© 2015 by The Oceanography Society. All rights reserved. All phytoplankton and higher plants perform photosynthesis, where carbon dioxide is incorporated into biomass during cell growth. Ocean acidification (OA) has the potential to affect photosynthetic kinetics due to increasing seawater pCO2 levels and lower pH. The effects of increased CO2 are difficult to predict because some species utilize carbon concentrating mechanisms that buffer their sensitivity to ambient CO2 levels and require variable energy investments. Here, we discuss the current state of knowledge about the effects of increased CO2 on photosynthesis across marine photosynthetic taxa from cyanobacteria and single-celled eukaryotes to marine macrophytes. The analysis shows that photosynthetic responses to OA are relatively small for most investigated species and highly variable throughout taxa. This could suggest that the photosynthetic benefits of high CO2 are minor relative to the cell’s overall energy and material balances, or that the benefit to photosynthesis is counteracted by other negative effects, such as possible respiratory costs from low pH. We conclude with recommendations for future research directions, such as probing how other physiological processes respond to OA, the effects of multiple stressors, and the potential evolutionary outcomes of longterm growth under ocean acidification.