Biofilm-driven flocculation in microalgae-bacteria consortia: Mechanisms and biotechnological implications

Abstract The ability of naturally selected microalgae-bacteria consortia (MBC) to form permanent flocs enables fast-sedimentation by gravitation and thereby low-cost biomass harvesting in phototrophic bioreactor systems. Despite this potential, the underlying flocculation mechanisms - particularly the role of extracellular polymeric substances (EPS) - remain poorly understood. This study provides a mechanistic insight into EPS-driven flocculation and its implications for biomass engineering. The found EPS matrix, primarily composed of proteins and polysaccharides (4.2 ± 2.6% EPS g TSS −1 ), was shown to be essential for floc integrity and reflocculation following EPS removal. The analysis of the sinking properties showed that the reflocculation was significantly inhibited by antibiotic and enzymatic treatments, resulting in recovery rates below 22% compared to 87% in the control, indicating the central role of EPS-producing bacteria. Furthermore, extracted EPS were capable of inducing flocculation in non-flocculating Chlorella vulgaris , increasing particle size from 5 to 97 µm and recovery rate from 5 to 35%, highlighting their potential as natural bioflocculants. These findings demonstrate that EPS might be suitable for the use in enhancing harvesting efficiency and bioengineering tailored consortia with desired functional traits. The results advance the understanding of cooperative interactions within MBC and could open up new avenues for scalable, sustainable, and selective biomass production in biotechnology.