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 axeni c cultures of the planktonic cyanobacterium, Oscillatoria laetevirens is re ported. . 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 addi tion. High algicide production, which was not dependent on optimal growth, correlated with deficiences of magnesium and phosphate, and high pH. Algici dal activity in the culture medium was not detected following organic nitro gen addition while optimal growth was sustained; however, ammonia reduced b iomass yield but did not affect algicide formation. The differential algici de concentration inside the cells and in the culture medium indicated secre tion 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 nu trients and allelopathy is discussed in species succession and dominance in the maintenance of freshwater blooms.