Stable carbon isotope biogeochemistry of monosaccharides in aquatic organisms and terrestrial plants

Abstract

Carbohydrates are one of the major biochemical groups in lakes and oceans. In cells, they are primary metabolic energy compounds, serving as storage for photosynthetically fixed carbon, and providing structural support. The cycling of carbohydrates in aquatic and terrestrial environments is important for determining food web interactions and for understanding fluxes of organic matter. We investigated compound-specific isotopic ratios of individual monomeric carbohydrates (i.e. sugars) from seven algal and cyanobacterial primary producers, three angiosperms and gymnosperms, two bacterial cultures, and seven marine invertebrates in order to determine the isotopic fractionation of these molecules relative to bulk carbon isotopic compositions. The carbon isotopic composition ( δ 13C) of individual sugars varied greatly within an organism (7.6 ± 4‰), with the greatest range in zooplankton where sugar δ 13C varied by 15‰. The calculated δ 13C of the total combined carbohydrate (TCC) pool for individual species was on average enriched in 13C by 4.6 ± 4‰ relative to the bulk C isotopic composition. However, whole cedar wood had TCC with isotopic compositions more depleted than bulk δ 13C value. Glucose produced by heterotrophic organisms was enriched in 13C relative to all other monosaccharides. These small, but potentially important, differences might be useful in tracing the flow of carbon in difficult to characterize pools of organic matter, such as the dissolved organic carbon in the ocean. The enrichment in 13C of glucose produced in abundance by heterotrophs may be a controlling factor in the stepwise enrichment in carbon isotopes with increasing trophic level in food webs.