Warming effect on nitrogen fixation in Mediterranean macrophyte sediments

Abstract

21The Mediterranean Sea is warming faster than the global Ocean, with important 22 consequences for organisms and biogeochemical cycles. Warming is a major stressor for key 23 marine benthic macrophytes. However, the effect of warming on marine N2 fixation remains 24 unknown, despite the high productivity of macrophytes in oligotrophic waters is partially 25 sustained by the input of new nitrogen (N) into the system by N2 fixation. Here, we assess the 26 impact of warming on N2 fixation rates on three key marine macrophytes: Posidonia oceanica, 27Cymodocea nodosa, and Caulerpa prolifera. We experimentally measured N2 fixation rates in 28 vegetated and bare sediments at temperatures encompassing current summer mean, projected 29 summer mean and projected summer maximum seawater surface temperatures (SST) by the end 30 of the century under a scenario of moderate greenhouse-gas emissions. We found that N2 fixation 31 rates in vegetated sediments were 2.8-fold higher than in bare sediments at current summer mean 32 SST, with no differences among macrophytes. Currently, the contribution of N2 fixation to 33 macrophytes productivity could account for up to 7 %, 13.8 % and 1.8 % of N requirements for 34 P. oceanica, C. nodosa, and C. prolifera, respectively. We show the temperature dependence of 35 sediment N2 fixation rates. However, the thermal response differed for vegetated sediments, 36 where rates showed an optimum at 31 o C followed by a sharp decrease at 33 o C, and bare 37 sediments, where rates increased along the range of experimental temperatures tested here. The 38 activation energy and Q10 were lower in vegetated sediments than in bare sediments. The 39 projected warming is expected to increase the contribution of N2 fixation to Mediterranean 40 macrophytes' productivity. Therefore, the thermal dependence of N2 fixation might have 41 important consequences for primary production in coastal ecosystems in the context of warming. 42Biogeosciences Discuss., https://doi

High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea

“…In addition, sediment slurries disturb the sediment structure and redox gradient, and, therefore, might prevent the coupled nitrification and denitrification in the transition layers from oxic to anoxic conditions (Eyre et al, 2002;Herbert, 1999). Since the coupled nitrification and denitrification has been reported to be important in continental shelf and coastal sediments (Herbert, 1999;Gardner and McCarthy, 2009;Christensen et al, 1987); the denitrification rates in this coastal lagoon could be higher than actual reported values.…”