Characterization of floc-forming, fast-sedimenting microalgae-bacteria consortia used for low-cost biomass harvesting in chemostatic bioreactors

Naturally selected microalgae-bacteria consortia (MBC) offer a promising solution for cost-efficient biomass harvesting in photosynthetic, chemostatic bioreactors, addressing a key challenge in current microalgal biotechnology. Through repeated selection of environmental samples for fast-sedimenting structures, a highly stable floc-based culture containing microalgae, cyanobacteria, and associated bacteria was obtained, exhibiting an exceptional sedimentation behavior by passive gravitation. Comparative analyses with unialgal reference strains confirmed the superior sinking properties of MBC cultures (recovery rate: 97.48 ± 2.30%; mean sinking velocity: 4.16 ± 0.52 m h −1 ; mean particle size: 417.59 ± 21.82 μm). Furthermore, cultivation on three wastewater-related substrates revealed a strong influence of the floc morphology and microbial composition, while maintaining the characteristic sinking behavior. Metagenomic sequencing identified a predominance of Fragilaria spp. (sewage), Ochromonas spp. (artificial medium) and Chlorella spp. (fish sewage) sample. This study provides the first comprehensive characterization of the flocculation and sedimentation behavior of these MBC and establishes a methodological foundation for future application-oriented research of this customizable, low-cost biomass production systems. • Naturally selected MBC enable gravity-based biomass harvesting. • Microbial flocs show exceptional recovery rates (>90%) without chemical flocculants. • Floc morphology adapts to applied substrates with conserved sedimentation. • Metagenomic sequencing reveals substrate-specific communities. • These MBC offer a low-cost, scalable alternative for microalgal biomass recover.