Linkages between decomposition of terrigenous dissolved organic matter (DOM) and microbial community structure: influences of chemical DOM characteristics.

The quality of dissolved organic matter (DOM) has been shown to influence bacterial community structure and subsequent the degradation of DOM. In this study we used a riverine DOM source to generate terrestrial DOM fractions of different quality which were mixed with a brackish bacterial community from the Baltic Sea. DOM was either added as particle-free river water (0.22 µm filtered) or modified by lyophilization and ultrafiltration. Dissolved organic carbon (DOC) consumption, DOM composition, microbial activities and shifts in the microbial community structure were compared to unamended Baltic Sea water and river water control experiments over a four week incubation period. The molecular composition of DOM, analyzed by ultrahigh-resolution mass spectrometry, indicated a clear difference of the DOM composition in the set-ups. During the course of the experiments we measured an average of 8% DOC loss. However, the molecular composition of DOM, which included several thousand molecular formulae, did not change significantly in any of the experiments. This may be due to degradation of colloidal or low-molecular weight DOM (<150 Da), that both escape the analytical window of the mass spectrometry. In contrast to DOM, bacterial numbers, activities and community compositions experienced drastic changes during the experiment. Whereas typical bacteria of the Baltic Sea were identified in the first days of the experiment, a gammaproteobacterial sequence previously found in hydrocarbon degrading studies became dominant after one week in all Baltic Sea treatments including the controls. The freshwater control experiment (where the added DOM originates from) experienced a shift towards a betaproteobacterial and flavobacterial dominated bacterial community that shows no similarity with the brackish bacterial community. This indicates that the addition of terrestrial DOM, probably due to the mainly refractory nature, has only minor effects on the change of the brackish bacterial community and that presumably only low-molecular weight DOM were utilized in the experiments.