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Bacterial Degradation of Dissolved Organic Matter from Two Northern Michigan Streams

This study used high-pressure size exclusion chromatography (HPSEC) to measure the changes in molecular weight distributions of dissolved organic matter (DOM) of two Northern Michigan streams following inoculation with bacterial concentrates from the same locations. During the initial 12 h of the experiment, weight average molecular weight (M w ) of DOM decreased, as high molecular weight components were lost from solution. After 12 h, the M w of DOM increased, primarily because of a loss of intermediate to lower molecular weight components. Leucine incorporation showed little or no bacterial metabolism during the first 12 h, but metabolism increased substantially after 12 h. The initial loss of high molecular weight components during the period of little or no bacterial metabolism suggests preferential adsorption of these components to the bacterial surfaces, perhaps followed by metabolism. This suggested interpretation is consistent with previous observations of preferential adsorption of higher molecular weight components to viable but non-metabolizing Bacillus subtilis and to mineral surfaces. The latter loss of lower molecular weight components was most likely due to bacterial metabolism of the DOM, which is consistent with previous observations that lower molecular weight components are more biodegradable. The HPSEC technique uses 254 nm wavelength for detection and focuses primarily on humic- and fulvic-type components rather than low molecular weight organic molecules, such as carbohydrates. Thus, results confirmed that humic/fulvic components are biodegradable, but did not address other DOM components.