| Summary: | Oral presentation. 19th International Bryozoological Association conference, Dublin 2022. Ocean warming and acidification may compromise calcifying species. However, their combined and long-term effects in bryozoans remain poorly understood. Here we compare the proportion of cover, skeletal structure and the microbiome composition of Pentapora ottomuelleriana population from a volcanic CO2 vent and a population living at ambient pH with lack of venting activity. The proportion of cover decreased from 2016 to 2020 at both sites but the trend was more negative at the CO2 vent site. The increasing seawater temperature in the last decade combined with acidified conditions have accelerated mortality rates at the CO2 vent site compared to those at ambient pH site, also shifting the microbiome towards the dominance of anaerobic microbial taxa. We also observed an increase of carbonate skeleton thickness and a decrease of zooid length in colonies from the CO2 vent site. Although these changes in the calcification of bryozoan individuals could be explained by the energy reallocation to calcification in periods of stress, they are not sufficient to overcome the synergistic impacts of both warming and acidification. Peer reviewed
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