Biogenic selenium (Se) emissions play a major role in the biogeochemical cycle of this essential micronutrient. Microalgae may be responsible for a large portion of these emissions via production of methylated Se compounds that volatilize into the atmosphere. However, the biochemical mechanisms underlying Se methylation in microalgae are poorly understood. Here, we study Se methylation by Chlamydomonas reinhardtii, a model freshwater alga, as a function of uptake and intracellular Se concentrations and present a biochemical model that quantitatively describes Se uptake and methylation. Both selenite and selenate, two major inorganic forms of Se, are readily internalized by C. reinhardtii, but selenite is accumulated around ten times more efficiently than selenate due to different membrane transporters. With either selenite or selenate as substrates, Se methylation was highly efficient (up to 89% of intracellular Se) and directly coupled to intracellular Se levels (R(2) > 0.92) over an intracellular concentration range exceeding an order of magnitude. At intracellular concentrations exceeding 10 mM, intracellular zerovalent Se was formed. The relationship between uptake, intracellular accumulation, and methylation was used by the biochemical model to successfully predict measured concentrations of methylated Se in natural waters. Therefore, biological Se methylation by microalgae could significantly contribute to environmental Se cycling.