Glyphosate may have dual effect on bloom algae as a phosphorus source or pesticide. The physiological and biochemical responses of Microcystis aeruginosa (M. aeruginosa) to glyphosate and its formulation in the common herbicide, Roundup((R)), were compared. The result suggested that both the cell numbers and Chl-a content of M. aeruginosa increased when the glyphosate concentration increased from 0.01 to 5 mg P L-1. However, Roundup((R)) showed low-dose (below I mg P L-1) stimulation and high-dose (above 1 mg P L-1) inhibition on M. aeruginosa cell density and Chl-a content (hormesis effect). Phosphate was more available than glyphosate or Roundup((R)), and Roundup((R)) was more toxic than glyphosate itself at 3 mg P L-1. Analysis of the maximum yield of PSII indicated that glyphosate stimulated the photosynthesis process while Roundup((R)) inhibited the photosynthesis of M. aeruginosa. The photosynthesis process was enhanced on the 21st day compared with that on the 14th day in all P mediums. The extracellular alkaline phosphatase activity (AAA) decreased with the increasing glyphosate or Roundup((R)) concentration. The change pattern of APA was similar in both the glyphosate and Roundup((R)) mediums. (C) 2013 Elsevier B.V. All rights reserved.