Carbon and hydrogen isotopic composition of sterols in natural marine brown and red macroalgae and associated shellfish

Carbon (δ13C) and hydrogen isotopic compositions (δD) of sterols in natural marine brown (Sargassum filicinum and Undaria pinnatifida), red macroalgae (Binghamia californica and Gelidium japonica), and in shellfish (Binghamia californica, Haliotis discus and Omphalius pfeifferi) feeding on the brown algae have been investigated as a first attempt to understand the isotopic compositions of sterols in natural algae and heterotrophs. Brown algae have 24-methylcholesta-5,24(28)-dien-3β-ol with δ13C value of −19.1 ± 0.1‰ and δD value of −298 ± 4‰, and 24-ethylcholesta-5,24(28)E-dien-3β-ol with δ13C value of −20.3 ± 0.2‰ and δD value of −301 ± 1‰. Red algae have cholest-5-en-3β-ol with δ13C value of −20.9 ± 3.0‰ and δD value of −289 ± 0‰. The δ13C and δD values of these sterols in algae are consistent with those of algal sterols found in marine sediments, suggesting that the isotopic compositions of algal sterols are well preserved in sediments with little change due to heterotrophic effects. The three shellfish have a wide diversity of sterol types but most sterols, except cholest-5-en-3β-ol, have quite similar isotope compositions (δ13C value of −19.5 ± 0.4‰ and δD value of −296 ± 5‰) to the dietary algal sterols. On the other hand, cholest-5-en-3β-ol is slightly depleted in 13C by up to 1.7‰ and significantly enriched in D by up to 49‰ relative to the other shellfish sterols. This may suggest that shellfish sterols are derived from dietary algal sterols with no substantial isotopic fractionation during either assimilation or modification, but cholest-5-en-3β-ol is derived from an admixture of processes including de novo biosynthesis in shellfish.