Since pre-industrial times, atmospheric CO2 concentrations have risen from 280 ppm to > 400 ppm causing a drop in surface ocean pH by 0.1 pH units, which corresponds to a ~ 30% increase in acidity. Ocean acidification (OA) is expected to negatively affect calcifying organisms like scleractinian corals. Most hermatypic, or reef building, corals live in photosymbiosis with small, single-celled algae (zooxanthellae) of the phylum Symbiodinium. The coral provides metabolic nutrients to the algae and benefits from its translocated photosynthetic energy. The algae are assumed to ease the negative effects of OA as they are able to fix excess CO2 during photosynthesis. The aim of this bachelor thesis was to analyze the role of the symbiotic algae on the physiological status of the scleractinian corals Porites lutea and Seriatopora hystrix under the events of OA and additional light deprivation. Coral fragments were collected from a volcanic carbon dioxide vent site within the coral triangle in Papua New Guinea, with seawater pCO2 values similar to those predicted for 2100 (pH 7.8). Corals from the adjoining reef with normal values of pCO2 (pH 8.1) served as controls. Pigment composition and content in the zooxanthellae of the sampled corals were analyzed (via HPLC), as well as biomass and protein content of both coral host and symbiont. The results confirmed former studies in which Porites lutea did hardly suffer from OA or even benefit. Pigment concentrations were clearly elevated at the vent compared to the control site and symbiont protein concentrations started to increase at the vent site at the end of the study. Seriatopora hystrix instead was significantly affected by OA. Pigment concentrations stayed unchanged but protein concentrations clearly decreased under the influence of OA, whereas biomass concentrations increased. But as biomass build up is a rather tedious process, these findings might not be related to the experiment. Under the additive stress of light exclusion, both corals were expected to suffer most due to the lack of supporting effects from the symbiosis. Surprisingly, Porites lutea was unaffected. Pigment concentrations decreased during darkness but there was no difference between vent and control site. In contrast, Seriatopora hystrix was clearly afflicted with both OA and light deprivation. Pigment concentrations declined at both sites but to lower values at the vent site. Interestingly, protein concentrations declined as well at the vent site whereas biomass concentrations were higher compared to those of light control until the mid of the experiment.