Carbon isotope variability in monosaccharides and lipids of aquatic algae and terrestrial plants

Abstract

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 13C by 0 to 9[per thou] compared to the total cell material and significantly enriched (1 to 16[per thou]) in 13C compared to lipids (fatty acids, phytol, sterols and alkenones) within single organisms. The depletion of 13C in n-alkyl lipids relative to monosaccharides was larger than the depletion of 13C in isoprenoid lipids relative to monosaccharides. In addition, an isotopic enrichment was observed in the 13C content of C5 monosaccharides compared to glucose in some of the organisms studied, indicating isotopic heterogeneity within carbohydrates. The magnitude of the differences between monosaccharides, total cell material and lipids was far greater than previously reported. Thus, selective assimilation of ingested carbohydrates can lead to isotopic enrichments of heterotrophic biomass and zooplankton and subsequent trophic levels. In addition, since dissolved organic carbon (DOC) and particulate organic carbon (POC) has a significant carbohydrate fraction, the d13CPOC and d13CDOC signal will be significantly influenced by the relative amounts and the d13C values of the carbohydrates present. This has significant implications for the isotopic integrity of the organic matter fractions during carbon cycling in food chains of aquatic ecosystems.