Tannin diagenesis in mangrove leaves from a tropical estuary: A novel molecular approach

Molecular-level condensed tannin analyses were conducted on a series of man grove (Rhizophora mangle) leaves at various stages of decomposition in a tr opical estuary. Total molecular tannin yields ranged from 0.5% ash-free dry weight (AFDW) in the most highly degraded black leaves (6-7 weeks in the w ater) up to >7% AFDW in fresh leaves (<1 week in the water). Total tannin e xhibits an intermediate lability in these leaves relative to other measured biochemicals. Leaching is an important mechanism in tannin removal from le aves as indicated by the 30% loss of measurable tannin during a leaching ex periment. Condensed tannin was >80% procyanidin (PC) with the remainder bei ng prodelphinidin (PD). PD tannin, with its higher degree of hydroxylation, proved to be more labile than PC tannin. Average chain length of condensed tannin (degree of polymerization) exhibited an initial increase in respons e to leaching, but later decreased in the subsequent shift toward abiotic o r microbially mediated chemical reactions. Several trends point toward a po ssible condensation reaction in which tannin plays a role in nitrogen immob ilization. These include an apparent inverse correlation between molecular tannin and nitrogen, a positive correlation between molecular tannin and pe rcent basic amino acids, C-13-NMR data indicating transformation of tannin as opposed to remineralization, and C-13-NMR data showing loss of condensed tannin B-ring phenolic carbons coupled with preservation of A-ring phenoli c carbon. In addition to condensed tannin, the molecular method used also y ielded several triterpenoids. Triterpenoids accounted for up to 3.5% AFDW o f the leaf material and exhibited a threefold increase between yellow senes cent leaves entering the estuary and black leaves. This trend is likely due to the weakening of protective cuticular membranes during leaf decompositi on, which leads to increased yields in the acidic conditions used for tanni n analyses. Copyright (C) 2001 Elsevier Science Ltd.