50-cm deep sediment cores were taken in saltmarsh, seagrass, mangroves and unvegetated areas around the German Bight, Malaysia and Columbia in 2022 and 2023. Up to 3 points per ecosystem were sampled along a transect, in total 93 cores were analysed. Carbohydrates were sequentially extracted using MilliQ-water and 0.3 M EDTA for later analyses. For more specific analysis enzyme-linked immunosorbent assay (ELISA) was used for detection of fucoidan and arabinogalactan-protein glycan as described in the following studies (Vidal-Melgosa et al., 2021 and Cornuault et al., 2014). In short, 100 µL of sediment extracts were added to a pre-coated 96-well plate and incubated overnight at 4°C. The signal was developed using primary antibodies, BAM1 (fucoidan) and JIM13 (arabinogalactan-protein glycan), diluted 1:10 in skim milk PBS solution, followed by anti-rat antibody at a 1:1000 dilution in the same solution. Absorbance was measured at 450 nm using a Spectramax Id3 plate reader (Molecular Devices).

The samples were analysed on 2024-08-07 including the processing blanks for ELISA analysis.