Abstract
2,4-Dichlorophenoxyacetic acid (2,4-D) is the third most applied pesticide in Brazil to control broadleaf weeds in crop cultivation and pastures. Due to 2,4-D’s high mobility and long half-life under anoxic conditions, this herbicide has high probability for groundwater contamination. Bioremediation is an attractive solution for 2,4-D contaminated anoxic environments, but there is limited understanding of anaerobic 2,4-D biodegradation. In this study, methanogenic enrichment cultures were obtained from Amazonian top soil (0—40 cm) and deep soil (50 -80 cm below ground) that biotransform 2,4-D (5 µM) to 4-chlorophenol and phenol. When these cultures were transferred (10% v/v) to fresh medium containing 40 µM or 160 µM 2,4-D, the rate of 2,4-D degradation decreased, and biotransformation did not proceed beyond 4-chlorophenol and 2,4-dichlorophenol in the top and deep soil cultures, respectively. 16S rRNA gene sequencing and qPCR of a selection of microbes revealed no significant enrichment of known organohalide-respiring bacteria. Furthermore, a member of the genus Cryptanaerobacter was identified as possibly responsible for phenol conversion to benzoate in the top soil inoculated culture. Overall, these results demonstrate the effect of 2,4-D concentration on biodegradation and microbial community composition, which are both important factors when developing pesticide bioremediation technologies.
Highlights
The Brazilian agriculture sector has undergone enormous growth in recent decades, with the country becoming one of the world’s largest supplier of soy, coffee, sugar cane and corn, among other commodities (FAO 2015)
2,4-D transforming microbial enrichments under methanogenic conditions were developed with soil samples from the Amazon (Rondonia, Brazil)
2,4-D depletion was observed in all microcosms, and top soil showed a faster depletion than deep soil (Fig. 3) Limited decrease of 2,4-D was observed in abiotic controls (Fig. 3)
Summary
The Brazilian agriculture sector has undergone enormous growth in recent decades, with the country becoming one of the world’s largest supplier of soy, coffee, sugar cane and corn, among other commodities (FAO 2015). This rapid growth has relied heavily on the use of pesticides. 2,4-D is classified toxicologically in Brazil as class I (highly toxic) (ANVISA 2016) and as class III for environmental hazards (product dangerous to the environment) (Rebelo et al 2010). Once present in anoxic environments, including saturated agricultural land and aquifers, 2,4-D is considered ‘persistent’ to ‘highly persistent’ with a half-life of 41–333 days (EPA 2005). The development of reliable and affordable technologies to remediate contaminated anoxic subsurface environments is needed to minimize the human health and environmental risks associated with 2,4-D
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