Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Frontiers in Marine Science 1388214 [13 p.] nitrogen fixation ocean warming ocean acidification nitrogen assimilation Crocosphaera minicosm experiment MEDITERRANEE 2024 fdi:010091090 ENG Frontiers in Marine Science ISI:001265563800001 10.3389/fmars.2024.1388214 https://www.documentation.ird.fr/hor/fdi:010091090 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/2024-08/010091090.pdf 11 Horizon (IRD) The response of N-2 fixation to projected future conditions in the ocean cannot be reliably predicted to date. We conducted a minicosm experiment with pre-acclimated cultures of the globally significant diazotroph Crocosphaera watsonii strain WH8501 ("Crocosphaera"). PH and temperature were altered simultaneously to match the RCP scenarios 4.5 and 6 and investigate a more realistic future scenario compared to studies that focus on changes of a single stressor only. The cell abundance and nitrogen metabolism of Crocosphaera was monitored over 5 days. Our results imply that Crocosphaera is able to simultaneously perform N-2 fixation and assimilate dissolved inorganic nitrogen (DIN, i.e., nitrate and ammonium) under all the conditions tested and implies a competition with non-diazotrophic phytoplankton for DIN, which should be further investigated. Using NanoSIMS analysis of single cells, our results point towards a preference for DIN assimilation over N-2 fixation under more acidic and warmer conditions. Overall, our results show that while the combined alteration of pH and temperature had a negative effect on the diazotroph's growth and N-2 fixation, Crocosphaera is likely to cope well with conditions in the future ocean. The high intra-population variability in nitrogen assimilation pathways may give this species the flexibility to quickly react to environmental changes. 034 ; 021 ; 084 UR235