Sequential bio- and phototransformation of the herbicide methabenzthiazuron in water

Abstract

We investigated the transformation of methabenzthiazuron in water by microorganisms and solar light. This compound was very slowly phototransformed when irradiated at lambda > 290 nm, but it could be successfully oxidized into 6-hydroxymethabenzthiazuron by Aspergillus niger, as shown by nuclear magnetic resonance experiments. The toxicity of this metabolite, as determined by the standardized Microtox test, was sixfold lower than that of the parent molecule. The 6-hydroxymethabenzthiazuron was not further metabolized by A. niger but was photooxidized with ring cleavage of the aromatic ring and photodimerized on irradiation at lambda > 290 nm. In the presence of humic substances, the photodegradation was slower. We demonstrate that the transformations of methabenzthiazuron, observed either with the fungus A. niger or by the action of solar light, do not proceed via the urea chain N-dealkylation, as usually reported, but only via hydroxylation or cleavage of the benzene ring. This work shows the complementarity of both approaches, photo- and biodegradation, to study the fate of herbicides in the environment.

Comparison of Microbial and Photochemical Processes and Their Combination for Degradation of 2-Aminobenzothiazole

“…Brownlee et al (7) studied the fate of 2-(thiocyanomethylthio)benzothiazole (TCMTB) in the aquatic compartment and showed that that active molecule of the fungicide BUSAN was rapidly photolysed. The herbicide MBTU, on the contrary, had a very low photoreactivity (24), but the addition of nitrate ions (photoinductors) in water increased MBTU photodegradation by a factor of 10 (26). More recently, Malouki (25) studied in detail the photolysis of 2-mercaptobenzothiazole (MBT) in water.…”

“…An original study on the sequential bio-and phototransformations was carried out on MBTU in water (24). As mentioned before, MBTU was very slowly phototransformed at wavelengths higher than 290 nm.…”

Auto-remediation of surface waters by solar-light: Photolysis of 1-naphthol, and two herbicides in pure and synthetic waters

“…All of them absorb above 300 nm and have thus the potential to undergo direct photolysis when exposed to solar-light. However, MBTU was previously reported to be very slowly photolyzed in solutions when irradiated above 290 nm alone and in the presence of natural organic matter [15,16]. Thus, we only investigated the photolysis of MBTU in the presence of nitrate ions.…”

Metabolism of 2-Mercaptobenzothiazole by Rhodococcus rhodochrous

“…3). This hydroxylation was previously shown to operate in ABT in R. rhodochrous (13), in BT and OBT in Rhodococcus erythropolis, R. rhodochrous, and R. pyridinivorans PA (2,14), and on N- [2-benzothiazolyl]-N,NЈ-dimethylurea (methabenzthiazuron) in Aspergillus niger (15) and Cunninghamella echinulata (23).…”

“…To our knowledge, this is the first report showing that MBT is potentially degradable by a pure culture and that the enzymatic equipment of R. rhodochrous OBT18 could operate under some environmental conditions. Also, it was shown that 6-OH-MBT is less toxic than MBT, according to the standardized Microtox test performed as previously described (15). For an exposure time of 15 min, the 50% effective concentrations were 9.5 M for 6-OH-MBT and 1.5 M for MBT.…”