Cultivation of phototrophic purple bacteria from fermented wine lees: effect of light regime and hydraulic retention time.

Phototrophic purple bacteria (PPB) represent a versatile microbial platform for simultaneous wastewater treatment and valuable bioproducts production. Herein, the performance of a PPB-based sequencing batch reactor i.e. PPB metabolism, photophysiology, and treatment efficiency, with a particular focus on biomass productivity, PHA/HV accumulation, pigment formation, removal of soluble COD, ammonium, and organic acids, was evaluated under three hydraulic retention times (HRT = 2, 4, and 6 days) and two illumination regimes (12-hour and 24-hour light). HRT was predominantly the primary driver of metabolic behavior and at HRT = 2 days, a rapid substrate turnover and mild metabolic stress favored PHA and HV accumulation, especially under intermittent illumination. At HRT = 4 days, the system reached its most balanced state, exhibiting the highest biomass productivity, stable pigment concentrations, and maximum removal efficiencies. Continuous illumination enhanced phototrophic activity and treatment performance at this retention time. At HRT = 6 days, the culture shifted toward maintenance metabolism, resulting in minimal polymer formation, reduced pigment levels, and lower removal efficiencies. Correlation analysis revealed coherent relationships among the measured parameters: high productivity was associated with high pigment levels and strong COD/OAs removal, while high PHA/HV fractions corresponded to reduced growth and lower treatment performance. These findings highlight a fundamental trade-off between growth and storage in PPB metabolism. Overall, the study demonstrates that intermediate retention time (HRT = 4 days) combined with continuous illumination provides the optimal balance between biomass production, phototrophic activity, and wastewater treatment efficiency. Short retention time is more suitable for biopolymer production, while long retention time limits both growth and treatment performance. These insights contribute to a deeper understanding of PPB physiology and offer practical guidance for optimizing PPB-based wastewater treatment and bioproduct recovery systems.