Aspects of the biogeochemistry of carbohydrates in aquatic environments

Abstract

The goal of this thesis is to examine the distribution and diagenesis of carbohydrates in aquatic environments. The following questions are studied: what is the carbohydrate composition of sediment in different environments (e.g., deep-sea oxic; shallow-sea oxic; deep-sea anoxic; fresh-water anoxic; brackish-water anoxic, etc.)? How does the environment at the sediment-water interface affect the composition of the carbohydrate input? How do sedimentary carbohydrates compare to plankton carbohydrates? How do metal-carbohydrate interactions and biological degradation affect the diagenesis of carbohydrates in recent sediments? Can fossil carbohydrates be used as a means to elucidate paleo-environments? In order to investigate these questions in a quantitative manner, a liquid chromatographic sugar analyzer sensitive to 10-10 moles was constructed. Various extraction techniques, involving acid hydrolysis and EDTA treatment, were thoroughly examined to determine lability of sugars, sources of contamination, maximum yields, and reproducibility. Furthermore, several experiments were performed to show that sugars extracted from sediment by EDTA were originally associated with in situ metal ion organic complexes. Although the carbohydrate compositions of sediment from different aquatic environments are remarkably similar, the degree of metal binding of carbohydrates varies between oxidizing and reducing sediments and appears to be related to the degree of biological degradation at the sedimentwater interface. In an oxic environment, biological degradation produces a highly metal-bound carbohydrate residue. In a reducing environment, the degree of biological activity is low (relative to oxic environments) and hence the degree of metal binding of the resulting carbohydrate residue is low. There is no evidence for further abiotic, alteration after burial in either environment. Sewage material dumped into a shallow oxic environment is degraded rapidly despite the high content of potentially toxic metals; these metals are probably tied-up in the metal bound carbohydrate residue. Metal binding appears to fix potentially soluble carbohydrates in situ, thereby inhibiting diffusion. This finding undercuts the previous belief that chromatographic separation of organic molecules along mineral surfaces is a significant diagenetic process. The relative abundances of sugars in acid extracts of sediment and plankton from different aquatic environments are similar; this similarity suggests that plankton is the main source of sedimentar carbohydrates. Carbohydrates in sediment may be used to interpret paleo-environmental fluctuations. For example, the degree of metal binding is indicative of the Eh at the sediment-water interface. The glucose and ribose contents of sediment may be used to estimate relative terrigenous and marine organic inputs, respectively. Paleo-eutrophication conditions in the surface waters also may be discerned.