Application of Pulsed Electric Fields in Algal and Bacterial Cocultures: Investigating Bacterial Responses and Purity of Algal Protein Extract

Introduction: Pulsed electric field (PEF) technology shows promise for microbial control in biorefinery applications. However, its effectiveness in mixed cultures remains poorly understood. This study investigated the differential effects of PEF treatment on bacterial inactivation and algal protein preservation in a coculture of Chlorella vulgaris algae and Delftia sp. bacteria. Methods: Algal and bacterial viable cell counts and viability were quantified using flow cytometry with differential fluorescent staining (SYTO 9, YO-PRO-1, fluorescein diacetate FDA), bacterial growth was monitored spectrophotometrically at 600 nm, protein extraction was determined by modified Lowry assay, protein profiles analyzed by SDS-PAGE, and extract antimicrobial activity was assessed by agar diffusion and growth inhibition assays. We compared PEF treatments at two energy levels (4 and 100 J/mL) against high-pressure homogenization (HPH) as a control, with assessments at different growth phases (days 1, 3, and 7). Results: While PEF consistently inactivated >95% of algal cells, regardless of the growth phase, bacterial inactivation varied significantly, with maximum susceptibility on day 3 (70–80% mortality) when bacteria entered the starvation phase. Unexpectedly, on day 7, PEF treatment of cocultures led to bacterial proliferation, with viable counts increasing up to 4-fold compared with untreated controls. Analysis of algal extracts showed no antimicrobial activity against bacteria, and instead supported bacterial proliferation, suggesting that cellular disruption releases compounds that can be metabolized by surviving bacteria. Furthermore, while PEF preserved the integrity of algal protein profiles regardless of bacterial presence, HPH treatment of cocultures introduced a novel ∼27 kDa protein band, suggesting bacterial contamination of the extract. Conclusion: These findings reveal the complex, growth phase-dependent dynamics inherent in PEF treatment of mixed microbial systems and provide critical insights into biorefinery applications in which microbiological control and product quality must be balanced.