Advances in understanding the molecular composition of dissolved organic matter and its reactivity in the environment

Abstract

Dissolved organic matter (DOM) is the ultimate product of Earth's systems dynamics. DOM chemical signature is strongly shaped by the interaction among Earth's spheres, such as the atmosphere, the geosphere, the biosphere, and the hydrosphere, but also life and human activity. DOM source, composition, photochemical alteration and availability affect freshwater ecosystems, their carbon and nitrogen fluxes and, thus, the global carbon and nitrogen cycles. The aim of this thesis was to gain an understanding of the molecular composition of DOM and its photochemical and biological reactivity in an environment impacted by anthropogenic disturbance. The York and James River systems within the Cheasapeake Bay watershed provided an excellent study site for such studies. The experiments included monitoring the alteration of DOM and its subcomponents dissolved organic nitrogen (DON) and carbon (DOC) from both natural and anthropogenic sources during photochemical and biological processes. My combined analytical and statistical approach identified the molecular signature of the photolabile and the photoproduced DOM and thebiolabile and the bioproduced DOM ding these processes. My approach depicted differences in DON assimilation by the York River biota depending on the DOM source, where anthropogenic DON showed more bioavailability than naturally derived DON. Furthermore, anthropogenically-derived DON showed an intense bioavailability in the freshwater end member of the James River, VA. These findings suggest that anthropogenic DON is highly reactive in the natural environment and that simple assays examining net consumption or production of bulk DON pools are inadequate for assessing its bioavailability. The studied photochemical alterations of natural and anthropogenic DOM induced production of newly dissolved organic nitrogen (DON) even from natural sources that are relatively N-poor. My experimental results demonstrated that photochemistry transforms DON from complex structural entities to ammonia, aliphatic molecules, and low carbon number molecules that might enhance microbial metabolism, and eventually increases CO2 emissions and reduces DOM concentrations in stream ecosystems.