Sulfur emission from marine phytoplankton has been recognized as an important factor for global climate and as an entry into the biogeochemical S cycle. Despite this significance, little is known about the cellular S metabolism in algae that forms the basis of this emission. Some biochemical and genetic evidence for regulation of S uptake and assimilation is available for the freshwater model alga Chlamydomonas. However, the marine environment is substantially different from most fresh waters, containing up to 50 times higher free sulfate concentrations and challenging the adaptive mechanisms of primary and secondary S metabolism in marine algae. This review intends to integrate ecological and physiological data to provide a comprehensive view of the role of S in the oceans.