Free ammonia inhibition of algal photosynthesis in intensive cultures

The effect of free NH(3) inhibition on short-term photosynthesis was investigated in three microalgal species: the freshwater chlorophyte Scenedesmus obliquus, the marine diatom Phaeodactylum tricornutum and the marine chlorophyte Dunaliella tertiolecta. By performing a series of assays at various concentrations of added NH(4)Cl and culture pH, we demonstrated that the inhibitory compound was free NH(3) and that pH played no role in determining the magnitude of inhibition, other than in establishing the degree of dissociation of nontoxic NH(4) to toxic NH(3). When corrections were made for pH, all three species displayed the same sigmoidal response curve to free NH(3) concentration; 1.2 mM NH(3) led to 50% reduction in photoassimilation of C. Based on literature values, some marine phytoplankton appear to be significantly more sensitive to free NH(3) than were the test species, which are noted for their excellent growth characteristics. However, the combination of low algal biomass and strong pH buffering commonly found in most marine and many freshwater environments probably limits the possibilities for NH(3) toxicity to low alkalinity freshwaters and intensive algal cultures in which NH(4) is the main source of N. Such conditions occur commonly in algal wastewater treatment systems.