Chemical characterization of dissolved organic matter (DOM) in seawater : structure, cycling, and the role of biology

Abstract

The goal of this thesis is to investigate three different areas relating to the characterization of dissolved organic matter (DOM): further determination of the chemical compounds present in high molecular weight DOM (HMWDOM), the cycling of individual compounds within the HMWDOM pool, and the biological controls on DOM release. The first section of this thesis provided additional molecular level characterization of HMWDOM. While some individual components have been identified, the total yield of compounds isolated is much smaller than the amount expected by spectroscopic analysis by nuclear magnetic resonance (NMR) spectroscopy. Since the majority of the as yet unidentified carbon was inferred to be carbohydrate, periodate over-oxidation was used to analyze this fraction. Analysis of both surface and deep water HMWDOM samples indicated that the majority of the carbon present was periodate over-oxidizable, including 70% of the aliphatic NMR signal. Comparison of the periodate demand for HMWDOM versus that for linear glucopolysaccharide standards indicated that HMWDOM had a greater degree of branching. Based on the changes in the 'H NMR spectra during the reaction, it was concluded that 6-deoxysugars were the primary compounds in the unidentified fraction of HMWDOM. Compound specific radiocarbon analysis can provide information about the cycling and relative ages for individual HMWDOM components. In the second section of this thesis, a new method was presented for the purification of individual underivatized amino acids hydrolyzed from HMWDOM. This new separation protocol utilized various chromatographic techniques, including cation exchange chromatography and high-pressure liquid chromatography (HPLC) using C18 and strong cation exchange (SCX) columns. Six amino acids were isolated from a HMWDOM sample with sufficient purity and quantity for radiocarbon analysis. These amino acids had a range of Δ14C values, from 121% to 454%. The final section investigates biological controls on the dissolved organic nitrogen (DON) pool. Total hydrolyzable amino acids (THAA), and nucleic acids were measured for four incubations: a control incubation, a grazer added incubation, a zero virus incubation, and a 10 times virus incubation. Comparison to the control showed THAA and nucleic acid release were influenced by viruses but not grazers.