Arsenic (As), one of the toxic metalloids, can be found in different forms, including organic or inorganic compounds in various redox states, each with their distinct mobilities and toxicities. For example, the most mobile form, arsenite (As(III)) is prevalent in rice paddies, which are often flooded under reducing conditions, leading to high As accumulates in rice plants. This has led to increased attention on the effective management of As in the field to mitigate As contamination, necessitating a better understanding of microbial reactions that govern As speciation in environments. However, what remains yet understudied is how As biotransformation and its prerequisite step, microbial uptake, are affected by extracellular aquatic chemistries (e.g., dissolved organic matter (DOM) and metals. Additionally, the relationship between As biotransformation and other prevalent metabolisms in rice paddies such as methanogenesis is less understood. The overarching objectives of this dissertation were to: (1) Investigate the effects of DOM with iron on the microbial uptake of As. (2) Explore the repression of microbial As(III) uptake by bio-labile carbon via a catabolite repression-like mechanism, leading to a decrease in As uptake. (3) Enhance our understanding of the relationship between As demethylation and methanogenesis. To address these objectives, four research projects were designed. Firstly, various types of environmental DOM were tested to examine their impact on As bioavailability using an E. coli biosensor. Secondly, the effects of mixtures of environmental DOM and iron at different As/C and Fe/C ratios were examined using a novel anaerobic E. coli biosensor. Thirdly, the effects of DOM on expression of glpF and arsM gene was investigated, which are responsible for As transport in E. coli and As methylation in A. rosenii. Finally, soil microcosm experiments were conducted to investigate the relationship between methylotrophic methanogenesis and As demethylation. The results of this thesis contribute to advancing our understanding of the effects of water chemistries on As uptake and downstream biotransformation in environments, offering the potential for better control of As contamination and also greenhouse gas emissions in agricultural fields.