Effects of a Sethoxydim‐based Herbicide on the Photosynthetic Capacity of Chlorella vulgaris, a Non‐target Organism
Authors: Smythers Amanda L. , Garmany Armin , Perry Nicole , Adkins Paris Ethan , Higginbotham Ethan , Kolling Derrick R.J. 
Published: June 2021

Herbicides are widely used in agriculture to promote the growth of target plants through the inhibition of competitors. These herbicides typically rely on compounds that target specific proteins or organelles in the undesired organisms and are commonly mixed with carriers, wetting agents, or spreaders to make commercial formulations. These formulations are optimized to make handling easier, improve the performance of the active ingredient, and to minimize the adverse effects on non‐target organisms. However, the reactivity of the herbicide formulation has the potential to be vastly different than that of the active ingredient without additives, causing unforeseen behaviors and interactions when introduced into the environment. These interactions can result in an increased toxicity as well as a decrease in species specificity that can be difficult to predict.[1]

In our lab, we have been using sethoxydim, an acetyl‐CoA carboxylase inhibitor that targets broadleaf grasses, as a selective stressor to induce increased expression of ACCase.[2]Instead, we have found that the commercial formulation causes catastrophic damage to the photosynthetic machinery and membrane integrity of Chlorella vulgaris, a unicellular green alga. Using a Clark‐type oxygen electrode, we have found that less than 0.1 % v/v of the herbicide disables the oxygen evolution complex of photosystem II within minutes of exposure, completely inhibiting all oxygen production. Simultaneously, by assessing the fast fluorescence transient, we have found the integral portion of the photosystem to remain intact, showing markers characteristic of a ‘K step,’ a phenomenon commonly shown in heat‐stressed plant cells that indicates damage of the OEC.[3]Following the OEC damage, we have observed over 70 % loss in cell viability after 30 min of herbicide exposure, followed by a degradation of pigments beginning at 4 h and a subsequent degradation of other biochemical components, which we have monitored in 12 h increments.

Although the herbicide formulation causes this devastating damage, the sethoxydim alone has no effect on the photosystem. We have also tested other components of the herbicide individually, including mineral spirits, naphthalene, and a detergent similar to the proprietary detergent found in the formulation – and found no effect. Our experiments suggest that there may be a synergistic effect of all the ingredients together that has caused Poast™ to be a potentially non‐specific and highly toxic herbicide that may cause catastrophic effects to aquatic ecosystems in the case of run‐off or spillage from agricultural industries.

Support or Funding Information

This material is based upon work supported by the National Science Foundation under Cooperative Agreement No. OIA‐1458952.

This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.