Insights on biotic and abiotic 2,4‐dichlorophenoxyacetic acid degradation by anaerobic iron‐cycling bacteria

Abstract

AbstractThe use of the phenoxy herbicide 2,4‐dichlorophenoxyacetic acid (2,4‐D) has been steadily increasing in recent years due to its selectivity against broad‐leafed weeds and use on genetically modified crops resistant to 2,4‐D. This increases the likelihood of 2,4‐D persisting in agriculturally impacted soils, sediments, and aquatic systems. Aerobic microorganisms are capable of degrading 2,4‐D enzymatically. Anaerobic degradation also occurs, though the enzymatic pathway is unclear. Iron‐reducing bacteria (FeRB) have been hypothesized to augment anaerobic degradation through the production of a chemically reactive Fe(II) adsorbed to Fe(III) oxyhydroxides. To test whether this iron species can catalyze abiotic degradation of 2,4‐D, an enrichment culture (BLA1) containing a photosynthetic Fe(II)‐oxidizing bacterium (FeOB) "Candidatus Chlorobium masyuteum" and the FeRB "Candidatus Pseudopelobacter ferreus", both of which lacked known 2,4‐D degradation genes was investigated. BLA1 produces Fe(II)‐adsorbed to Fe(III) oxyhydroxides during alternating photoautotrophic iron oxidation and dark iron reduction (amended with acetate) cycles. No 2,4‐D degradation occurred during iron oxidation by FeOB Ca. C. masyuteum or during iron reduction by FeRB Ca. P. ferreus under any incubation conditions tested (i.e., +/−Fe(II), +/−cells, and +/−light), or due to the presence of Fe(II) adsorbed to Fe(III) oxyhydroxides. Our results cast doubt on the hypothesis that the mineral‐bound Fe(II) species augments the anaerobic degradation of 2,4‐D in anoxic soils and waters by iron‐cycling bacteria, and further justify the need to identify the genetic underpinnings of anaerobic 2,4‐D degradation.