Alga are widely distributed in aquatic ecosystems and play an important role in arsenic (As) bioavailability and biogeochemical cycling. As one of important freshwater algae, Microcystis aeruginosa is generally tolerant to As(V) and exhibits a stronger As bioaccumulation capacity compared to other freshwater algae. Many abiotic factors affect the metabolic functions of alga contaminated by As, such as As levels, hydrogen ion levels (pH), and key nutrient concentrations of nitrogen (N) and phosphorus (P) in culture media. To date, very limited information is available on how and to what extent environmental factors influence arsenic (As) biotransformation and release in freshwater algae. To further understand environmental factors that impact As(V) bioaccumulation, we investigated As bioavailability and biotransformation in M. aeruginosa, aspects of its growth, intracellular As accumulation in algae cells, and release after algae death. Also, we examined the influences of three different DOP forms (β-sodium glycerophosphate (βP), adenosine 5'-triphosphate (ATP) and D-Glucose-6-phosphate disodiumsalt (GP)) and inorganic P of phosphate (IP) on As(V) toxicity, accumulation and biotransformation in Microcystis aeruginosa. We found that N is critical for M. aeruginosa As(V) biotransformation to occur, particularly with regard to As(III) transformation. Also, As accumulation benefited from low P levels when combined with high N concentrations. Phosphate was second to As(V) as the primary factor to affect As accumulation. Additionally, we found that the small amounts of As that accumulated under low concentrations of As and high P were tightly stored in living algal cells and were easily released after cell death. Meanwhile, arsenic accumulation in algal cells were thus greatly enhanced under DOP conditions especially βP since there was highest intracellular TP under βP treatment. Furthermore, As(V) reduction and subsequent As(III) methylation was greatly facilitated in M. aeruginosa by DOP, particularly GP, which confirmed by its higher relative expression of two functional genes including arsC and arsM. Furthermore, higher P distribution in MSF under IP treatment could explain that the tested algae have lower toxic stress compared to DOP treatments. Meanwhile, higher MSF distribution of As in M. aeruginosa under DOP conditions can explain higher toxicity with lower 96 EC50 values. Our results will be helpful for the understanding, practical applications, and overall control of the key environmental factors, particularly those associated with algal bioremediation for As-polluted water. This study was jointly supported by the National Nature Science Foundation of China (project nos. 41401552, 41271484 and 21277136) and the Nature Science Foundation of Fujian Province (2016J01691).