Predicted warming intensifies the negative effects of nutrient increase on tropical seagrass: A physiological and fatty acid approach

15 pages, 7 figures, 3 tables.-- Under a Creative Commons licenseThe Gulf of Aqaba (GoA; northern Red Sea) supports extensive seagrass meadows, dominated by the small tropical seagrass species, Halophila stipulacea. Due to its semi-closed structure, in the GoA, regional nutrient loading and global warming are considered the biggest threats to local seagrass meadows, and their combination can potentially amplify their negative impacts. Using a seagrass-dedicated mesocosm, we exposed two seagrass populations with different local “nutrient history” to control (27 °C) and simulated warming (31 °C), with and without nutrients (20 µM DIN). Following four weeks of these treatments (“stress phase”), all plants were returned to control conditions (“recovery phase”) for another three weeks. Results showed that exposure to only thermal stress favoured growth, compared to exposure to only nutrient increase that reduced Fv/Fm and growth but favoured algae proliferation. Exposure to the combined thermal and nutrient stress, negatively affected seagrass performance resulting in high mortality observed after four weeks of combined exposure. The negative effects of combined stressors were stronger in populations with low “nutrient history”. Additionally, we propose two novel fatty acid (FA) biomarkers, one based on FA unsaturation, 16:3n-3/16:2n-6, and the other on FA elongation processes, 18:2n-6/16:2n-6. Fatty acid analyses showed a significant decrease in 16:3n-3/16:2n-6 and 18:3n-3/18:2n-6 with increases in temperature and nutrients; a more drastic decline was found under the interaction of both stressors. Our results point out that C16 PUFAs, that are synthesized within the “prokaryotic pathway”, are more sensitive to thermal and the combined thermal + nutrients stressors than C18 PUFAs, which are synthesized within the “eukaryotic pathway”. In general, following a month of control conditions, a clear recovery of most of the seagrass descriptors was observed, highlighting the great capability of Halophila stipulacea to recover from stress conditions. Our results have important ecological and management implications to the seagrass meadows in the GoA and elsewhere. For seagrasses to survive climate change, managers must put efforts into limiting other stressors such as eutrophication that would potentially reduce the seagrass resilience to climate changeThis research and SEASTRESS project was funded by The Israeli Ministry of Science and Technology (MoST), Israeli-Italian binational Grant Number 3-15152 (GW, PBC) and ICA in Israel, Grant 03-16-06a (GW). This work was also supported by the SEANARIOS project (SEAgrass sceNARIOS under thermal and nutrient stress: FKZ 03F0826A), an Israeli-German Scientific Cooperation, funded by the German Federal Ministry of Education and Research (BMBF), jointly with the SEASTRESS project, an Israeli-Italian Scientific Cooperation, funded by the Ministry of Science and Technology of Israel (MOST). Lastly, this project was also supported by the project IN607B 2020/04 funded by Xunta de Galicia, Galician Agency for Innovation, GAIN (Spain)Peer reviewe