Mangrove tannins in aquatic ecosystems: Their fate and possible influence on dissolved organic carbon and nitrogen cycling

Abstract

We describe the fate of mangrove leaf tannins in aquatic ecosystems and their possible influence on dissolved organic nitrogen (DON) cycling. Tannins were extracted and purified from senescent yellow leaves of the red mangrove (Rhizophora mangle) and used for a series of model experiments to investigate their physical and chemical reactivity in natural environments. Physical processes investigated included aggregation, adsorption to organic matter-rich sediments, and co-aggregation with DON in natural waters. Chemical reactions included structural change, which was determined by excitation-emission matrix fluorescence spectra, and the release of proteins from tannin-protein complexes under solar-simulated light exposure. A large portion of tannins can be physically eliminated from aquatic environments by precipitation in saline water and also by binding to sediments. A portion of DON in natural water can coprecipitate with tannins, indicating that mangrove swamps can influence DON cycling in estuarine environments. The chemical reactivity of tannins in natural waters was also very high, with a half-life of less than 1 d. Proteins were released gradually from tannin-protein complexes incubated under light conditions but not under dark conditions, indicating a potentially buffering role of tanninprotein complexes on DON recycling in mangrove estuaries. Although tannins are not detected at a significant level in natural waters, they play an important ecological role by preserving nitrogen and buffering its cycling in estuarine ecosystems through the prevention of rapid DON export/loss from mangrove fringe areas and/or from rapid microbial mineralization.

Assessing the dynamics of dissolved organic matter (DOM) in coastal environments by excitation emission matrix fluorescence and parallel factor analysis (EEM‐PARAFAC)

“…Since the fluorescence characteristics of component 4 were similar to those of tannins (Maie et al 2007(Maie et al , 2008 as well as to a blue-shifted tryptophan-like fluorophore (Maie et al 2006), it is difficult to determine the precise origin of component 4 based only on the fluorescence characteristics. However, the relationship with salinity as well as the transect distribution of component 4 were almost identical to those of component 5 (Figs.…”

“…Tryptophan-like fluorophores shifted to shorter emission wavelength were also found concurrently with tyrosine-like fluorophores in the ultrafiltered DOM of the Florida coastal Everglades (Maie et al 2006). In addition to aromatic amino acid residues in DOM, polyphenols such as galic acids and tannins exhibited such fluorescence characteristics (Maie et al 2007). The fluorescence characteristics of component 4 were also similar to those of tannins, showing a peak at excitation/emission of 275/ 313 nm (Maie et al 2007(Maie et al , 2008.…”

Basin scale distribution of chromophoric dissolved organic matter in the Pacific Ocean

“…Sarpal et al (1995) suggested that the shoulder on the absorption spectrum at 280-290 nm was caused in part by proteins. In addition to proteins, other bio-components have absorption mostly at wavelengths shorter than 300 nm (Zika 1981;Maie et al 2007Maie et al , 2008.…”

Changes of tannin and nutrients during decomposition of branchlets of Casuarina equisetifolia plantation in subtropical coastal areas of China

“…On the one hand, the increase in nitrogen concentrations in leaf litter may be due to greater loss of carbon and mass than nitrogen, and on the other hand, the immobilization of external nitrogen, such as microbial biomass (He et al 1988, Wang et al 2012) and dissolved organic nitrogen (Maie et al 2008), and internal nitrogen, such as leaf litter protein and amino acids (Gallardo and Merino 1992). We observed a negative correlation between TP and nitrogen contents at various stages of decay (Table 1), which is indicative of humification reactions.…”