Chemical Structure Controls Soil Biotransformation Kinetics of Bacillus Lipopeptide Biopesticides

Bacillus lipopeptides (surfactins, iturins, and fengycins) are an emerging class of biopesticides. However, their fate in agricultural soils, and specifically how variations in the chemical structure such as alkyl chain length, amino acid sequence, and cyclization affect biotransformation, remains poorly understood. Here, we assessed biotransformation kinetics and pathways of a representative set of lipopeptides by coupling laboratory soil incubation experiments with HPLC-HRMS analyses. Half-lives varied strongly across lipopeptide families, with fastest degradation for iturins (1-8 d), followed by fengycins (5-15 d) and surfactins (6-67 d). In one of the three tested soils, surfactins exhibited biphasic removal with a substantial fraction (i.e., ∼21% of the spiked amount) remaining after >100 days of incubation, indicating formation of a poorly bioavailable fraction. Additionally, our results revealed a strong effect of the alkyl chain length for certain lipopeptides, with a half-life increase per additional CH2 unit up to ∼45% for iturins and ∼110% for surfactins. Lastly, we report that linearized surfactins were transformed substantially faster than cyclic surfactins. Together, the reported links between chemical structure and soil biotransformation kinetics can inform the selection, and possibly also the (re)design, of lipopeptide biopesticides for improved biodegradability in agricultural soils.