Selective culture of Arthrospira platensis by continuous NH3 exposure to control microalgal contaminants and grazers, exemplified by Synechococcus leopoliensis

Abstract In the commercial production of Arthrospira platensis in outdoor environments, contamination by other microorganisms remains a major challenge. To exploit the high free ammonia (NH 3 ) tolerance of A. platensis , we investigated whether continuous NH₃ exposure could enable its selective cultivation by suppressing the growth of biological contaminants. The cyanobacterium Synechococcus leopoliensis , which exhibits relatively high NH₃ tolerance, albeit lower than that of A. platensis , was used as a model contaminant. The absolute half-maximal effective concentrations (EC 50 ) of NH 3 for A. platensis and S. leopoliensis in batch monocultures were 3.7 and 1.2 mmol L − 1 , respectively. In the subsequent semi-continuous co-culture of A. platensis and S. leopoliensis , the selective growth of A. platensis was achieved at an NH 3 concentration of 2.5 mmol L −1 , which was predicted to completely suppress the growth of S. leopoliensis . Although the inhibitory effect of NH 3 on A. platensis tended to be enhanced when co-cultured with S. leopoliensis compared to the monoculture, A. platensis continued to proliferate at a cell yield of 9.30×10 8 cells L -1 day -1 . Since the NH 3 tolerance of S. leopoliensis is comparable to or even higher than that of many other contaminants and grazers, these lab-scale results suggest the feasibility of the selective culture of A. platensis in the presence of biological contamination through appropriate control of NH 3 concentration.