ABSTRACT

Organic molecules dissolved in aquatic ecosystems represent the largest pool of organic matter transported through river networks and one of the most complex mixtures on Earth. The processing of these organic molecules by microorganisms resulting in their conversion to CO2 contributes to the large exchange of CO2 between the aquatic environment and the atmosphere with implications for global climate change. Despite the importance of this phenomenon, as much as 80% of the organic carbon susceptible to microbial metabolism remains unidentified. The goals of this study are to use recent advances in low through-put, ultra-high resolution analytical organic chemistry to identify and characterize the pool of biologically reactive molecules and to extend the ability of commonly used high through-put, low-resolution optical techniques to provide information about the temporal dynamics of these molecules in river networks. The outcomes of this research could advance the understanding of the link between the composition of organic molecules in a river network and the evasion of CO2 to the atmosphere, answering the question of what makes an organic molecule biodegradable. The researchers will work with educators who teach K-12 students in making processes that are invisible to the naked eye accessible and compelling as the educators develop curricula that depict the influence of molecular geochemistry and microbial geobiology to life on Earth.

The researchers hypothesize that: (1) biologically reactive but molecularly uncharacterized humic molecules account for the majority of dissolved organic matter that bacteria respire to CO2; (2) the constituents of colored or fluorescent dissolved organic matter can be associated with groups of individual molecular formulas through the use of advanced statistical analyses, including Spearman Rank and 2-D correlation analyses; and (3) dissolved organic matter molecules that are ubiquitous across distant watersheds span the biological reactivity spectrum, while molecules unique to a watershed are predominantly reactive and readily converted to CO2. The research will be performed in streams within 2 well characterized river basins, one in the temperate forests of Pennsylvania and one in the tropical evergreen forests of Costa Rica. Water samples will be collected under baseflow and storm flow conditions, across stream orders and seasons, and separated into biological reactivity classes using stream water-fed plug flow bioreactors. The samples will be molecularly characterized using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry as well as UV-visible absorbance and fluorescence spectra with excitation emission matrices. The successful completion of the research should advance the ability to use optical sensors to understand carbon flow through river networks and advance the understanding of the molecular nature of the dissolved organic matter that fuels the evasion of CO2 from streams and rivers to the atmosphere.

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