Methanogenic inhibition by roxarsone (4-hydroxy-3-nitrophenylarsonic acid) and related aromatic arsenic compounds

Roxarsone (4-hydroxy-3-nitro-phenylarsonic acid) and p-arsanilic acid (4-aminophenylarsonic acid) are feed additives widely used in the broiler and swine industry. This study evaluated the inhibitory effect of roxarsone, p-arsanilic, and other phenylarsonic compounds on the activity of acetate- and H₂-utilizing methanogenic microorganisms. Roxarsone, p-arsanilic, and 4-hydroxy-3-aminophenylarsonic acid (HAPA) inhibited acetoclastic and hydrogenotrophic methanogens when supplemented at concentrations of 1 mM, and their inhibitory effect increased sharply with incubation time. Phenylarsonic acid (1 mM) inhibited acetoclastic but not H₂-utilizing methanogens. HAPA, a metabolite from the anaerobic biodegradation of roxarsone, was found to be sensitive to autooxidation by oxygen. The compound (2.6 mM) caused low methanogenic inhibition (only 14.2%) in short-term assays of 12 h when autooxidation was prevented by supplementing HAPA solutions with ascorbate. However, ascorbate-free HAPA solutions underwent spontaneous autooxidation in the presence of oxygen, leading to the formation of highly inhibitory compounds. These results confirm the microbial toxicity of organoarsenic compounds, and they indicate that biotic as well as abiotic transformations can potentially impact the fate and microbial toxicity of these contaminants in the environment.