## 3 Results and Discussion
### Environmental Aspects  
Since roxarsone is considered to be relatively stable and inert towards degradation in the animal body, it is presumed to be and non-toxic and excreted mainly unmetabolized [23]. However, roxarsone can be rapidly converted into more toxic products, including inorganic arsenic (As), DMA, 3-AHPAA, etc., after exposure to the environment or during the composting of animal manure, leading to high environmental and human risk. From poultry urine, feces, and manures, roxarsone might be released into the environment and could be dispersed through surface water and/or groundwater, resulting in environmental pollution.  
Bacterial degradation of roxarsone, as suggested in this study, might be one of the feasible solutions for alleviating the possible contamination in the environment. As the strains MR-1 and Cellu-2a showed a reduction of roxarsone concentrations in the anaerobic conditions, microorganisms performing similar anaerobic reactions in the environment may contribute to the reduction of the contaminant. In this study, strain MR-1 showed a gradual decrease of roxarsone with almost complete reduction (0.1 mM) in 30 days (Figure 1A), while strain Cellu-2a showed a rapid direct reduction of roxarsone with decreasing approximately 80% roxarsone in 5 days under the experimental condition (Figure 1B). With HFO, a commonly found iron mineral type in the ambient environment, roxarsone decreased more rapidly than without HFO in the incubation by strain MR-1, which reduced Fe(III) in HFO to Fe(II), in turn, reducing roxarsone indirectly (Figure 2A). Solid phase Fe(II) has been known to act as an efficient reductant, providing interfacial surfaces, reactive to aqueous species of oxidants and transfer electrons between them [24]. The reduced iron Fe(II) on HFO surfaces by MR-1 might have acted as an efficient reductant for roxarsone. This type of heterogeneous reaction on solid surfaces may have facilitated a faster roxarsone decrease rather than a direct decrease by the cells.  
The strain Cellu-2a indicated somewhat different patterns of roxarsone decrease, which might not have been affected by heterogenous indirect reactions of the reduced HFO. Since Cellu-2a was able to grow to much larger cell densities by sucrose fermentation than MR-1, in advance to HFO reduction, roxarsone was reduced directly by the large numbers of Cellu-2a cells. These direct and/or indirect bacterial reductions of roxarsone may be considered as main degradation reactions in appropriate environmental conditions by anaerobic cellular respiration and/or fermentation.  
There is still limited information on the byproducts formed from the reduction of roxarsone by strains MR-1 and Cellu-2a. Many researchers reported that roxarsone was transformed to an aromatic amine of 3-AHPAA by a facile reduction of the nitro group and other inorganic arsenic products such as As(III) and As(V) [25; 26; 27]. Furthermore, [PERSON] et al. [15] have reported that _S. oneidensis_ MR-1 showed the transformation of roxarsone to 3-AHPAA in the presence of dissolved Fe(III). Although identification of the byproducts or metabolites was not successful, like the above previous research, it will be important to reveal the byproducts formed by strain Cellu-2a as well as MR-1 in both direct and indirect reduction conditions. This would help the understanding of environmental behaviors of roxarsone in the environment. By the assessment of bacterial degradation of roxarsone, this study provides a better idea of roxarsone behavior in anaerobic conditions by bacterial respiration and fermentation, and also by biotic and abiotic processes.