To endure, coral reefs must accumulate CaCO₃ at a rate greater or equal than the sum of mechanically, biologically, and chemically mediated erosion rates. We investigated the potential role of holothurians on the CaCO₃ balance of a coral reef. These deposit feeders process carbonate sand and rubble through their digestive tract and dissolve CaCO₃ as part of their digestive process. In aquarium incubations with Stichopus herrmanni and Holothuria leucospilota total alkalinity increased by 97 ± 13 and 47 ± 7 μmol kg⁻¹, respectively. This increase was due to CaCO₃ dissolution, 81 ± 13 and 34 ± 6 μmol kg⁻¹ and ammonia secretion, 16 ± 2 and 14 ± 2μmol kg⁻¹, respectively, for these species. Surveys conducted at a long-term monitoring site of community calcification (DK13) on One Tree Reef indicated that the density of sea cucumbers was approximately 1 individual m⁻². We used these data and data from surveys at Shark Alley to estimate the dissolution of CaCO₃ by the sea cucumbers at both sites. At DK13 the sea cucumber population was estimated to be responsible for nearly 50% of the nighttime CaCO₃ dissolution, while in Shark Alley for most of the nighttime dissolution. Thus, in a healthy reef, bioeroders dissolution of CaCO₃ sediment appears to be an important component of the natural CaCO₃ turnover and a substantial source of alkalinity as well. This additional alkalinity could partially buffer changes in seawater pH associated with increasing atmospheric CO₂ locally, thus reducing the impact of ocean acidification on coral growth.