Cycling of dissolved and particulate organic matter at station Aloha: Insights from 13C NMR spectroscopy coupled with elemental, isotopic and molecular analyses

Abstract

Compositions of ultrafiltered dissolved organic matter (UDOM) and ultrafiltered particulate organic matter (UPOM) were characterized in samples collected from a depth profile (20–4000 m) in the North Pacific at Station Aloha. 13C Nuclear Magnetic Resonance (NMR) analyses together with δ 13C values, carbon/nitrogen (C/N) ratios and molecular characterizations of UDOM and UPOM indicate different bulk chemical compositions and sources for these two size fractions. Carbohydrates and amino acids are the major biomolecules present in UDOM and UPOM. At all depths, UPOM had higher amino acid and lower carbohydrate contents compared to UDOM. UDOM and UPOM samples showed a decrease in the relative contribution of carbohydrates to the total organic carbon with increasing depth, whereas the contributions of lipids increased. Amino acids did not show any clear depth trends for UDOM, but decreased in UPOM. The compositional trends with depth indicate that selective degradation processes, which preferentially remineralize reactive biomolecules such as carbohydrates, affect UDOM and UPOM compositions in the water column. Molecular analyses of carbohydrates and amino acids characterized ∼9% of the C in UDOM and ∼28% of the C in UPOM. Although a relatively small proportion of the total C was characterizable with molecular analyses, the depth trends and the ratio of amino acids to carbohydrates in UPOM and UDOM were similar to those determined by 13C NMR. This suggests that the organic matter fraction characterized by molecular analyses is representative of the molecularly uncharacterized fraction of organic matter. It is postulated that the molecularly uncharacterized fraction is encapsulated or shielded by a hydrolysis-resistant matrix or the individual amino acid and sugar units have been altered to form a chemical entity unrecognizable by the molecular analyses.