Free ammonia tolerance of cyanobacteria depends on intracellular pH

Algae-based wastewater treatment can remove nutrients from wastewater and simultaneously produce valuable products. However, inhibition of microalgae by NH3 in the wastewater poses a challenge to this process. It is desirable to understand the mechanism of inhibition by NH3 for physiological characterization of microalgae and their application. NH3 tolerance, which varies among species, is assumed to depend on intracellular pH. To verify the adequacy of the assumption, in this study, the 50% inhibitory concentration (EC50) of NH3 for four cyanobacterial species was evaluated as an indicator of NH3 tolerance and compared to intracellular pH. The EC50 of NH3 was 4.95, 5.05, 4.38, and 0.74 mM and the intracellular pH was 8.0, 8.4, 8.2, and 7.3 for Arthrospira platensis, Arthrospira fusiformis, Synechococcus leopoliensis, and Anabaena cylindrica, respectively. A significant correlation was obtained between intracellular pH and EC50. Analysis of the chlorophyll fluorescence of cells exposed to NH3 for 1.5 h showed that photochemical quenching was reduced rather than Fv'/Fm' under NH3 concentrations lower than EC50, suggesting that the electron transfer system downstream of photosystem II (PSII) was more sensitive than PSII itself. Because several sites downstream of PSII utilize H+, including ATP synthase, the inhibition may be caused as a result of H+ consumption by NH3 diffused inside the cells. As consumption of H+ in NH3 ionization is suppressed under high pH, species with higher intracellular pH probably have higher NH3 tolerance. These results should help understand the free NH3 tolerance of cyanobacteria.