Abstract

Heterotrophic bacteria are responsible for degrading dissolved organic matter (DOM), and process 50% or more of Earth’s net primary production. Although integral to global nutrient cycling, the complexity of bacterial communities makes it difficult to resolve the mechanisms by which they degrade DOM. Adding to the complexity of this interaction is the compositional diversity of DOM. The St. Lawrence Estuary (SLE) is an important repository for DOM, produced both internally by phytoplankton and externally by terrestrial plants. I aim to identify the bacterial taxa that respond to differential DOM inputs using 16S rRNA abundance as a proxy for metabolic activity. A microcosm experiment was conducted in the SLE in which marine DOM and terrestrial DOM were extracted by ultrafiltration and solid-phase extraction. DOM extracts were amended to microcosms of raw SLE water and incubated at 7°C and 25°C for 32 hours. The Gammaproteobacterial lineage Pseudoalteromonas experienced a 70% increase in metabolic activity in response to HMW marine DOM at both 7°C and 25°C, which was not observed in any other DOM treatment. Terrestrial DOM treatments resulted in a significant increase in alpha-diversity within the bacterial community at 25ºC, indicating a relative increase in the activity of rare bacteria in response to freshwater DOM. Microcosm experiments such as this aim to provide a better understanding of how DOM composition can influence bacterial community structure and metabolism. Considerations for future experiments include transcriptomics analysis to describe the metabolic pathways involved in DOM degradation.