Impact of bacteria-to-algae cell ratios on organic matter production and bioflocculation of Chlorella vulgaris

Algal cultures used in wastewater treatment typically coexist with bacterial consortia. These heterotrophic bacteria assist biofloc formation, supporting nutrient removal, but may also compete with algae for nutrients and contribute to culture crashes. Both algae and bacteria release extracellular organic matter (EOM), which plays a key role in floc development. While prior studies have explored algal-bacterial interactions, the specific role of algae-associated bacteria in EOM production and bioflocculation remains poorly characterised. This study addresses that gap by examining how differences in the initial bacteria-to-algae cell ratios affect early-stage co-culture outcomes, including nutrient removal, biomass characteristics (viability and integrity), EOM profiles, and bioflocculation in Chlorella vulgaris cultures by the end of the exponential growth phase, which is a key period for biomass harvesting in algal bioprocessing systems. Co-existing bacteria were isolated and reintroduced into algal cultures at different ratios in a photobioreactor. Algal growth and viability remained stable across ratios from 0.2:1–15:1, with all cultures converging to a 0.1:1 ratio by day 6. However, bacterial viability dropped below 50 %, likely due to the absence of organic carbon in the algal medium. Higher initial bacteria-to-algae ratios led to greater production of 0.45–1.2 µm-sized AOM (AOM1.2), hydrophobic organic carbon (HOC), and low molecular weight neutral substances. Notably, biofloc formation increased significantly at initial ratios above 8:1, correlating positively with bacterial load and specific EOM fractions. These findings suggest that optimising bacteria-to-algae ratios can enhance bioflocculation, offering a pathway to improved algal biomass recovery in wastewater treatment and algal bioprocessing systems. • Algal growth remained stable across bacteria-to-algae ratios from 0.2:1–15:1. • Bacterial viability reduced < 50 % due to organic carbon limitation in the medium. • Biofloc formation was significant at bacteria-to-algae ratios above 8:1.