| Summary: | International Symposium on Foraminifera (FORAMS 2023), 26-30 June 2023, Perugia, Italy Since the beginning of the Industrial Revolution, the use of fossil fuel has resulted in an atmospheric increase in carbon dioxide (pCO2) and a concurrent alteration of ocean chemistry with a decrease in ocean pH and of the carbonate saturation state. These changes in water chemistry, referred to as Ocean Acidification (OA), are also leading to biodiversity loss and an impact on key marine organisms that rely on calcium carbonate for building their skeletons or shells. Future OA scenarios can be studied and modelled in hydrothermal areas, particularly in correspondence of vents such as in Ischia Island (NW sector of the Gulf of Naples, Southern Italy) that represents one of these natural laboratories. In this research, the variations of benthic foraminiferal (i.e., morphological and molecular) communities were investigated in fourteen samples collected along two transects at North and South of the Castello Aragonese bridge, where clear pH gradients were reported. The sites with the lowest pH values are dominated by agglutinated specimens that are less to not prone to dissolution. On the other hand, calcareous specimens, both hyaline and porcelaneous, are mostly found at sites with normal marine pH values. Variations of benthic foraminiferal parameters like density or living/dead specimens are not only driven by pH values but also by the occurrence of the Posidonia oceanica, a seagrass, and the sediment grain-size (i.e., mud). The wall-type changes of the morphological community along the pH gradients correspond well to those identified in the molecular community. These observations suggest that OA negatively affects the benthic foraminiferal communities and support the application of the foraminiferal metabarcoding even in the assessment of the effects of climate changes Peer reviewed
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