Nutrients and pH regulate algicide accumulation in cultures of the cyanobacterium Oscillatoria laetevirens

• The change in algicide synthesis in response to nutrients and pH in axenic cultures of the planktonic cyanobacterium, Oscillatoria laetevirens is reported. • Analytical and bioassay techniques were used to determine the effects of nitrogen (N), sulpher (S), phosphorus (P), magnesium (Mg²⁺ ), calcium (Ca²⁺ ) and hydrogen (H⁺ ) on Oscillatoria laetevirens growth, secondary metabolite accumulation and minimum inhibitory concentration in culture filtrates. • Growth and secondary metabolite production were affected by nutrient addition. High algicide production, which was not dependent on optimal growth, correlated with deficiences of magnesium and phosphate, and high pH. Algicidal activity in the culture medium was not detected following organic nitrogen addition while optimal growth was sustained; however, ammonia reduced biomass yield but did not affect algicide formation. The differential algicide concentration inside the cells and in the culture medium indicated secretion by an efflux mechanism rather than leakage from aging cells. • Algicide production by Oscillatoria laetevirens was negatively regulated by both phosphate and magnesium, and by pH. The interactive influence of nutrients and allelopathy is discussed in species succession and dominance in the maintenance of freshwater blooms.