The effects of sodium bicarbonate on freshwater green alga Neochloris oleoabundans were evaluated with respect to cell growth, cell morphology, and control of protozoa contamination under non-sterile conditions co-inoculated with protozoa pre-cultures. While culture collapse occurred in the control and the cultures containing 40mmol/L NaHCO3 due to protozoa contamination, the addition of 160 or 200mmol/L NaHCO3 could completely inhibit the growth of microalgae-preying protozoa and allow safe cultivation of N. oleoabundans for co-incubation with protozoa in non-sterile media. These results suggest that well-formulated media containing appropriate amounts of dissolved inorganic carbon (DIC) may enable low-cost cultivation of freshwater microalgae in non-sterile open systems.