Mercury and its compounds are a kind of persistent toxic pollutants. Methylmercury (MeHg) is the most concerned specie of mercury in the environment due to its high toxicity and quick accumulation along the food chain. Methylation and demethylation are the two key processes controlling the level of MeHg in aquatic environments. As the major primary producer in the ocean, microalgae are expected to play an important role in the cycling and accumulation of Hg in marine ecosystems by either absorbing Hg species from seawater or involving in the transformations of Hg species. Despite the large quantities of studies on the uptake of Hg by microalgae, there is still lack of clear knowledge on whether marine microalgae can induce the methylation and demethylation of mercury in aquatic environments. In this study, mercury isotope dilution and isotope tracer techniques were adopted to determine the methylation and demethylation of Hg at concentrations comparable to that in natural environments by 16 common marine microalgae. The tested microalgae include 8 species of diatoms, 5 species of dinoflagellates, 2 species of green algae and 1 species of golden algae. Methylation of inorganic Hg was found to be negligible in the culture of all 16 tested marine microalgae, while 7 microalgae could significantly induce the demethylation of MeHg. The rates of microalgae mediated MeHg demethylation are at the same order of magnitude as that of photodemethylation, indicating that marine microalgae may play an important role in the degradation of MeHg in the ocean. Further studies suggest that the demethylation of MeHg by the microalgae may be mainly caused by their extracellular secretions and associated bacteria, rather than the direct demethylation of MeHg by microalgal cells.