Coral reef ecosystems develop best in high flow environments but their fragile frameworks are also vulnerable high wave energy. Wave resistant algal rims, predominantly made up of the crustose coralline algae (CCA) Porolithon onkodes and P. pachydermum, are therefore critical structural elements for the survival of many shallow coral reefs. Concerns are growing about the susceptibility of CCA rims to ocean acidification (OA). because CCA Mg-calcite skeletons are more susceptible to dissolution under low pH conditions than coral aragonite skeletons. Here we show, via a dissolution experiment, that dried dolomite rich CCA have 6-10 times lower rates of dissolution than predominantly Mg-calcite CCA in both high CO2 (~700 ppm) and control (~380 ppm) environments, respectively. We reveal this stabilizing mechanism to be a combination of reduced porosity due to dolomite in-filling and selective dissolution of other carbonate minerals. Physical breakup proceeds by dissolution of Mg-calcite walls until the dolomitized cell eventually drops out intact.