The dissolved fraction of organic sulfur in the ocean is the most abundant form of sulfur there by a factor of ten, a new study finds. The results, which represent the first basin-wide measurements of dissolved organic sulfur in the ocean, challenge current views of marine sulfur dynamics, suggesting gaps in researchers' understanding. Although sulfur is an essential element for phytoplankton survival and for various climate processes, including aerosol formation and climate regulation, little is known about the oceanic pool of dissolved organic sulfur, a form that is particularly hard to detect. Here, using water samples collected along a transect in the East Atlantic and Southern Ocean between November and December 2008, Kerstin Ksionzek and colleagues calculated an inventory of dissolved organic sulfur stock. Their results, which they extrapolate to global oceans, suggest that dissolved organic sulfur is by far the largest oceanic reservoir of organic sulfur, exceeding all other marine organic sulfur reservoirs by an order of magnitude. What's more, dissolved organic sulfur is also actively involved in the microbial loop, quite similar to organic nitrogen.