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Study on effects of glyphosate on microbiomes.
The Soil Physics and Land Management Group (SLM) of Wageningen University (Prof. Coen Ritsema and Prof. Violette Geissen), together with the Emerging Pathogens Institute of the University of Florida (Prof. Ariena van Bruggen), the Department of Ecological Plant Protection of the University of Kassel (Prof. Maria Finckh) and the Department of Life and Environment Science of Hangzhou Normal University in Zhejiang (Prof. Miaomiao He) have published a remarkable review article. This article on the effects of glyphosate, the active ingredient in Roundup, on microbial communities and plant, animal and human health is timely: this year, the European Commission (EC) will decide on the possible extension of glyphosate's approval as a herbicide in the EU.
In 2017, glyphosate was re-approved for five years in the EU because the European Food Safety Authority (EFSA) and the EC judged the substance to be safe for humans and animals. However, in 2015, the World Health Organization's (WHO) International Agency for Research on Cancer (IARC) determined that there was sufficient evidence that glyphosate causes cancer in laboratory animals and is likely to cause cancer in humans. Since that time, much scientific research has been conducted on the effects of glyphosate on animal health and, indirectly, human health. The company that developed glyphosate- and Roundup-resistant crops (Monsanto, now owned by Bayer Crop Science) has lost a number of lawsuits in the U.S. against people who developed non-Hodgkin's lymphoma (cancer of the lymphatic system) after long-term heavy exposure to glyphosate.
However, the current work by van Bruggen et al. is not about the direct health problems associated with glyphosate, but rather the potential indirect problems via the "microbiome." Soil, plant roots and spaces between plant cells, the surfaces and internal cavities of animals and humans (such as the gut) all have characteristic "microbiomes." These consist of bacteria, fungi, and all kinds of microscopic animals. The composition of these microorganisms has evolved evolutionarily over the centuries to achieve optimal cooperation between the "microbiome" and the host (macroorganisms). These specialized "microbiomes" largely determine disease resistance and normal development of the macroorganisms. Of course, the ultimate microbiome of any macroorganism (e.g., human) also depends on diet, drug use, and exposure to contaminants.
To control weeds, approximately 0.11 g of active ingredient is applied per m2 (1080 g/ha). This is sufficient to kill non-resistant plants. The application concentration in liquid is (at 200 l/ha) 5.4 mg/ml, which increases rapidly as it dries on the leaf. Now, the reality is that glyphosate is not only lethal to plants (with the exception of Roundup-resistant genetically engineered plants), but also to many microorganisms, so the natural composition of the microbiome is altered by exposure to concentrations of about 0.5-50 mg/ml glyphosate, the concentrations that are applied to leaf surfaces.
The current article provides an overview of residues in soil, water, and all types of food for animals and humans. Measured residues are compared to maximum allowable residues; the latter levels are regularly increased because measured concentrations may no longer meet standards due to the accumulation of glyphosate in the environment. Concentrations are particularly high in animal feed (up to 0.530 mg/g in grass). Glyphosate is indeed largely excreted in the stool and urine after partial absorption through the intestine and circulation through the blood. However, glyphosate also accumulates in bone, liver, and kidney and persists in the intestine (tested in rats). Humans come into contact with it through ground byproducts from slaughtered animals, but also through plant products that have been sprayed with glyphosate. Until recently, the effects of relatively low residue concentrations on microbiomes in soil, on and in plants, and in animals appeared inconclusive in the scientific literature. Some researchers found "no effect," while others found clear negative effects on specific microorganisms.
The authors of the review article noted that the results of the study depend on how detailed DNA data are analyzed. When only orders, classes and families of organisms or biomass and diversity are considered, no effect is usually found. However, when genera and species or specific processes are considered, effects become apparent. For example, nitrogen fixation by bacteria in legumes was affected. This is also true for the development of the immune system in bees, which subsequently became more sensitive to a parasite and a virus. The development of the nervous system of baby rats was also impaired and the rats showed ADHD-like symptoms. Immune and nervous system development is controlled in part by the gut microbiome. The review article also provides a long list of minimum inhibitory concentrations (MICs) for all types of bacteria.
Until recently, there was scientific "bickering" about these numbers. Differences would be due to incubation conditions with or without oxygen. The authors of the review show that the differences are due to the method used to measure bacterial concentration rather than the presence or absence of oxygen. This means that the very low concentrations previously thought to be harmful to benign or even beneficial bacteria are correct. In contrast, many pathogenic bacteria can resist higher concentrations of glyphosate. Recent DNA research has shown that up to 26% of bacteria in the human gut are sensitive to glyphosate. Thus, a shift in the microbiome may occur. The authors therefore conclude that tolerable residues in human and animal diets should be lowered to prevent or at least minimize damage to the microbiome and thus the host. Glyphosate residues in soil result not only from spraying crops with glyphosate, but also from the application of manure from animals that have been fed glyphosate-resistant feed containing high levels of residues through the international feed market. Currently, concentrations of glyphosate and the first degradation product (which is also toxic) in soil and surface and groundwater are already so high (due in part to slow total degradation) that there are concerns for future drinking water health . In addition, normal crops are less likely to thrive in soils that have been fertilized with glyphosate-containing fertilizers, putting circular agriculture at risk in the long run. The authors therefore argue that all these factors should be seriously considered before a decision is made on whether or not to allow glyphosate in the (near) future.
Text: Prof. Dr. Maria R. Finckh
Prof. Dr. Maria R. Finckh
Department of Organic Plant Protection