Mercurio, P., Flores, F., Mueller, J. F., Carter, S., & Negri, A. P. (2014). Glyphosate persistence in seawater. Marine pollution bulletin. DOI: 10.1016/j.marpolbul.2014.01.021
What is glyphosate?
Glyphosate is a form of broad spectrum herbicide commonly found in “Roundup” which was brought into popularity in the U.S. during the 1970’s by Monsanto. Glyphosate kills plants via contact with foliage or roots; in small doses, it can control growth.
Glyphosate reached greater popularity when Monsanto released glyphosate-resistant crops. Farmers could now apply the herbicide broadly to their fields without harming crops. Glyphosate is now an integral part of worldwide agriculture used in massive quantities annually. With such a long history of use, it is no wonder that several glyphosate-resistant weeds have developed under agricultural selection pressure. In 1996, the first resistant weed was reported in Australia followed by many others in Canada and the U.S.
The Environmental Protection Agency (EPA) states that glyphosate has relatively low dermal and oral toxicity. Studies with rats indicate that it does not bioaccumulate in the food chain and there is insufficient evidence to suggest glyphosate causes cancer. However, rats directly exposed to the herbicide did experience blood, liver, and pancreatic problems. In high doses, glyphosate may also act as an endocrine disruptor. Dietary studies used in the EPA assessment do not indicate significant risks from consuming plants exposed to glyphosate at current levels of usage. A 2002 study commissioned by the European Union reached the same conclusions.
The half-life (time needed for the initial concentration to degrade by half) of glyphosate in soil or freshwater can vary from just a few days to nearly 200 days (Table 1). Glyphosate is broken down fairly quickly to a major metabolite aminophosphonic acid (AMPA) through organic processes. However, it is also applied in great quantities during the summer season when heavy precipitation washes much of the herbicide into coastal waters. Water quality in near shore waters is of particular concern for sensitive corals and seagrass in the Great Barrier Reef (GBR) system. Glyphosate is the mostly widely used herbicide in the Australia with about 15,000 tons of glyphosate applied annually.
Researchers from Australia set out to quantify how long it takes for glyphosate to biodegrade in the Great Barrier Reef (GBR) system. This is the first study to measure the persistence of glyphosate in seawater.
Glyphosate degradation was measured in a flask “simulation tests” over the course of 330 days. Tests used natural seawater containing native bacteria present in the Great Barrier Reef (GBR). No nutients or food were added to the flasks. Three treatments were used to mimic conditions in the GBR lagoon: (1) 25°C in the dark (mean annual seawater temperature in the GBR), (2) 25°C in low light, and (3) 31°C in dark (maximum summer temperature for near-shore waters in the GBR).
The half-lives of glyphosate in each treatment were calculated using the slope of the data for the natural log of glyphosate concentration over time.
Results and Significance
Half-lives were much greater for both dark treatments than the light treatment. Although the light treatment was comparable to other freshwater studies (Figure 2; Table 1), the dark treatments at 25°C and 31°C (267 days and 315 days) persisted much longer than previous reports. These results suggest that glyphosate is highly persistent in dark conditions with a modest temperature effect between the two treatments.
Although the methods used in this study mimicked the natural environment, the authors state more work is needed to determine details of glyphosate’s fate in the marine environment. For example, glyphosate is known to bind to organic material which suggest glyphosate would stay near to shore. However, this binding potential also prevents degradation and may translate to very long distance transport.
Glyphosate is not incorporated in most marine monitoring programs. This is surprising given that is one of the most widely used herbicides in the world. Other recent work also suggests that the toxicity of glyphosate is tied to the surfactants it is often combined with in commercial products such as “Roundup”. Further work is needed to examine glyphosate’s potential in the marine environment.
The authors also measured the metabolite product of glyphosate degradation aminophosphonic acid (AMPA) in the flasks. As glyphosate was metabolized by bacteria, the concentrations of AMPA increased (Mercurio et al. 2014, Figure 1).