Seagrass meadows form highly productive and diverse ecosystems in coastal areas worldwide, where they are increasingly exposed to ocean acidification (OA). Efficient nitrogen (N) cycling and uptake are essential to maintain plant productivity, but the effects of OA on N transformations in these systems are poorly understood. Here we show that complete N cycling occurs on leaves of the Mediterranean seagrass Posidonia oceanica at a volcanic CO2 vent near Ischia Island (Italy), with OA affecting both N gain and loss while the epiphytic microbial community structure remains largely unaffected. Daily leaf-associated N2 fixation contributes to 35% of the plant's N demand under ambient pH, while it contributes to 45% under OA. Nitrification potential is only detected under OA, and N-loss via N2 production increases, although the balance remains decisively in favor of enhanced N gain. Our work highlights the role of the N-cycling microbiome in seagrass adaptation to OA, with key N transformations accelerating towards increased N gain.

Biology Centre of the Czech Academy of Sciences České Budějovice Czech Republic

Center for water technology Department of Biology Aarhus University Aarhus Denmark

Department of Ecosystem Biology Faculty of Science University of South Bohemia České Budějovice Czech Republic

Department of Integrative Marine Ecology Stazione Zoologica Anton Dohrn National Institute of Marine Biology Ecology and Biotechnology Naples Italy

Department of Marine Ecology University of Bremen Bremen Germany

Department of Research Infrastructures for marine biological resources Stazione Zoologica Anton Dohrn National Institute of Marine Biology Ecology and Biotechnology Naples Italy

Genoa Marine Centre Stazione Zoologica Anton Dohrn National Institute of Marine Biology Ecology and Biotechnology Genova Italy

Institute for Marine Biological Resources and Biotechnology National Research Council Ancona Italy

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