Glyphosate Weedkiller Hits Algae Harder Than Its Active Ingredient Alone, Study Finds

Glyphosate is the most heavily deployed herbicide on the planet, sprayed across millions of hectares of farmland every growing season. Wherever it is applied, rain and irrigation eventually carry it into streams, rivers, and lakes, where it encounters organisms that were never the intended target. Among the most vulnerable of these unintended recipients are the microscopic green algae that form the base of aquatic food webs. A new study published in Environmental Science and Pollution Research by María Mercedes Iummato and colleagues in Argentina has now delivered one of the most detailed portraits yet of what happens inside a freshwater algal cell when glyphosate arrives, and the results carry a warning that goes beyond glyphosate itself: the commercial product is far more damaging than the pure active ingredient it contains.

The research team, drawn from the University of Buenos Aires and Argentina’s National Scientific and Technical Research Council (CONICET), exposed the green microalga Scenedesmus vacuolatus to two different treatments over a 96-hour period. One treatment used the isopropylamine salt of glyphosate, or IPAG, which is the active ingredient in many glyphosate-based products, at concentrations of 150, 250, and 350 milligrams per liter. The other used a commercial glyphosate-based formulation, referred to in the study as CFG, at concentrations of 4, 6, and 8 milligrams of acid equivalent per liter. This paired design allowed the researchers to separate the toxicity of glyphosate itself from the toxicity of the so-called inert ingredients that manufacturers blend into commercial products to improve their performance in the field.

The headline finding is stark. The concentration that suppressed algal growth by 50 percent, a standard toxicological benchmark known as the IC50, was 326.11 milligrams per liter for the pure active ingredient but only 4.43 milligrams of acid equivalent per liter for the commercial formulation. In other words, the formulated product achieved the same level of growth inhibition at roughly one-seventieth of the glyphosate concentration required by the active ingredient alone. That enormous gap strongly suggests that the co-formulants, the surfactants and adjuvants that help the herbicide stick to leaves and penetrate plant tissue, are doing much of the biological damage once the product reaches the water.

Growth was only the beginning. When Iummato’s team looked at photosynthetic pigments, they found that both treatments reduced total chlorophyll content relative to untreated controls, indicating that the herbicide interferes with the alga’s photosynthetic machinery. The commercial formulation added a further twist: exposed cells increased in volume. Cell enlargement without proportional growth in cell number is a classic sign of disrupted cell division, a pattern that has been documented in other algae exposed to herbicides and that points to an arrest of the reproductive cycle while individual cells continue to expand. For a phytoplankton population, that combination means the community can no longer replace itself at the rate the ecosystem depends on.

To understand the biochemical mechanisms behind these visible effects, the researchers measured a battery of molecular biomarkers that reveal the state of the cell’s antioxidant defenses and the extent of oxidative damage. When organisms are exposed to chemical stressors, their metabolism generates reactive oxygen species, unstable molecules that attack proteins, lipids, and DNA. Cells respond by ramping up antioxidant enzymes such as superoxide dismutase (SOD), which converts superoxide radicals into hydrogen peroxide, and catalase (CAT), which breaks the hydrogen peroxide down into water and oxygen. A third enzyme, glutathione-S-transferase (GST), helps detoxify harmful compounds by conjugating them with glutathione, a versatile cellular antioxidant. The levels and activities of these molecules serve as an early-warning system, registering stress long before populations collapse.

The two treatments told strikingly different biochemical stories. Algae exposed to the pure active ingredient, IPAG, responded with a coordinated defensive effort: levels of oxidized proteins rose, the ratio of carotene to chlorophyll a increased, glutathione content climbed, and the activities of SOD, CAT, and GST all went up compared with controls. This is the signature of a cell that has detected the threat and mobilized its full antioxidant arsenal against it. The elevated carotene-to-chlorophyll ratio suggests the cells were also reinforcing their pigment shields, since carotenoids quench reactive oxygen species generated in the photosynthetic membranes and protect chlorophyll from photo-oxidative destruction.

The commercial formulation produced a more alarming pattern. Exposed algae showed increased thiobarbituric acid reactive substances, or TBARS, a widely used indicator of lipid peroxidation, meaning the herbicide was literally damaging the fatty membranes that enclose the cell and its organelles. Oxidized proteins accumulated, the carotene-to-chlorophyll ratio rose, glutathione and carotene levels increased, but here the antioxidant enzymes told the opposite story: SOD, CAT, and GST activities all decreased relative to controls. The combination of rising oxidative damage markers and falling antioxidant enzyme activity suggests that the formulation did not merely provoke a defensive response but overwhelmed or impaired it. When the enzymatic defenses falter while lipid peroxidation accelerates, the cell is losing the oxidative battle.

This contrast between the active ingredient and the formulated product is the study’s most consequential message for environmental regulation. Glyphosate risk assessments have traditionally focused on the active ingredient, treating co-formulants as biologically inert helpers. A growing body of ecotoxicological work, including comparative studies of pesticides on aquatic and soil organisms, has challenged that assumption, and the present findings add algal primary producers to the list of victims. Because algae sit at the foundation of freshwater food webs, suppressing their growth and photosynthesis can ripple upward to zooplankton, fish, and everything that depends on them. The finding that a commercial formulation can be toxic at concentrations tens of times lower than the active ingredient means that water quality criteria based on glyphosate alone may dramatically underestimate the ecological risk of real-world herbicide applications.

The study also carries geographic weight. Glyphosate use is intensive in the agricultural regions of Argentina, particularly the soy-growing Pampas, and monitoring programs have repeatedly detected the herbicide and its degradation product aminomethylphosphonic acid, or AMPA, in surface waters and groundwater across the region. Scenedesmus vacuolatus, the species used in these experiments, is a common freshwater chlorophyte and a standard test organism in algal growth inhibition assays conducted under international testing guidelines, which makes the results directly relevant to regulatory frameworks. The researchers note that both the formulation and the active ingredient negatively affected the alga, altering growth and metabolic parameters including oxidative stress markers, with the commercial formulation causing greater alterations at lower glyphosate concentrations.

What emerges from this multi-biomarker approach is a picture of glyphosate-based herbicides as more than simple weed killers acting on a single plant-specific enzyme. In these algal cells, exposure triggered cascades of oxidative stress, pigment remodeling, protein oxidation, and membrane damage, with the formulated product inflicting a heavier and less recoverable blow than its active ingredient alone. As debates over glyphosate’s environmental fate continue worldwide, this study adds a technically rigorous data point to the argument that the toxicity of a pesticide product cannot be judged by its active ingredient in isolation. The co-formulants that make the spray effective in the field may be the same compounds that quietly undermine the microscopic engines of aquatic ecosystems, and regulators, the authors’ findings imply, would do well to evaluate the products as they are actually sold and sprayed, not just the molecule on the label.

Subject of Research: Toxic effects of a glyphosate-based commercial formulation and its active ingredient on the green microalga Scenedesmus vacuolatus

Article Title: Multi-biomarker evaluation of the effects of a glyphosate-based commercial formulation and its active ingredient on the green microalga Scenedesmus vacuolatus

Article References: Iummato, M. M., Graziano, M., Ríos de Molina, M. D. C., & Juárez, Á. B. (2026). Multi-biomarker evaluation of the effects of a glyphosate-based commercial formulation and its active ingredient on the green microalga Scenedesmus vacuolatus. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38265-y

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38265-y

Keywords: glyphosate, herbicide, Scenedesmus vacuolatus, microalgae, ecotoxicology, oxidative stress, antioxidant enzymes, biomarkers, commercial formulation, co-formulants, freshwater, lipid peroxidation