Early diagenesis of plant-derived dissolved organic matter along a wetland, mangrove, estuary ecotone
Limnol. Oceanogr., 49(5), 2004, 1667-1678 | DOI: 10.4319/lo.2004.49.5.1667
ABSTRACT: We studied the role of photochemical and microbial processes in contributing to the transformation of dissolved organic matter (DOM) derived from various plants that dominate the Florida Everglades. Plant-derived DOM leachate samples were exposed to photochemical and microbial degradation and the optical, chemical, and molecular weight characteristics measured over time. Optical parameters such as the synchronous fluorescence intensity between 270 and 290 nm (Fnpeak I), a strong indicator of protein and/or polyphenol content, decreased exponentially in all plant leachate samples, with microbial decay constants ranging from -1.0 d-1 for seagrass to -0.11 d-1 for mangrove (half-life [t1/2] = 0.7-6.3 d). Similar decreases in polyphenol content and dissolved organic carbon (DOC) concentration also occurred but were generally an order of magnitude lower or did not change significantly over time. The initial molecular weight composition was reflected in the rate of Fnpeak I decay and suggests that plant derived DOM with a large proportion of high molecular weight structures, such as seagrass derived DOM, contain high concentrations of easily microbially degradable proteinaceous components. For samples exposed to extended simulated solar radiation, polyphenol and Fnpeak I photochemical decay constants were on average -0.7 d-1 (t1/2 1.0 d). Our data suggest that polyphenol structures of plant-derived DOM are particularly sensitive to photolysis, whereas high molecular weight protein-like structures are degraded primarily through physical-chemical and microbial processes. Furthermore, microbial and physical processes initiated the formation of recalcitrant, highly colored high molecular weight polymeric structures in mangrove-derived DOM. Thus, partial, biogeochemical transformation of plant-derived DOM from coastal areas is rapid and is likely to influence carbon and nutrient cycling, especially in areas dominated by seagrass and mangrove forests.