Mercury is emitted into the atmosphere through natural sources such as volcanoes, but more extensively through human activities such as burning coal and fossil fuels, industrial waste, and small-scale gold mining. The gaseous form of mercury in the atmosphere is gradually deposited into the oceans in the form of dissolved mercury, or methylmercury. Human-caused mercury emissions have been, for example, responsible for a threefold increase in mercury concentrations dissolved in the surface layers of the world’s oceans since the industrial revolution (Lamborg et al. 2014). A fraction of the dissolved mercury is naturally converted into methylmercury by sulphate-reducing bacteria, in which case the process is referred to as mercury methylation. This conversion is particularly intense in the less oxygenated deeper ocean waters (between depths of 400 m and 800 m). Also, in the surface layers, the dissolved methylmercury and mercury are degraded by light and re-emitted into the atmosphere in a gaseous mercury form (photo-reduction process). The production of methylmercury in the oceans, therefore, depends on the balance between methylation, which is more intense in less oxygenated zones (deeper ocean layers), and photo-reduction, which is more intense in surface ocean layers. The balance between these reactions explains the trend towards an increase in methylmercury concentrations with depth.