| Summary: | [eng] Marine ecosystems are affected negatively by climate change and this can be especially true in the Mediterranean Sea, where due to its hydrography, climate change can act disproportionally more intense compared to global averages. It is expected that the endemic Mediterranean seagrass Posidonia oceanica meadows will disappear due to the effects of global warming and increases in seawater CO2 concentration. For this reason, we studied the effects of increasing seawater temperature and CO2 concentration and the interaction of these two factors in P. oceanica and its associated microbiome, the atmospheric nitrogen (N2) fixers. P. oceanica shoots were subjected to ambient temperatures and increases of 3 ºC in winter and 4 ºC in the summer of 2019, and ambient (435 ppm) and increased (1200 ppm) pCO2 values to investigate the effect of predicted seawater temperature and CO2 increases by the end of the century. Measured response variables included N2 fixation rates associated with the different plant parts, photosynthetic activity, respiration, primary production and the plant´s ability to metabolize dissolved organic phosphorus (DOP) to release dissolved inorganic phosphorus (DIP) through alkaline phosphatase activity (APA) assays. A significant (p < 0.05) negative response had been shown in APA under elevated temperature in winter old leaf and positive response in rhizome. However, each part of the plants reacted differently under elevated CO2, with a significant (p < 0.05) negative response in the rhizome and roots during summer, compared with under ambient CO2 treatment. During the winter experiment, primary production (gross primary production and net primary production) and respiration rate showed significant (p < 0.05) positive response under elevated CO2 and temperature. A significant (p < 0.05) positive response in primary production (net primary production and gross primary production) and whole leaf net primary production were observed under control conditions during summer. Photosynthetic activity based on PAM measurements showed significant (p < 0.05) positive response under elevated CO2 and elevated temperature. N2 fixation rates of roots and sterile roots showed significant (p < 0.05) negative response under elevated temperature conditions.
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