Since the first occurrence of the Great Atlantic Sargassum Belt (GASB) in 2011, scientists have been searching for explanations for this yearly recurring phenomenon. Climate warming-induced changes in winds and currents, in combination with increased anthropogenic nutrient inputs to the tropical Atlantic, have been identified as potential drivers of bloom formation. Yet, a detailed understanding of the physiological response of holopelagic sargassum to varying nutrient conditions is lacking. In summer 2022, we studied the effect of different nitrate (0 – 35 µM) and phosphate (0 – 2.2 µM) conditions on carbon fixation and growth of Sargassum fluitans III in ex situ culture systems. Stable isotope incubation experiments (¹³C-labelled dissolved inorganic carbon and ¹⁵N-labelled nitrate) were performed to quantify carbon and nutrient uptake rates under different conditions. While nutrient uptake rates were directly linked to nutrient availability, increasing nutrient conditions did not result in increasing carbon fixation and growth rates. Instead, it let to a decreasing biomass carbon to nitrogen ratio, potentially hinting at internal nutrient storage. Our results revealed that the effect of nitrate and phosphate on the growth of sargassum is more complex than previously assumed, posing new questions regarding the role of anthropogenic nutrient inputs in the GASB formation. Further, our results provide new insights into the regulation of carbon fixation and therewith the overall carbon dioxide removal and climate mitigation potential of holopelagic sargassum.

Primary Presenter: Miriam Philippi, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (miriam.philippi@awi.de)

Authors:

Miriam Philippi, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany  (miriam.philippi@awi.de)

Philipp F. Hach,   (phach@gmx.de)

Sarah Lena Eggers, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany  (lena.eggers@awi.de)

Franziska Elmer, Seafields Solutions Ltd  (Franziska.Elmer@seafields.eco)

Hagen Buck-Wiese, Max Planck Institute for Marine Microbiology, Bremen, Germany and Marum Centre for Marine Environmental Research, University of Bremen, Bremen, Germany  (hbuck@mpi-bremen.de)

Jacqueline Stefels, Groningen Institute of Evolutionary Life Sciences, University of Groningen, Groningen, The Netherlands  (j.stefels@rug.nl)

Evelyn Raquel Salas-Acosta, Unidad Académica de Sistemas Arrecifales, Instituto de Ciencias del Mar y Limnología-UNAM, Puerto Morelos, Q. Roo, Mexico  (evelynraquel.salas@gmail.com)

Eden Magaña-Gallegos, Unidad Académica de Sistemas Arrecifales, Instituto de Ciencias del Mar y Limnología-UNAM, Puerto Morelos, Q. Roo, Mexico  (eden.maga@cmarl.unam.mx)

Brigitta I. van Tussenbroek, Unidad Académica de Sistemas Arrecifales, Instituto de Ciencias del Mar y Limnología-UNAM, Puerto Morelos, Q. Roo, Mexico  (vantuss@cmarl.unam.mx)

Mar Fernández-Méndez, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany  (mar.fernandez-mendez@awi.de)