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. The Mediterranean Sea is warming faster than the global ocean, with importantconsequences for organisms and biogeochemical cycles. Warming is a majorstressor for key marine benthic macrophytes. However, the effect of warmingon marine N2 fixation remains unknown, despite the fact that the highproductivity of macrophytes in oligotrophic waters is partially sustained bythe input of new nitrogen (N) into the system by N2 fixation. Here,we assess the impact of warming on the N2 fixation rates of three keymarine macrophytes: Posidonia oceanica, Cymodocea nodosa,and Caulerpa prolifera. We experimentally measured N2 fixation rates in vegetated and bare sediments at temperatures encompassingcurrent summer mean (25 and 27  ∘ C), projected summer mean (29 and31  ∘ C), and projected summer maximum (33  ∘ C) seawatersurface temperatures (SSTs) by the end of the century under a scenario ofmoderate greenhouse gas emissions. We found that N2 fixation ratesin vegetated sediments were 2.8-fold higher than in bare sediments at currentsummer mean SST, with no differences among macrophytes. Currently, thecontribution of N2 fixation to macrophyte productivity couldaccount for up to 7 %, 13.8 %, and 1.8 % of N requirements forP. oceanica, C. nodosa, and C. prolifera,respectively. We show the temperature dependence of sediment N2 fixation rates. However, the thermal response differed for vegetatedsediments, in which rates showed an optimum at 31  ∘ C followed by asharp decrease at 33  ∘ C, and bare sediments, in which rates increasedalong the range of the experimental temperatures. The activation energy and Q10 were lower in vegetated than bare sediments, indicating the lowerthermal sensitivity of vegetated sediments. The projected warming is expectedto increase the contribution of N2 fixation to Mediterraneanmacrophyte productivity. Therefore, the thermal dependence of N2 fixation might have important consequences for primary production in coastalecosystems in the context of warming.

Warming effect on nitrogen fixation in Mediterranean macrophyte sediments