Marine mercury emission plays an important role in the atmospheric mercury budget. It is caused by the transformation of ionic mercury to volatile elemental mercury (Hg(0)) and the subsequent release of the latter from surface waters. In this study, we investigated mercury transformation using three approaches: incubation experiments, statistical analyses of phytoplankton and Hg(0) data, and microbiological determinations. The incubation experiments revealed that (1) biotic-light-dependent transformation accounted for an average of 30% of the total natural transformation, (2) photochemistry also contributed 30%, such that its contribution was less important than previously considered, and (3) low-light production accounted for 40%. Field experiments and the microbiological investigations suggested the cyanobacterial genera Synechococcus and Aphanizomenon as the main transformers of mercury. On the basis of the rough balance of biotic mercury transformation with mercury emission during summer, on average, only a small portion of a few percent of the mixed layer participates in active transformation.