It is now widely accepted that limitation of phytoplankton growth by a single limiting resource in the ocean is usually the exception, rather than the rule.  In fact, we now know that phytoplankton are often co-limited in group-specific and region-specific ways by various combinations of iron, nitrogen, phosphorus and vitamin availability, with major implications for their ability to store carbon the ocean. The existence of multiple simultaneously limiting factors has obvious potential consequences for carbon sequestration by phytoplankton blooms following natural and artificially-induced iron fertilization and upwelling of major nutrients. Other effects of co-limitation can be less intuitive, such as the observation that diazotrophic cyanobacteria co-limited by iron and phosphorus often grow and fix nitrogen better than when they are limited by either nutrient alone. Co-limitation can also affect the balance between organic carbon fixation and calcification in coccolithophores, potentially altering carbon export and air/sea carbon dioxide fluxes. This talk will explore the physiological and molecular mechanisms behind phytoplankton co-limitation, and how they can ultimately lead to global biogeochemical consequences in our changing ocean.