Marine macroalgae are important components of coastal ecosystems, providing food and habitat for numerous species. Since the atmospheric CO2 concentration increases, the CO2 concentration of the upper surface layers of the global oceans increases as well, causing an alteration of the seawater chemistry. This shift can affect marine macroalgae which depend on carbon to efficiently run photosynthesis. Thus, the performance of marine macroalgae might be influenced by ocean acidification with as yet unpredictable consequences for the entire ecosystem. Intertidal macroalgae are not only affected by enhanced atmospheric CO2 concentrations but also by severe alterations in their abiotic environment due to the diurnal tidal cycle (e.g. tidal emergence). This study aimed to provide the first data on the combined effects of enhanced CO2 and tidal emergence on the physiological performance and the expression of specific enzymes involved in carbon fixation in the common intertidal brown macroalga Fucus serratus. Furthermore, the applicability of molecular tools for this macroalgal species should be tested. F. serratus was cultured for two weeks at two different CO2 concentrations (280 and 1200 ppm) and two different tidal regimes (regular emergence and permanent submersion). Physiological traits were unaffected by enhanced CO2 concentrations and tidal emergence. Photosynthesis, growth and chlorophyll a content remained constant in each of the tested treatments. The insensitivity of physiological traits might be the result of an actively running carbon concentrating mechanism (CCM). By this CCM, photosynthesis of F. serratus is already carbon saturated at present CO2 concentrations. Gene expression analysis was performed by a quantitative real-time polymerase chain reaction (qRT-PCR), investigating the expression of genes encoding for carbonic anhydrase (CA), ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO) and phosphoenolpyruvate carboxykinase (PEPCK). Enhanced CO2 and tidal emergence did not affect the expression of the tested genes. The combination of both parameters led, however, to an up-regulation of all tested genes. Gene expression was unaffected by either CO2 or desiccation what might be due to the activity of the CCM. The up-regulation of the genes under the combined influence of the two factors cannot be explained by the present study. However, this study proved that (1) molecular tools are applicable to F. serratus and (2) that the two tested abiotic parameters interact, leading to a change in the transcriptional abundance of the tested enzymes. Although gene expression was affected by the interaction of the abiotic parameters, physiological traits remained unchanged, indicating that F. serratus is well adapted to its abiotic environments by a dynamical reaction without a change in fitness. The present study did not investigate enzyme activity and content. Future investigations should consider proteomic analysis to explain the effects of changing environments and different abiotic stresses in a more comprehensive way.