Mangrove Leaf Tannins: Their Fate and Role in Dissolved Organic Nitrogen Cycling in Subtropical Coastal Environment.
Nagamitsu Maie1, Rudolf Jaffé1, and Oliva Pisani2. (1) Southeast Environmental Research Center, Florida International Univ, University Park Campus, Miami, FL 33199, (2) Department of Chemistry & Biochemistry, Florida International University, University Park, Miami, FL 33199
Mangroves are important vegetation in tropic and subtropical coastal regions in the world. Litterfall plays a crucial role in the nutrient cycling of mangrove systems due to the large amount of organic mater returned to the aquatic system through leaf senescence. The abscised leaves release substantial amounts of inorganic nutrients and Dissolved Organic Materials (DOM), which contribute sugars, proteins, and polyphenols to the surrounding water environment within a relatively short time period (1). Sugars and proteins are very susceptible to microbial degradation, and thus can be quickly incorporated into food webs. However, tannins, a class of polyphenols, are known to suppress nutrient utilization for microorganisms through complexation and precipitation of N-containing compounds. Since mangrove leaves contain a large amount of condensed tannins (CT) (1, 2), it is expected that CT leached from mangrove leaves may sequester proteinaceous materials, preventing them from rapid loss from the mangrove forest. The chemical structure of CT is modified with time, leading to a decrease in the protein binding capacity (3). As such, labile organic N could be slowly released to estuarine/coastal ecosystems resulting in a more effective, long term usage of N-containing compounds in mangrove fringe areas. While the ecological importance of CT in forest and heath environments is widely recognized (4-6), their fate and role in N cycling in estuarine/coastal environments are unknown. Our previous studies on the characterization of DOM in the subtropical, oligotrophic coastal wetland, Florida Coastal Everglades, USA, suggest that CT concentration is quite low in natural water even in the areas surrounded by mangrove forests (1). What is the environmental fate of mangrove-derived CT, and how do tannins affect the biogeochemical cycling of Dissolved Organic Nitrogen (DON) in the coastal ecosystems? To answer these questions, we addressed possible biogeochemical processes of mangrove CT in coastal environments, which include: (1) self-aggregation of CT at different salinities, (2) sequestration of DON in natural water by CT, (3) adsorption of CT onto sediment, (4) biological and (photo)chemical alteration of CT, and subsequent change in the protein binding capacity. These processes were investigated by a series of model experiments using purified CT extracted from senescent red mangrove (Rhizophora mangle) leaves. Our results suggest that a major portion of CT leached in the aquatic environment can be promptly eliminated through self-aggregation at high salinity (> 15‰) and also by adsorption onto sediment. A portion of DON in natural water is found to co-precipitate with CT, suggesting the possibility of mangrove forest can be a sink of DON in adjacent marine water. The chemical structure of CT alters in natural water with time, and the process is accelerated with exposure to sunlight (solar-simulated light). Our experiment indicates that the diagenetic products of CT are important precursors of chromophoric DOM (CDOM) in aquatic environments of coastal ecosystems. Investigation of the stability of CT-protein complex in an incubation experiment showed that CT-protein complex was not dissociated during 28-d of dark incubation at room temperature. However, sunlight exposure helped the dissociation of the complex by releasing proteins into the ambient water body after 2-d of incubation (which corresponds to 6-d of natural sun light dose). The dissolved protein concentrations returned to the control level (without CT treatment) after 7-d of incubation (21-d of natural light dose). This is the first indication that leached tannins may control protein (and DON) dynamics in mangrove forests and adjacent aquatic systems. References: (1) Maie, N. et al. Biogeochem. in press. (2) Benner, R. et al. 1990. Geochim. Cosmochim. Acta 54: 2003-2013. (3) Maie, N. et al. 2003. Soil Biol. & Biochem. 35: 577-589. (4) Northup et al. 1998. Biogeochem. 42: 189-220. (5) Schimel et al. 1998. Biogeochem 42: 221-234. (6) Preston et al. 1999. In: Plant polyphenols 2: Chemistry, Biology, Pharmacology, Ecology, Kluwer, 825-841.