Multiscale characterization of a microalgae-yeast co-culture: combining physiological performance assessment and...

Microalgae and yeasts can establish a mutualistic relationship based on reciprocal oxygen/carbon dioxide exchange, offering a strategy to overcome productivity limits caused by the slow growth of microalgae. This study examines the physiological and biophysical responses of Parachlorella kessleri when co-cultured with Saccharomyces cerevisiae to enhance lipid productivity while maintaining cost-effectiveness for large-scale applications. A multi-scale analytical approach was used to clarify the mechanisms driving this interaction. Under co-culture conditions, biomass and lipid production increased 4-fold compared to microalgal monoculture grown in BBM + NO₃ medium. Yeast sustained microalgal growth during the transition to phototrophy after glucose depletion (day 6) and promoted lipid accumulation through nutrient competition. Despite prolonged nutrient limitation (approximately 8 days), yeast viability remained above 68%, indicating possible metabolic support from microalgae via exopolysaccharides (EPS) - derived molecules. At the biophysical level, atomic force microscopy (AFM) analyses showed increased microalgal cell size, visible EPS production, and clear surface structural modifications in yeast cells, together with a significant reduction in yeast cell wall stiffness. Single-cell force spectroscopy using fluidic force microscopy (FluidFM) revealed stronger and longer microalgae-yeast interactions in co-culture, likely due to enhanced EPS production when cocultured. Overall, combining physiological and biophysical insights provides a deepened understanding of co-culture dynamics across scales, supporting the rational design and optimization of microalgae-yeast systems for sustainable and economically viable lipid production.