Symbiodinium contains a prokaryotic-type II RuBisCO, which has a low affinity for CO2. High concentrations of CO2 are therefore necessary to promote carbon assimilation and to meet the hosts’ energetic demand for symbiontderived photosynthates. Holobiont respiration may present an additional internal CO2 source contributing to the complex carbon exchange and transfer system within corals. Chlororespiration, involving plastoquinone oxidation with O2 and a terminal oxidase can be active within the chloroplasts of Symbiodinium. Furthermore, calcification occurring in the calicodermis of the coral and host mitochondrial respiration can further contribute to the internal CO2 supply in the holobiont. Coral host respiration is just one source of inorganic carbon for symbiont photosynthesis ; external inorganic carbon sources such as seawater are also utilised. However, the supply of inorganic carbon via passive diffusion from the surrounding seawater and host tissue is restricted by several factors: 1) the generally low CO2 content of seawater, 2) the presence of a diffusive boundary layer, and 3) the presence of multiple membranes of the host tissue surrounding the endodermal Symbiodinium cells, which need to be traversed. Both, coral host and symbionts employ a range of carbon concentrating mechanisms to enhance the carbon supply from the external medium and thus increase CO2 availability to the Symbiodinium chloroplasts as well as for calcification purposes. The rate of photosynthesis by the symbionts and therefore their carbon demand is closely correlated with photon irradiance, and may become carbon limited under high irradiance. As the delivery of carbon to the algal symbionts is controlled by the activity of CCMs, as well as host respiration, the host metabolism can thus have a strong impact on symbiont photosynthesis, e.g., by supplying sufficient inorganic carbon under high irradiance. While demands on the host-supplied carbon shift with irradiance, e.g., due to extra demand in light-enhanced calcification, there are only few experimental investigations of such responses in the literature. We investigated if respiratory-dependent processes in the coral would follow a typical asymptotic rise with increasing irradiance, as it is known for photosynthetic processes. Photosynthesis and calcification require carbon as substrate ; photosynthesis is directly dependent on light and coral calcification is known to be light-enhanced. Indeed, there is a close interplay of internal utilization of high throughput screening metabolically derived carbon for both processes. The exchange of respiratory gases in photosynthetic symbioses is difficult to study in the light because respiratory O2 uptake is masked by the O2 production from photosynthesis. At low irradiance, where symbiont photosynthesis is lower than respiratory activity in the coral, i.e., below the irradiance compensation point net O2 uptake and CO2 release can be measured.