Sethoxydim is a commercially available herbicide targeting acetyl-CoA carboxylase, the catalyst for the rate-limiting step of lipid synthesis, in annual and perennial grass weeds. When applied in the appropriate concentration, sethoxydim binds to the enzyme preventing carboxyl transfer and halting lipid synthesis, and thus killing the plant. Diluted sethoxydim is generally applied in a field or landscaped area where it can be absorbed through the soil. In water, however, sethoxydim treatment tells a different story. With a half-life of one hour, sethoxydim rapidly degrades in water to form at least seven intermediate products, which have been characterized within the last five years. It has also been proposed that in the presence of UV radiation from sunlight, a radical intermediate is formed that facilitates this breakdown. Despite this phenomenon, the impact of high concentrations of sethoxydim and the resulting intermediates has not been studied. We have conducted studies on the impact of sethoxydim-based herbicide on Chlorella vulgaris, a unicellular green microalga found in streams that has the potential to be affected by herbicide runoff. Using a concentration of 300 µM, we have observed a complete shutdown of oxygen production indicative of photosynthetic damage within 10 min of exposure. More interesting, however, is that simultaneous pulse amplitude modulated fluorescence measurements have shown that the protein is still partly functional; the Qa and Qb site appear to be intact while the oxygen evolving complex is either removed or otherwise inhibited. Furthermore, the OJIP transient fluorescence curve exhibits a ‘K-step,’ a phenomenon that has been previously shown to indicate heat shock. Lastly, using biochemical assays as well as microscopy, we have found evidence that under large concentrations of sethoxydim-based herbicide, C. vulgaris undergoes a process reminiscent of programmed cell death as seen in Chlorella saccharophila under heat stress.

Characterizing the effects of sethoxydim on the photosynthetic activity and biochemical composition of Chlorella vulgaris

255th National Meeting (2018)

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