Surface activity of Chlorella exudates

Chlorella sorokiniana is a protein-rich microalga whose extracellular exudates are increasingly recognized for their potential as natural surface-active compounds. Although dissolved organic matter (DOM) and microbial exudates are known to affect soil wettability and heavy metal mobility, the specific interfacial behavior of C. sorokiniana exudates has not yet been quantitatively studied. In this research, we examined the combined physicochemical properties of these exudates and investigated how their molecular composition influences surface activity. The algae's growth was monitored daily through optical density at 750 nm (OD₇₅₀), chlorophyll levels, and biomass measurements. After three weeks, during the late exponential to early stationary phase, exudate production was quantified via Non-Purgeable Organic Carbon (NPOC) and Total Nitrogen (TN), which reached 12 mg/L and 430 mg/L respectively, indicating active organic matter release. Surface tension measurements using pendant drop techniques showed a reduction from 72 mN/m to 53 mN/m, confirming biosurfactant activity. Contact angles on hydrophobic polystyrene measured by the Wilhelmy plate method were advancing at 71.5° and receding at 60°, reflecting increased wettability caused by the exudates. The amphiphilic nature and effects on interfacial interactions were characterized through surface free energy components based on the Owens-Wendt-Rabel-Kaelbel (OWRK) model. Fluorescence analysis with Excitation–Emission Matrix (EEM) and PARAFAC identified two main protein-like fluorophores—tyrosine-like (Ex/Em ≈ 275/305 nm) and tryptophan-like (Ex/Em ≈ 275/340 nm)—confirming their protein origin. Overall, this study highlights C. sorokiniana exudates as natural biosurfactants, directly connecting their molecular makeup to their surface activity.