Removal of Mg 2+ inhibition benefited the growth and isolation of ammonia-oxidizing bacteria: An inspiration from bacterial interaction

Ammonia-oxidizing bacteria (AOB) play an important role in the global nitrogen cycle and have broad applications in the nitrogen removal from wastewater. However, the AOB species are sensitive to environmental factors and usually form tight relationships with other microbes, making the AOB isolation and maintenance are difficult and time-consuming. In this study, the relationship that occurred between AOB and their bacterial partners was found to be able to improve the ammonia oxidation; during the co-cultivation, the magnesium ions (Mg 2+ ) with removal rate as high as 36.7% was removed from culture medium by the concomitant bacterial species, which was regarded as the main reason for improving ammonia oxidation. During the pure cultivation of AOB isolate, when the concentration of Mg 2+ reduced to low levels, the ammonia-oxidizing activity was more than 5 times and the amoA gene expression was more than 12 times higher than that grown in the initial culture medium. Based on a newly designed culture medium, the ammonia oxidation of AOB isolate grown in liquid culture was significantly promoted and the visible AOB colonies with much more number and larger diameter were observed to form on agar plates. With the addition of high concentration of calcium carbonate (CaCO 3 ), AOB colonies could be easily and specifically identified by following the hydrolytic zones that formed around AOB colonies. Another AOB isolates were successively obtained from different samples and within a short time, suggesting the feasibility and effectivity of this culture medium and strategy on the AOB isolation from environments.