Evaluating pesticide degradation in the environment: Blind spots and emerging opportunities.

The benefits of global pesticide use come at the cost of their widespread occurrence in the environment. An array of abiotic and biotic transformations effectively removes pesticides from the environment, but may give rise to potentially hazardous transformation products. Despite a large body of pesticide degradation data from regulatory testing and decades of pesticide research, it remains difficult to anticipate the extent and pathways of pesticide degradation under specific field conditions. Here, we review the major scientific challenges in doing so and discuss emerging opportunities to identify pesticide degradation processes in the field

Characteristic isotope fractionation patterns in <em>s</em>-triazine degradation have their origin in multiple protonation options in the <em>s</em>-triazine hydrolase trzn.

s-Triazine herbicides (atrazine, ametryn) are groundwater contaminants which may undergo microbial hydrolysis. Previously, inverse nitrogen isotope effects in atrazine degradation by Arthrobacter aurescens TC1 (i) delivered highly characteristic (13C/12C, 15N/14N) fractionation trends for pathway identification and (ii) suggested that the s-triazine ring nitrogen was protonated in the enzyme s-triazine hydrolase (TrzN) where (iii) TrzN crystal structure and mutagenesis indicated H+-transfer from the residue E241. This study tested the general validity of these conclusions for atrazine and ametryn with purified TrzN and a TrzN-E241Q site-directed mutant. TrzN-E241Q lacked activity with ametryn; otherwise, degradation consistently showed normal carbon isotope effects (&epsilon;carbon = -5.0&permil; &plusmn; 0.2&permil; (atrazine/TrzN), &epsilon;carbon = -4.2&permil; &plusmn; 0.5&permil; (atrazine/TrzN-E241Q), &epsilon;carbon = -2.4&permil; &plusmn; 0.3&permil; (ametryn/TrzN)) and inverse nitrogen isotope effects (&epsilon;nitrogen = 2.5&permil; &plusmn; 0.1&permil; (atrazine/TrzN), &epsilon;nitrogen = 2.1&permil; &plusmn; 0.3&permil; (atrazine/TrzN-E241Q), &epsilon;nitrogen = 3.6&permil; &plusmn; 0.4&permil; (ametryn/TrzN)). Surprisingly, TrzN-E241Q therefore still activated substrates through protonation implicating another proton donor besides E241. Sulfur isotope effects were larger in enzymatic (&epsilon;sulfur = -14.7&permil; &plusmn; 1.0&permil;, ametryn/TrzN) than in acidic ametryn hydrolysis (&epsilon;sulfur = -0.2&permil; &plusmn; 0.0&permil;, pH 1.75), indicating rate-determining C-S bond cleavage in TrzN. Our results highlight a robust inverse 15N/14N fractionation pattern for identifying microbial s-triazine hydrolysis in the environment caused by multiple protonation options in TrzN