## 1 Introduction  
Organoarsenic compounds have been implemented in a variety of industries: As pesticides, poultry feed additives, herbicides, fungicides, etc. Despite their extensive use, the fact that they are associated with arsenic cannot be neglected for their toxicity due to their speciation. Among organoarsenic compounds, roxarsone (3-nitro 4-hydroxyphenyl) arsenic acid) has been used extensively on poultry farms as a feed additive for the promotion of chicken growth [1, 2]. Poultry houses and litter are known to be the main source for the accumulation and spread of roxarsone in soil and water resources [3]. Roxarsone has also been used as an antocicodial agent [4]. The use of this compound has been banned by many countries such as the USA, Canada, Australia, and the EU. In general, these applications are declining in step with growing concerns about their impact on the environment and human health, but roxarsone and other phenylarsonic acid derivates are still used in many parts of Asia and in South American countries [5, 6].  
Roxarsone is highly soluble in water and hence its capacity for mobility in the environment is also higher. In turn, the chances of its contaminating the environment via transport from soil to groundwater are high [3]. Roxarsone can be converted into toxic products such as inorganic arsenic species upon exposure to the environment, resulting in human health risks [7]. Thus, monitoring these types of arsenic-based compounds and the microbes responsible for their transformation in the environment is essential from an environmental as well as a health perspective.  
Researchers have reported that the compounds related to the transformation of roxarsone in the environment range from soil and water to a list of more specific compounds: Arsenite, arsenate, dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and 3-amino-4-hydroxyphenylarsonic acid (3-AHPAA) with the facile reduction of a nitro group [3; 7; 8; 9; 10]. Roxarsone transformation may occur through biotic or abiotic processes [11]. Moreover, several studies have shown that roxarsone can be adsorbed onto iron oxides forming immobilized arsenic compounds, and this can control the mobility of roxarsone from soil to groundwater and thus minimize its pollution [12; 13; 14]. In addition, it has also been reported that _Shevanella oneidensis_ MR-1, a widely distributed metal-reducing bacterium, facilitated the reduction of roxarsone [15]. Other microbes such as _Bacillus_ spp., _Paenibacillus_ spp., _Arthrobacter_ spp., _Lysobacter_ spp., and _Alkaliphilus_ spp. have also been reported to be involved in the degradation of roxarsone, especially in brown soil [16]. Recently, the biotransformation of roxarsone by the aerobic soil bacterium _Enterobacter_ spp. CZ-1 has also been reported [17]. Even though the biotic, as well as abiotic degradation and speciation of roxarsone in aerobic and anaerobic conditions, have been studied, the information on the microorganisms responsible for the reduction in anaerobic conditions is limited. The main objectives of this study were to assess reduction of roxarsone in anaerobic conditions by _Cellulomonas_ sp. strain Cellu-2a in fermentation condition with or without hydrous ferric oxide (HFO), a class of naturally occurring ubiquitous minerals, and to compare the direct and indirect reduction of roxarsone with the extensively studied _S. oneidensis_ MR-1.