Isotopic labeling of cultured macroalgae and isolation of 13C-labeled cell wall polysaccharides for trophic investigations

Macroalgae are considered as major primary producers in coastal environments, acting as a global carbon sink. This abundant biomass contains up to 50% of storage or cell wall polysaccharides, which therefore represent a reservoir of organic matter for potential algal consumers. Tracking of natural isotopic abundance (delta C-13 vs. delta N-15) in macroalgae-colonized habitats such as kelp forests and rocky shores previously evidenced the importance of the algal resource to support local and adjacent trophic webs mostly via the microbial detrital pathway. However, such bulk isotopic measures of natural abundances cannot inform precisely on the microbial actors and processes at play for the degradation of selected algal compounds. To overcome these limitations, we developed a stable isotope labeling procedure for cultures of the brown alga Laminaria digitata and tested its ability to yield labeled polysaccharides. Sporophytes of L. digitata were grown in controlled conditions for three months in seawater regularly amended with C-13-labeled sodium bicarbonate. Elemental analysis - isotope ratio mass spectrometry of algal specimens showed a significant enrichment after 10 days of treatment, reaching a maximum of At%C-13 = 3.5321% after 80 days. Sequential polysaccharide extraction from this labeled algal biomass allowed retrieving both alginate and fucose-containing sulfated polysaccharide fractions with high isotopic enrichment (At%C-13 = 3.6279% and 3.5868%, respectively). This labeling protocol opens the way for future studies combining coastal ecosystems trophic interactions and microbial activities toward macroalgal biomass degradation.