This work investigated degradation (measured by qPCR) and biological deactivation (measured by culture-based natural transformation) of extra- and intracellular antibiotic resistance genes (eARGs and iARGs) by free available chlorine (FAC), NH₂Cl, O₃, ClO₂, and UV light (254 nm), and of eARGs by ^•OH, using a chromosomal ARG (blt) of multidrug-resistant Bacillus subtilis 1A189. Rate constants for degradation of four 266–1017 bp amplicons adjacent to or encompassing the acfA mutation enabling blt overexpression increased in proportion to #AT+GC bps/amplicon, or in proportion to #5′-GG-3′ or 5′-TT-3′ doublets/amplicon, with respective values ranging from 0.59 to 2.3 (×10¹¹ M^–1 s^–1) for ^•OH, 1.8–6.9 (×10⁴ M^–1 s^–1) for O₃, 3.9–9.2 (×10³ M^–1 s^–1) for FAC, 0.35–1.2(×10¹ M^–1 s^–1) for ClO₂, and 2.0–8.8 (×10^–2 cm²/mJ) for UV at pH 7, and from 1.7–4.4 M^–1 s^–1 for NH₂Cl at pH 8. For FAC, NH₂Cl, O₃, ClO₂, and UV, ARG deactivation paralleled degradation of amplicons approximating a ∼800–1000 bp acfA-flanking sequence required for natural transformation in B. subtilis, whereas deactivation outpaced degradation for ^•OH. At practical disinfectant exposures, eARGs and iARGs were ≥90% degraded/deactivated by FAC, O₃, and UV, but recalcitrant to NH₂Cl and ClO₂. iARG degradation/deactivation always lagged cell inactivation. These findings provide a quantitative framework for evaluating ARG fate during disinfection/oxidation, and support using qPCR as a proxy for tracking ARG deactivation under carefully selected circumstances.

Reactions of dissolved organic matter (DOM) with photochemically generated reactive halogen species (RHS) may represent an important natural source of organohalogens within surface seawaters. However, investigation of such processes has been limited by difficulties in quantifying low dissolved organohalogen concentrations in the presence of background inorganic halides. In this work, sequential solid phase extraction (SPE) and silver-form cation exchange filtration were utilized to desalt and preconcentrate seawater DOM prior to nonspecific organohalogen analysis by ICP-MS. Using this approach, native organobromine and organoiodine contents were found to range from 3.2−6.4 × 10^–4 mol Br/mol C and 1.1–3.8 × 10^–4 mol I/mol C (or 19–160 nmol Br L^–1 and 6–36 nmol I L^–1) within a wide variety of natural seawater samples, compared with 0.6–1.2 × 10^–4 mol Br/mol C and 0.6–1.1 × 10^–5 mol I/mol C in terrestrial natural organic matter (NOM) isolates. Together with a chemical probe method specific for RHS, the SPE+ICP-MS approach was also employed to demonstrate formation of nanomolar levels of organobromine and organoiodine during simulated and natural solar irradiation of DOM in artificial and natural seawaters. In a typical experiment, the organobromine content of 2.1 × 10^–4 mol C L^–1 (2.5 mg C L^–1) of Suwannee River NOM in artificial seawater increased by 69% (from 5.9 × 10^–5 to 1.0 × 10^–4 mol Br/mol C) during exposure to 24 h of simulated sunlight. Increasing I^– concentrations (up to 2.0 × 10^–7 mol L^–1) promoted increases of up to 460% in organoiodine content (from 8.5 × 10^–6 to 4.8 × 10^–5 mol I/mol C) at the expense of organobromine formation under the same conditions. The results reported herein suggest that sunlight-driven reactions of RHS with DOM may play a significant role in marine bromine and iodine cycling.