To better understand the calcification mechanisms of coralline algae it is critical to deter- mine the extent to which seawater carbonate chemistry influences their physiology, most notably the chemistry of the calcifying fluids (CF), where the skeleton forms. In addition to carbonate chemistry, natural variability in pH, flow, and light could all potentially modulate the response of reef organisms to ocean acidification. We specifically designed three laboratory experiments to investigate and understand the role of all of these drivers for the physiology of coralline algae. One experiment investigated the role of pH variability, one the combined effect of water velocity and light, and the last one was created to separate the effects of the different species of the carbonate systems. Flow, Light, pH variability and carbonate chemistry speciation affected the physiology of the organisms to a certain degree. We found that there were complex interactions between chemical conditions in the calcifying fluid and conditions in the surrounding seawater. We also found that despite the role played by these environmental drivers, the rhodoliths had a great control on their internal chemistry and were able to withstand a large range of environ- mental conditions. Understanding the key mechanisms controlling coralline algae calcification is critical to making further progress in determining their future in a more acidic ocean.