Biodegradation of Pesticides at the Limit: Kinetics and Microbial Substrate Use at Low Concentrations

Abstract

The objective of our study was to test whether limited microbial degradation at low pesticide concentrations could explain the discrepancy between overall degradability demonstrated in laboratory tests and their actual persistence in the environment. Studies on pesticide degradation are often performed using unrealistically high application rates seldom found in natural environments. Nevertheless, biodegradation rates determined for higher pesticide doses cannot necessarily be extrapolated to lower concentrations. In this context, we wanted to (i) compare the kinetics of pesticide degradation at different concentrations in arable land and (ii) clarify whether there is a concentration threshold below which the expression of the functional genes involved in the degradation pathway is inhibited without further pesticide degradation taking place. We set up an incubation experiment for four weeks using 14C-ring labeled 2-methyl-4chlorophenoxyacetic acid (MCPA) as a model compound in concentrations from 30 to 20,000 µg kg −1 soil. To quantify the abundance of putative microorganisms involved in MCPA degradation and their degradation activity, tfdA gene copy numbers (DNA) and transcripts (mRNA) were determined by quantitative real-time PCR. Mineralization dynamics of MCPA derived-C were analyzed by monitoring 14CO 2 production and 14C assimilation by soil microorganisms. We identified two different concentration thresholds for growth and activity with respect to MCPA degradation using tfdA gene and mRNA transcript abundance as growth and activity indices, respectively. The tfdA gene expression started to increase between 1,000 and 5,000 µg MCPA kg −1 dry soil, but an actual increase in tfdA sequences could only be determined at a concentration of 20,000 µg. Accordingly, we observed a clear shift from catabolic to anabolic utilization of MCPA-derived C in the concentration range of 1,000 to 5,000 µg kg −1. Concentrations ≥1,000 µg kg −1 were mainly associated with delayed mineralization, while concentrations ≤1,000 µg kg −1 showed rapid absolute dissipation. The persistence of pesticides at low concentrations cannot, therefore,

Enantiomer-Selective Characterization of the Adsorption, Dissipation, and Phytotoxicity of the Plant Monoterpene Pulegone in Soils

“…Accordingly, a three parameter-sigmoidal equation (Equation (3)) accurately described the experimental dissipation data (R 2 > 0.993) and was used to calculate the time required for the concentration of R- or S-pulegone to decrease to half of their initial concentration (DT 50 ) ( Figure 3 , Supplementary Materials, Table S2 ). Sigmoidal dissipation patterns are typical of compounds at relatively high concentrations in soil that are utilized as energy and carbon sources by soil microorganisms [ 44 ].…”

Spatial Control of Microbial Pesticide Degradation in Soil: A Model-Based Scenario Analysis

“…[16][17][18][19][20][21]23 Additionally, MCPA degradation kinetics were recently analyzed in the topsoil of the reference soil. 37 We simulated the recommended MCPA application rate (2 kg/ha). MCPA was subject to reactive transport in the soil column and only dissipated by biodegradation.…”

“…Measured tfdA gene abundance in the reference soil 37 was converted into microbial biomass B [μmol C/kg] using the conversion factor f m/g [μmol C/gene] as calibrated by Chavez Rodriguez et al 39 Microbes were considered to be immobile, and their distributions were assigned using COMSOL's Domain ODEs and ADEs module. Initial and Boundary Conditions.…”

“…where B TS [μmol C/kg] is the average tfdA gene abundance in the topsoil of the reference soil (1.11 × 10 8 genes/kg soil, 37 converted with the factor f m/g [μmol C/gene]), taken as proxy for the degrader abundance. Note that the depth y is 0 m at the surface and is taken positively downward.…”

Biodegradation Kinetics of Fragrances, Plasticizers, UV Filters, and PAHs in a Mixture─Changing Test Concentrations over 5 Orders of Magnitude

“…It is an important fate process for many chemicals and biodegradation data are, therefore, needed for the risk assessment and regulation of chemicals. Many formats exist for biodegradation tests differing in complexity, ranging from standardized screening tests for ready biodegradability, simulation tests for biodegradation in different environmental compartments, , to non-standardized tests used for specific research purposes or difficult-to-test chemicals. − Interest in the differences between standard tests and the more complex conditions in the field is not new , but has increased lately. − The environment is characterized by the presence of complex microbial communities, multiple substrates, and xenobiotics at low concentrations. Although, the relationship between the concentration of a single substrate and growth of a degrader population has long been described, much less is known about the concentration effect on biodegradation kinetics for mixtures at very low chemical concentrations in complex microbial communities.…”