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The stable carbon-isotope compositions of individual monosaccharides and lipids, as well as the bulk stable carbon-isotope composition of total cell material from different aquatic and terrestrial plants were determined. With the exception of a Phaeocystis sp. bloom sample, monosaccharides were generally enriched in 13 C by 0 to 9” compared to the total cell material and significantly enriched (1 to 16”) in 13 C compared to lipids (fatty acids, phytol, sterols and alkenones) within single organisms. The depletion of 13 C in n-alkyl lipids relative to monosaccharides was larger than the depletion of 13 C in isoprenoid lipids relative to monosaccharides. In addition, an isotopic enrichment was observed in the 13 C content of C 5 monosaccharides compared to glucose in some of the organisms studied, indicating isotopic heterogeneity within carbohydrates. The magnitude of the differences between monosaccharides …