Liquid digestate treatment potential of five green microalgae in non-axenic cultures

The research examined the growth of non-axenic microalgal cultures in liquid digestate for the removal of nutrients and organic contaminants. Five environmental isolates (Chlorella sorokiniana, Tetradesmus obliquus, Desmodesmus subspicatus, Chloroidium saccharophilum, Desmodesmus communis) were cultivated for a period of 120 days comprising two operational phases: Phase 1 without pH control and Phase 2 with pH adjustment. In Phase 1, the microalgal-bacterial cultures achieved up to 69.7 ± 5.1% of total nitrogen (TN) reduction rate, while orthophosphate (PO₄³⁻) removal reached only 46.5 ± 3.8%. In contrast, the pH adjustment in Phase 2 had a dramatic effect on the microalgae growth and treatment efficiency: the TN and orthophosphate removal rates were significantly higher, whereas sCOD reduction lowered. Among the tested non‑axenic cultures, the trials inoculated with Desmodesmus communis achieved the highest overall TN removal rates in both phases (69.7 ± 5.1% in Phase 1 and 89.4 ± 0.8% in Phase 2), whereas PO₄³⁻ reduction was relatively low in Phase 1 but reached 93.8 ± 0.3% in Phase 2. By contrast, cultures inoculated with Tetradesmus obliquus and Chlorella sorokiniana consistently exhibited the lowest TN reduction rates (55.4 ± 1.7–57.9 ± 5.8% in Phase 1 and 85.2 ± 1.6–85.8 ± 1.0% in Phase 2). By comparison, Desmodesmus subspicatus generally performed best in terms of sCOD reduction (69.0 ± 1.9–75.2 ± 0.7%). The pH adjustment significantly increased the density of microalgal coenobia and chlorophyll a concentration and also led to considerable changes in the relative abundance of bacteria. In Phase 1 three bacterial phyla dominated the non‑axenic cultures: Proteobacteria (40.9–75.0%), Bacteroidota (11.5–19.3%) and Actinobacteriota (2.9–27.1%). In Phase 2, NaOH‑driven pH increase clearly restructured the bacterial community: Proteobacteria remained dominant (59.0–78.4%), Bacteroidota declined markedly (1.5–9.0%), and Firmicutes became the second most abundant phylum (2.1–30.4%), while Actinobacteriota were strongly reduced. Overall, the removal of nutrients and organic pollutants was not solely attributable to microalgal activity but likely resulted from complex interactions within the microalgal–bacterial consortia under the contrasting pH regimes of Phases 1 and 2.