Primary production and calcification responses to irradiance were investigated in Lithophyllum cabiochae, a crustose coralline alga from Mediterranean coralligenous communities, collected at c. 25 m depth in the Bay of Villefranche. Algae were maintained in aquaria at temperature and irradiance levels close to in situ conditions. Physiological measurements were performed using incubation chambers in the dark and in the light at different irradiance levels within the range of those measured in situ. Both photosynthesis and calcification rates in L. cabiochae were strongly related to irradiance. Dark respiration averaged 0.2-0.3 µmol cm −2 thallus h −1 in terms of both O 2 consumption and CO 2 release and maximal gross photosynthesis averaged 1.0 µmol cm −2 h −1 in terms of both O 2 production and CO 2 uptake. Mean rate of net calcification was 0.1 µmol CaCO 3 cm −2 h −1 in the dark and reached 0.4 µmol CaCO 3 cm −2 h −1 in the light. Diel net and gross organic C productions were estimated to be 3 and 7 µmol C cm −2 thallus d −1, respectively. Diel net inorganic C production was estimated to be 3 µmol CaCO 3 cm −2 thallus d −1. Despite the low light conditions experienced by the algae at c. 25 m depth, L. cabiochae can be considered as a major contributor to primary productivity and calcium carbonate deposition, making coralligenous communities a major carbon and carbonate producer in the Mediterranean Sea.