CHARACTERIZING THE MOLECULAR MECHANISMS FOR THE BACTERIAL TRANSFORMATION OF RECALCITRANT ORGANIC MATTER IN COASTAL SALT MARSHES

Terrestrially-derived dissolved organic matter (t-DOM) is one of the largest reservoirs of reduced carbon on the planet with approximately 2.5x10¹⁴ g-C entering the global coastal oceans annually. This carbon pool is largely derived from the products of vascular plant degradation, such as humic and fulvic acids, which are recalcitrant due to their enrichment in aromatic moieties. As t-DOM travels through riverine systems to the coastal oceans it becomes increasingly refractory as riverine microbial communities preferentially oxidize the most bioavailable components. However, most chemical tracers diagnostic of t-DOM (e.g. lignin-derived phenols) are removed before reaching the open oceans, suggesting that this material is transformed in the coastal margins. Microbial transformations are expected to contribute to the disappearance of t-DOM in these systems, but the mediating factors and mechanisms are presently unknown. The work described in this dissertation aims to understand how coastal microbial communities degrade t-DOM. Within, I show that a concept well studied in soil systems, the priming effect (PE), may help explain the disappearance of t-DOM in these systems. PE occurs when the presence of a labile carbon source and/or nutrients stimulates the degradation of a recalcitrant carbon source. I focused my studies on coastal margins of the Southeastern United States. These systems represent a transition zone between riverine systems, carrying recalcitrant t-DOM, and marsh systems, which are sites of new production and contain small yet labile carbon pools. As such, they are ideal study sites in which to investigate whether coastal microbial communities can indeed be primed. A first study employed a mesocosm experiment to demonstrate that PE could be invoked in a natural coastal microbial community. A second study demonstrates that concentration and type of labile carbon influence the magnitude, timing and sign of PE in a species-specific manner. Finally, metatranscriptome-based field studies provide evidence that coastal marsh microbial communities are actively catabolizing aromatics and carbohydrates common to the systems. Collectively, these studies enhance our understanding of the mechanisms through which estuarine communities transform t-DOM and provide insights and future directions for continued study in the field.