Hagen Buck-Wiese1,2, Mona A. Andskog3, Nguyen P. Nguyen1,2, Margot Bligh1,2, Eero Asmala4,5, Silvia Vidal-Melgosa1,2, Manuel Liebeke1, Camilla Gustafsson4, and Jan-Hendrik Hehemann1,2*
1 Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany
2 University of Bremen, Center for Marine Environmental Sciences, MARUM, 28359 Bremen, Germany
3 Centre for Coastal Biogeochemistry, Southern Cross University, 2480 Lismore, Australia
4 University of Helsinki, Tvärminne Zoological Station, 10900 Hanko, Finland
5 Geological Survey of Finland, Environmental Solutions, 02151 Espoo, Finland
Brown algae annually convert gigatons of carbon dioxide into carbohydrates, including the complex extracellular matrix polysaccharide fucoidan. Fucoidan sequesters brown algal carbon due to its persistence in the marine environment. The lack of techniques to identify and quantify complex polysaccharides in seawater has impeded accounting for fucoidan carbon in the ocean. We adapted the techniques of anion exchange chromatography, enzyme-linked immunosorbent assay and biocatalytic enzyme-based assay for detection and quantification of fucoidan in seawater. They revealed that fucoidan constitutes up to half of brown algal exudates, a considerable fraction of primary production. Fucus vesiculosus in the Baltic Sea secreted 0.3% of their biomass as fucoidan per day. Dissolved fucoidan concentrations in seawater adjacent to algae reached up to 0.48 mg L-1. F. vesiculosus secreted fucoidan at a rate of 28-40 mg C kg-1 h-1, accounting for 44-50% of all exuded dissolved organic carbon. Our most recent results indicate that both Laminariales and Fucales enrich the surrounding seawater with fucoidan, which shows little no degradation over weeks. Composed only of carbon, oxygen, hydrogen and sulfur, fucoidan secretion does not consume nutrients enabling carbon sequestration independent of algal growth. Extrapolated over a year, the algae sequester more carbon into secreted fucoidan than their biomass. The global utility of fucoidan secretion is an alternative pathway for carbon dioxide removal by brown algae without the need to harvest or bury algal biomass.