Ocean acidification, the decrease in the pH and carbonate ion concentration due to the uptake of anthropogenic CO2, threatens coral reefs worldwide as studies predict the decline of calcifiers and increasing growth of non−calcified macroalgae. However, the flux of carbonate species partly depends on the benthic community composition. On the north shore of Moorea, French Polynesia, SeaFETS, ocean pH sensors, were placed at 5 locations across the reef. The pH was relatively stable on the fore reef while large diel fluctuations were recorded directly behind the reef crest, which is dominated by the macrophyte Sargassum pacificum. The fleshy alga, through photosynthesis and respiration, may be driving the pH fluctuations on the back reef. Previous studies explore the negative effects of coral algal interactions from shading, abrasion or allelopathy. In future elevated CO2 conditions, macroalgae could indirectly alleviate some of the effects of ocean acidification on associated corals by increasing the pH. I tested the combined and potentially interactive effects of algae and ocean acidification on the coral, Acropora pulchra, in a fully factorial field and mesocosm experiment. At the Richard B. Gump South Pacific Research Station, I exposed Acropora fragments to a mesocosm experiment crossing 3 CO2 treatments (ambient, high and fluctuating) with 2 light levels (shaded and unshaded) mimicking the light environment in Sargassum canopies. Future recovery of coral reefs may depend on the ability of corals to survive in association with macroalgae. Through this research, I hope to elucidate any interactive effects of macroalgae and ocean acidification on coral calcification, growth and survival.