Differential responses of Type I and Type II methanotrophs to nonanoic (pelargonic) acid and glyphosate

Glyphosate (GLY) is the most widely used herbicide worldwide. However, in organic and conventional farming systems, a natural alternative — nonanoic (pelargonic) acid (PA) — is gaining importance. PA is a non-selective herbicide whose mode of action is through disruption of the cellular membranes. PA is also added to commercial glyphosate formulations as surfactant, thereby increasing the effectiveness of this herbicide. Through agricultural application, both compounds enter the soil environment, which serves as a habitat for methanotrophic bacteria (MB). Methane (CH4) contributes to approximately 20% of the greenhouse effect, and by limiting its net emissions from the environment, MB play a crucial role in mitigating global warming. Therefore, potential disruption of MB activity by herbicides is of great concern. This is the first study to examine the effects of PA, GLY, and their combinations on MB. We demonstrate the negative impact of field-relevant concentration of PA (but not GLY) on Methylomonas methanica and methanotrophic soil enrichment cultures (ECs) dominated by Type I (γ-proteobacteria). Type II (α-proteobacteria) MB were more resilient to PA exposure than Type I MB. The underlying mechanism of this differential response was the variable susceptibility of cellular membranes to disintegration by PA.