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 distribu-tions 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 (Mw) of DOM decreased, as high molec-ular weight components were lost from solution. After 12 h, the Mw of DOM increased, primarily because of a loss of intermedi-ate 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 com-ponents to the bacterial surfaces, perhaps followed by metabolism. This suggested interpretation is consistent with previous observa-tions of preferential adsorption of higher molecular weight compo-nents to viable but non-metabolizing Bacillus subtilis and to min-eral 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 use