable carriers remains largely empirical, with a lack of predictive, mechanism-driven criteria. Moreover, most carrier screening efforts have been conducted using horizontally oriented configurations, which provide limited relevance to the rapidly emerging vertical microalgal biofilm cultivation systems. To address these gaps, this study systematically examined the initial adhesion and long-term biofilm development of Chlorella pyrenoidosa on five vertical-oriented carrier materials (polyester, polyurethane sponge, nylon, polyvinyl chloride, and polypropylene), focusing on interfacial thermodynamics and the secretion of extracellular polymeric substances (EPS). Thermodynamic analysis combined with the extended Derjaguin-Landau-Verwey-Overbeek theory revealed that the Lewis acid-base interaction, primarily governed by carrier hydrophobicity, is the dominant driving force for the initial adhesion of microalgae. During biofilm development, polyester (PET) achieved the highest biomass productivity (7.84 g/m 2 /day), outperforming the other carriers by 1.29–4.56 times. Concurrently, C. pyrenoidosa employed an adaptive EPS secretion strategy, increasing total EPS content by 4.11–8.35 times compared to suspended culture. Notably, the protein content increased more substantially, raising the protein-to-polysaccharide (PN/PS) ratio by 5.26–8.62 times, thereby improving biofilm stability across various carriers. Crucially, optimal carrier selection requires a trade-off between EPS secretion and biofilm growth performance, as these parameters were inversely related. Based on these criteria, PET was identified as the optimal carrier, exhibiting stable biomass productivities of 9.18–9.65 g/m 2 /day over three regrowth cycles. Overall, our findings offered a theoretical framework for carrier screening and rational design, thereby advancing the technological maturation and industrial-scale development of high-efficiency vertical microalgal biofilm cultivation systems. • The hydrophobicity of carriers was a key factor affecting the microalgal adhesion. • AB force dominated interfacial interaction energy between microalgae and carriers. • Microalgae adaptively secreted protein-rich EPS for biofilm stability on carriers. • Suitable carriers should balance EPS secretion and biofilm growth. • The optimal polyester (PET) carrier yielded a productivity of 9.18–9.65 g/m 2 /day.