Photosynthetic growth of 9 of the 11 known species of the family Rhodospirillaceae was strongly inhibited by atrazine. Growth of a wild-type strain of the bacterium Rhodopseudomonas sphaeroides van Niel was almost completely inhibited by 100 μM concentrations of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. These herbicides gave 50% inhibition of growth at concentrations of 35 and 25 μM, respectively. Through continuous culture of the wild-type R. sphaeroides in the presence of 100 μM atrazine, strains of the organism were selected in which photosynthetic growth was highly resistant to both atrazine and diuron. Members of the genus Rhodopseudomonas isolated from two natural ecosystems showed various levels of resistance to 100 μM atrazine, but Rhodopseudomonas showing a high level of resistance were more common among the isolates from a pond-mud sample which had been exposed to runoff from atrazine-treated cropland than from a sample of a lake mud never exposed to triazines.
