Uptake of dissolved inorganic nitrogen and N2 fixation by Crocosphaera watsonii under climate change scenarios

• Published in: Frontiers in Marine Science
• Main Authors: Umbricht, Jacqueline, Filella, Alba, Klett, Angelina, Vogts, Angela, Benavides, Mar, Voss, Maren
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Media SA 2024
• Subjects: nitrogen fixation; ocean warming; ocean acidification; nitrogen assimilation; Crocosphaera; minicosm experiment; Ocean acidification
• Online Access: https://archimer.ifremer.fr/doc/00908/101954/113100.pdf; https://archimer.ifremer.fr/doc/00908/101954/113101.xlsx; https://archimer.ifremer.fr/doc/00908/101954/113102.pdf; https://doi.org/10.3389/fmars.2024.1388214; https://archimer.ifremer.fr/doc/00908/101954/

The response of N2 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 N2 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 N2 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 N2 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.