Atrazine tolerance of algae isolated from two agricultural streams

Based on photosynthesis (oxygen evolution) EC50s determined from short (5 min) assays, green algae (Chlorella sp. and other species of Chlorococcales) isolated at random from an atrazine‒contaminated stream (Big Spring) in midsummer exhibited similar tolerances to atrazine (range of EC50s: 42–125 μg/L) compared to algae isolated at random from a noncontaminated stream (Osage Spring) (range of EC50s: 35–162 μg/L). A laboratory strain of Chlamydomonas reinhardii known to be susceptible to atrazine responded similarly (EC50: 45 μg/L). The EC50s of these algae are similar to those previously reported for laboratory strains. In contrast, an alga (Franceia sp., Chlorococcales) isolated at random during late spring from Big Spring responded similarly to a laboratory strain of Chlamydomonas reinhardii that is resistant to atrazine (Franceia sp. EC50s: 430–774 μg/L, C. reinhardii EC50: 466 μg/L). The EC50 values of these algae are near the high end of the range of EC50s reported for green algae.