Microbial performance impacted by surfactants from glyphosate application

Informations projet

MICROSURF

N° de convention de subvention: 101075581

DOI

10.3030/101075581

Date de signature de la CE 31 Janvier 2023

Date de début 1 Septembre 2023

Date de fin 31 Août 2028

Financé au titre de

• European Research Council (ERC)

Coût total

€ 1 499 375,00

Contribution de l’UE

€ 1 499 375,00

Coordonné par

UNIVERSITY OF STUTTGART
Germany

Periodic Reporting for period 1 - MICROSURF (Microbial performance impacted by surfactants from glyphosate application)

Période du rapport: 2023-09-01 au 2026-02-28

Résumé du contexte et des objectifs généraux du projet

Glyphosate is the most applied herbicide worldwide with approximately 700,000 tons applied globally per year. While also used in other sectors, such as by railway operators to remove weeds from railroad tracks, the agricultural sector accounts for the largest share of its application. The rise of glyphosate in agriculture is due to the development of glyphosate-resistant genetically modified crops, which allows weed control during crop growth. However, despite increased crop yields and reduced field maintenance costs, research has shown that glyphosate may exert toxic effects on a range of organisms, including humans, animals, and soil microorganisms. Additionally, glyphosate can persist in agricultural soils and may be further transported into rivers and lakes where it can remain detectable for hundreds of days.

Glyphosate is typically not applied as a pure substance but rather as a mixture containing glyphosate as the active ingredient along with various other compounds. This mixture is commonly referred to as a formulation or glyphosate-based herbicide (GBH). Most of these additional compounds are surfactants with the composition of GBHs typically consisting of 40-60% of glyphosate, 5-15% of surfactants, and the remainder being water. Surfactants facilitate the spreading and penetration of glyphosate into plant tissues, leading to more effective weed control by GBHs compared to pure glyphosate.

While surfactants are considered non-active ingredients in GBHs, previous studies have shown that surfactants are not inert, and they may themselves exhibit toxic effects. For example, a type of surfactant called polyethoxylated tallow amines has been phased out due to its toxicity. Additionally, research in marine environments has shown that surfactants applied to clean up oil spills can reduce the ability of certain microorganisms to degrade the oil. However, the effects of surfactants on microorganisms inhabiting other environments, such as agricultural soil, remain largely unknown.

The main goal of this project is to provide a better understanding of the effects of surfactants used in GBHs on microorganisms. This is important for several reasons. First, some microorganisms are capable of degrading glyphosate. Second, microorganisms play a key role in maintaining soil health, for example, by driving the cycling of vital nutrients, i.e. carbon and nitrogen. Third, these cycles also impact greenhouse gas (GHG) emissions as soil microbes can act as a source and a sink of GHGs depending on the environmental conditions. However, it is currently unclear whether surfactants stimulate or inhibit these processes.

The findings of this project will be of great importance for stakeholders involved in decision-making regarding the future use of glyphosate. They will also provide guidance on whether greater attention should be given to the application of surfactants in the context of sustainable agriculture objectives.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

We conducted several experiments addressing different aspects of our main objectives of understanding the effects of surfactants in GBHs on microorganisms. These included studies on the effects on both individual microorganisms and complex communities through the examination of carbon and nitrogen cycles (including GHG formation) and glyphosate biodegradation.

Surfactant effects on individual microorganisms:
We studied two glyphosate-degrading strains: Ochrobactrum pituisoum (strain GPr1 13) and Achromobacter insolitus (strain Kg 19). For strain GPr1 13, microbial responses depended on the type of GBH used. High concentrations of one formulation (termed “GBH1”) led to a reduced growth of strain GPr1 13 compared to a second formulation (termed “GBH2”), which increased growth. This suggests that the composition of GBHs, particularly the types of surfactants used, may have opposing effects on microbial growth. For strain Kg 19, growth rates were similar when consuming either pure glyphosate or GBH1, suggesting that the surfactants in GBH1 neither stimulated nor inhibited this strain. In addition, we tested the growth response of strain Kg 19 to pelargonic acid (PA), a surfactant present in some GBHs and also used as a glyphosate-free alternative to GBHs. Low concentrations of PA stimulated the growth similar to GBHs or pure glyphosate, but higher concentrations of PA inhibited growth of this strain.

Surfactant effects on carbon and nitrogen cycling:
To assess surfactant impacts on microbial carbon cycling in soils, we established lab-scale experiments, so-called microcosms, using agricultural soils under conditions simulating those of agricultural fields. Two types of microcosms were set up: oxic (oxygen-rich) and anoxic (oxygen-depleted) to simulate both top soil and deeper or waterlogged soil layers. Treatments included either pure glyphosate, a GBH, or PA. In the oxic microcosms, we observed an increase in the activity of multiple genes involved in carbon cycling, suggesting that microorganisms either utilized these compounds as nutrients or that the compounds generally stimulated microbial carbon cycling capacity. In the anoxic microcosms, PA increased methane production, a more potent GHG than carbon dioxide. GBH and pure glyphosate treatments also showed methane production, but at substantially lower levels compared to the PA treatments. These findings highlight the potential negative effects that surfactants may have on carbon cycling under different soil oxygen conditions.

Surfactant effects on glyphosate biodegradation in a complex community:
Microcosm data was furthermore used to assess whether surfactants in GBHs affect glyphosate degradation rates. In the oxic microcosms, glyphosate degradation rates did not differ between the pure glyphosate and GBH treatments. However, in the anoxic microcosms, GBHs increased glyphosate biodegradation rates compared to those measured in pure glyphosate treatments. The difference between oxic and anoxic conditions may be explained by the fact that microorganisms typically exhibit faster metabolisms in the presence of oxygen, allowing them to degrade glyphosate regardless of the treatment. However, in the anoxic microcosms, surfactants probably reduced glyphosate sorption to soil particles and increased its bioavailability, potentially along with other nutrients, for microbial communities. This enhanced availability might be sufficient to accelerate the metabolism of microorganisms under anoxic conditions.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

The major scientific breakthrough achieved in this project is the evidence suggesting that microorganisms may be capable of degrading glyphosate under anoxic conditions. While previous studies have indicated that this process might occur, specific genetic mechanisms have not yet been described. Ongoing work aims to identify the potential genes involved in this process. Additionally, our results indicate that surfactants in GBHs may improve glyphosate degradation under anoxic conditions, highlighting an important role that surfactants may exert in soils.

Our findings have important implications for regulatory and practical aspects. They may inform discussions on whether surfactants should be labelled as inert ingredient in GBHs. Additionally, the results may provide guidance to farmers regarding the use of PA and/or other surfactants, including recommended concentrations to minimize negative effects for soil health.

Schematic of the experimental design of the oxic and anoxic microcosms