Tannin geochemistry of natural systems: method development and application
Polyphenolic tannin comprises a significant proportion of the leaves, needles, and barks of vascular plant tissues---all major contributors to terrigenous organic matter cycling in aquatic systems. However, virtually nothing is known at the molecular level about tannin diagenesis despite the increased sensitivity and unambiguity of molecular characterizations vs. bulk techniques. Due to the analytical challenges in measuring tannin molecularly, there has been little crossover to ecology from the large body of research in natural products. This thesis presents the development of a new method for molecular-level condensed tannin analysis, a survey of source tissues, and a geochemical application focussed on mangrove leaf diagenesis in a tropical estuary.The developed method consists of acid hydrolysis (HCl) of samples in acetone, water, and phloroglucinol as a nucleophile for carbocation capture. Analytes are partitioned into ethyl acetate, dried, trimethylsilyl derivatized, and quantified by gas chromatography. The method reproducibly yields intact terminal units and phloroglucinol-adducted extender units, in addition to flavones, flavanones, triterpenoids, and carboxylic acids.Molecular tannin signatures were obtained from 117 source tissues, including leaves, needles, woods, and barks from tropical and temperate forests, coasts, and grasslands. Conifer needles were distinguished by high prodelphinidin content while dicotyledon leaves alone yielded both triterpenoids and flavones. Barks were distinguished by flavanones and tetracosanoic acid.Molecular tannin in a degradation sequence of mangrove leaves ( Rhizophora mangle) revealed several important trends not evident by bulk techniques. Degree of polymerization (average chain length) highlighted early leaching processes and a subsequent shift toward abiotic or microbially-mediated chemical reactions. An increased degree of hydroxylation corresponded to increased degradation rate, a trend that may be indicative of changes in redox or pH conditions leading to quinone formation. The apparent inverse correlation between basic amino acids and molecular tannin might signal Schiff base reactions partially responsible for nitrogen immobilization. Measured molecular tannin was second in abundance only to carbohydrates in the senescent yellow leaves entering the estuarine system. Leaching losses of 30% tannin highlight the potential importance of tannin in dissolved organic matter.Based on these studies, molecular level tannin shows great promise as a tool for geochemical studies.
- Oceanography