Bacterial production of particulate and dissolved organic matter in aquatic environments

Dissolved organic matter (DOM) is one of the largest active carbon reservoirs on the earth, which is nearly equivalent to all land plants biomass and atmospheric CO2. Primary production is a main source of DOM, but recent studies show heterotrophic bacteria could be major sources of marine DOM. In this dissertation, production mechanisms and chemical composition of DOM produced by bacteria as well as the contribution of bacteria to marine organic matter were investigated. Heterotrophic bacterial growth and the chemical composition of DOM produced by bacteria were monitored during experiments with artificial media containing glucose as only the carbon source. Glucose was quickly consumed and DOM was released during growth. DOM release indicates bacterial production and growth efficiency are underestimated when only cellular carbon is measured. Selective release of D-alanine occurred during cell growth and division when peptidoglycan is cleaved and newly synthesized subunits are incorporated into the cell wall via transpeptidation. The observed abundances of D-amino acids and amino sugars in DOM from diverse aquatic environments indicate a bacterial source and common decomposition processes. Another incubation experiments which monitored microbial degradation of simple and complex biomolecules were conducted. Both glucose and B. subtilis cells were quickly degraded. The changes in the chemical composition of DOM occurred rapidly during the first two days of the incubation experiment, indicating heterotrophic bacteria were primarily responsible for shaping DOM composition. Accurate measurements of the carbon content of heterotrophic bacteria are essential for quantitative assessments of bacterial contributions to biogeochemical cycles, but considerable variability exists among estimates. Carbon-normalized yields of amino acids in size-fractionated seawater samples and bacterial cultures were used to estimate an average carbon content of marine heterotrophic bacteria in the coastal (10.9 fg C cell-1) and open (5.8 fg C cell-1) ocean. The contributions of living autotrophic (9-14%) and heterotrophic bacteria (2-4%) to suspended particulate organic carbon (POC) at Station ALOHA indicate the contributions of heterotrophic bacteria to suspended POC are much lower than previously thought. Bacterial detritus was abundant and comprised 7-20% of suspended POC at Station ALOHA, indicating that they comprise a large fraction of the submicron particles and colloids in the ocean.