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To determine whether circulation of dairy wastewater induces the growth of phototrophic purple sulfur bacteria (PSB).Two dairy wastewater lagoons that were similar in size, geographic location, number and type of cattle loading the lagoons were chosen. The only obvious visual difference between them was that one was stagnant and the water was brown in colour (Farm 1), and the other was circulated and the water was red in colour because of the presence of PSB that contained carotenoid pigments (Farm 2). Both wastewaters were sampled monthly for 3 months and assayed for PSB and extractable carotenoid pigments (ECP). After this point, circulators were placed in the wastewater lagoon on Farm 1, and samples were taken monthly for 9 months and assayed for PSB and ECP. Before the installation of circulators, no PSB-like 16S rRNA sequences or ECP were observed in the wastewater from Farm 1; however, both were observed in the wastewater from Farm 2. After the installation of circulators, statistically greater levels of PSB and extractable carotenoid pigments were observed in the wastewater from Farm 1.Circulation enhances the growth of PSB in dairy wastewater.Because PSB utilize H2S and volatile organic acids (VOA) as an electron source for photosynthesis, and VOA and alcohols as a carbon source for growth, the increase in these bacteria should reduce H2S, volatile organic compounds and alcohol emissions from the lagoons, enhancing the air quality in dairy farming areas.