A mesocosm experiment was conducted to investigate whether three levels of decreasing seawater pH could alter the activity of key enzymes involved in the energy metabolism in Millepora alcicornis. Hydrocorals were acclimated to marine mesocosm conditions for 20 days and then exposed to different seawater pH levels [ambient pH (8.1) and experimental pH (7.8, 7.5 and 7.2)] for 16 and 30 days. Endpoints analyzed included the activity of key enzymes involved in the regulation of the glycolytic pathway (hexokinase and pyruvate kinase), aerobic energy production via the Krebs cycle (citrate synthase) and anaerobic energy production via lactate formation (lactate dehydrogenase).This dataset is included in the OA-ICC data compilation maintained in the framework of the IAEA Ocean Acidification International Coordination Centre (see https://oa-icc.ipsl.fr). Original data were extracted from tables in the related paper (see Related to) by the OA-ICC data curator. In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2024) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2025-02-27.