Technical note: Development of an extraction protocol and colorimetric analysis for alginate in marine sediment

Abstract. Transportation of organic carbon from coastal macroalgae, particularly brown algae, to deeper ocean layers has recently attracted attention as an effective mechanism for carbon sequestration. However, no observational data are currently available on the amounts of organic carbon derived from brown algae in marine environments such as seawater or sediments. In this study, we developed an extraction protocol for alginate, a polysaccharide unique to brown algae that accounts for 20–30 % of their dry weight, to provide quantitative and direct evidence of brown algal carbon sequestered in ocean sediments. Alginate extraction and colorimetric analysis methods are well established in food chemistry: we modified these techniques to apply them to marine sediments, which are characterized by high concentrations of cations (e.g., Ca, Mg, Fe) and humic substance-like high-molecular-weight organic compounds. We applied this new method to sediment samples collected from coastal waters around Hokkaido, Japan. Alginate contents were quantified as 6.11–26.0 mg m⁻² in Funka Bay, 39.0–41.3 mg m⁻² in Hakodate Bay, 11.8–14.7 mg m⁻² off Cape Esan, and 58.3–74.1 mg m⁻² off Muroran.