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

Abstract

In this study, we developed a novel extraction method for alginate, a major organic component derived from brown algae, in marine sediments and evaluated its applicability for quantitative analysis. Alginate analytical methods have been established in food chemistry: we modified these techniques to apply them to marine sediments, which are characterized by the cation composition (e.g., Ca, Mg, Fe) and humic substance-like high-molecular-weight organic compounds. By modifying the protocol through the addition of EDTA-2Na and the omission of ethanol precipitation, we improved the extraction efficiency of alginate from marine sediments, as demonstrated by spike recovery tests where recoveries of Na+-Alg significantly increased from 38.7 % with the conventional method to 64.7 %–82.6 %. Enzymatic degradation tests using alginate lyase confirmed that a portion of the extracted uronic acids from marine sediments (at least 34 %) was derived from alginate, verifying its presence in the natural samples. Using this modified method, alginate contents in sediment samples from coastal waters around Hokkaido, Japan, 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. However, these values may be overestimated due to the presence of other uronic acids; therefore, they should be interpreted with caution rather than being directly equated with absolute alginate content.