Physiological responses of floc-forming microalgae-bacteria consortia to environmental perturbations.

Fast-sedimenting microalgae-bacteria consortia (MBC) offer a cost-efficient pathway for biomass harvesting while remediating polluted water bodies in chemostatic photobioreactors. Understanding how abiotic parameters affect floc morphology, sinking properties and metagenomic species composition remains critical for optimization of these specific bioreactors. This study investigated whether fast-sedimenting MBC maintain structural resilience under moderate stress but lose stability beyond critical physiological tipping points. By investigating the physiological boundaries of five environmental factors, we identified clear operational thresholds. Moderate perturbations including light intensities up to 1500 µmol m-2 s-1, salinities from 0 to 35 PSU and low antibiotic concentrations showed no statistically significant impact on settling efficiency. In contrast, extreme pH at 12 and temperatures at 45 °C reduced recovery rates by more than 50 % compared to controls maintaining above 87 % efficiency. The surface charge decreased from -27.94 mV to -4.83 mV under acidic conditions at pH 3, indicating electrostatic destabilization of the floc matrix. Dominance of the cyanobacterium Thermoleptolyngbya spp. persisted above 70 % abundance across all treatments. These findings define a safe operating envelope between pH 6-9 and temperatures from 15 to 35 °C necessary to maintain gravity-driven sedimentation. This work provides quantitative boundaries where biological buffering fails, enabling predictive reactor design that avoids biomass washout in continuous cultivation systems.