Since the Industrial Revolution, increasing atmospheric CO2 has been causing a rise in the concentration of carbon dioxide dissolved in seawater. This process results in seawater acidification, which has a major impact on the physical and chemical parameters of the oceans, consequently affecting the numerous calcifying organisms in the marine environment. Calcifying organisms secrete calcium carbonate in their inner or outer skeleton and include plankton (e.g. coccolithophores and foraminifera), corals, mussels and some of the macroalgae. Calcifying macroalgae make a critical contribution to the structure and function of marine ecosystems in several coastal biotas, providing food and shelter to diverse organisms. The present review summarizes the current information about the brown alga Padina sp. and its ecophysiology, focusing on the environmental control of the calcification process; suggests possible benefits that seaweeds may derive from their calcium carbonate cover, and discuss different future Intergovernmental Panel on Climate Change scenarios of ocean acidification and their likely impact on calcifying algae and on the ecosystems in which they are a key component.