Interactions between filamentous turf algae and coralline algae are modified under ocean acidification

•We examined the effects of high CO2 and overgrowth by turf algae on coralline algae.•Overgrowth by turf algae complicated coralline algal metabolic response to high CO2.•We have shown that species interactions may change with ocean acidification.

Ocean acidification is a decrease in seawater pH and carbonate ion concentration due to increased uptake of atmospheric carbon dioxide by the world's oceans. This has major implications for many marine organisms, particularly the calcifiers. Crustose coralline algae (CCA) are among the most sensitive calcifying organisms to ocean acidification. In contrast, filamentous turf algae, which compete with CCA for space on the substratum, could potentially benefit from high pCO2 conditions, suggesting that the effects of filamentous turf on coralline algae may be amplified in a high pCO2 environment. The effect of ocean acidification on the growth of coralline algae, however, has rarely been investigated in combination with ecological interactions such as competition with filamentous turfing algae. Here we tested the combined effects of ocean acidification and overgrowth by filamentous turf algae on CCA calcification, photosynthetic capacity and quantum yield of photosynthesis. We observed a positive effect of algal turfs on CCA calcification but a negative effect on photosynthesis in the high pCO2 treatments, however, these effects were variable over time. Our results have demonstrated the importance of investigating how inter-species interactions such as competition will complicate the impacts of ocean acidification.