Utilization of Natural Organic Matter (NOM) Substrates by Bacteria

Natural organic matter (NOM) is ubiquitous in terrestrial and aquatic ecosystems, is an important source of carbon and other nutrients for microorganisms, and plays a role in many biogeochemical reactions. Despite its abundance and importance, much about NOM remains unknown. The research presented in this dissertation examines bacterial utilization of NOM and how it affects NOM physicochemical properties. In one study, physicochemical properties of a group of raw filtered water samples are compared with the properties of a group of NOM isolates. Though previous studies have shown a strong and predictive correlation between NOM molecular weight and molar absorptivity in isolates, this study shows that a strong correlation does not exist for RFW samples, thus emphasizing that any comparison of isolates and RFW samples must take into account their inherent differences. The second and third studies combine field and laboratory work to: 1) examine how NOM bioavailability varies with microbial community structure, quantity, and chemical characteristics of NOM, and, 2) to determine how adsorption may affect NOM physicochemical properties. Natural bacterial consortia were isolated from field sites and were used to inoculate NOM. One study showed changes in bacterial productivity and NOM over the course of the experiment that suggest the relative importance of NOM quantity and chemical characteristics to bioavailability is dependent on microbial community composition. A similar study utilized high-pressure size exclusion chromatography (HPSEC) to measure changes in molecular weight distributions of NOM after inoculation. The data show that weight average molecular weight (Mw) of NOM initially decreases, as high molecular weight components are lost from solution, likely due to preferential adsorption of these components. As biodegradation increases, low molecular weight components are lost from solution and Mw of the remaining NOM increases. The final study determines whether bacteria are able to utilize Fe from NOM complexes by quantifying P. mendocina growth under Fe-limited conditions as a function of Fe source: as supplied in the form of natural Fe-containing various NOM isolates, as a dissolved form (FeEDTA), and with a no-Fe-added control. This work also compares bacterial growth with total initial Fe in a variety of NOM samples.