| The coastal sediments of Cape Lookout Bight (CLB) contain a complex mixture of organic matter (OM) from different sources with each contributing in varying ways to carbon remineralization and sequestration. This study was undertaken to quantitatively investigate the role of two OM sources---phytoplankton and seagrass---on carbon preservation in coastal sediments and to estimate their availability to the anaerobic sedimentary microbial consortia.;In NC, two species of seagrasses (Zostera marina and Halodule wrightii) predominate, possessing similar isotopic compositions (delta13CTOC = -8.31 to -10.60‰, and delta15NTN = 0.32 to 1.83‰). Mass balance calculations indicate phytoplankton and seagrasses each contribute between 25--60% and terrestrial sources contribute ∼20--30% of the total OM.;The delta13C abundances of hydroxy-fatty acids (FA's) from three chemically-defined pools were examined in seagrasses and sediments as potential indicators for seagrass OM. delta13C values ranged broadly (e.g., alpha-hydroxy FA's ranged from -11.9 to -22.5‰ and -16.32 to -31.85‰ for seagrasses and sediment, respectively) with sedimentary FA's being depleted by ∼3‰ relative to seagrass FA's. Although the range of delta13C values of hydroxy-FA in sediments suggested a mixed source, the unique hydroxy-FA distribution (C18 to C28 alpha-hydroxy FA's) compared to the more common terrestrial distribution (C16 and C18 alpha-hydroxy FA's) suggested little terrestrial input.;To quantify the relationship between the quality of OM and it's degradation by natural microbial consortia, CLB sediment was amended with phytoplankton or seagrass OM and incubated under sulfate-reducing or methanogenic conditions. Organic carbon consumption and production were traced by measuring concentrations and delta13C of particulate organic carbon (POC), total dissolved organic carbon, total organic acids, dissolved inorganic carbon, and methane. Although rates of degradation were rapid for all treatments, seagrass OM was much more refractory than phytoplankton OM. After 341 days, ∼60% of the phytoplankton OM was remineralized whereas only ∼10% of the seagrass OM was remineralized. The rates and extents of remineralization of the phytoplankton OM were nearly the same for both consortia. However, during degradation of seagrass OM, the methanogenic consortium degraded POC slower than the sulfate-reducing consortium, indicating that the methanogenic: consortium may be less efficient in the degradation of more complex organic matter. |
