Methods of treating contamination, particularly bacterial contamination, in an open culture of microalgae with organic carbon are described herein. The methods comprise reducing the pH of a culture for a period of time and then raising the pH of the culture, wherein the culture comprises the presence of a first acid and the culture is contacted with a second acid to reduce the culture pH from a first pH value to a second pH value. The culture is maintained at the second pH value for at least 5 minutes before the culture pH is raised from the second pH value by contact with a base.
