Dissolved organic matter (DOM) is the largest pool of reduced organic carbon in most aquatic ecosystems (Hedges 1992) and the main source of substrates fueling bacterial metabolism. In aquatic ecosystems, DOM has different origins: allochthonous, such as from soil runoff, or autochthonous, such as from phytoplankton exudates. Allochthonous and autochthonous DOM differs in several optical (McKnight et al. 1994) and chemical characteristics (Benner 2002). Generally, autochthonous DOM is considered as more labile, whereas organic compounds from terrestrial sources are more recalcitrant to bacterial degradation (Del Giorgio and Davis 2003). This difference in their availability to microbial communities can be explained by the nature of the chemical identifiable substances in both DOM sources. For example, planktonderived DOM is usually enriched in protein and labile polysaccharides, whereas terrestrial DOM contains more humic substances and structural polysaccharides (Benner 2002; Benner 2003). These compositional differences are thought to influence the structure of natural bacterial assemblages (Crump et al. 2003). The heterotrophic assemblage of planktonic bacteria is often dominated by only a few bacterial groups. In most freshwater systems, ß-Proteobacteria is the prevailing group (Glo¨ckner et al. 1999). Actinobacteria have also been found to be numerically important in freshwaters ecosystems of different productivity (Glo¨ckner et al. 2000; Burkert et al. 2003; Warnecke et al. 2005). The third group that seems to be a consistent component of freshwater and marine bacterial communities is the Cytophaga-like bacteria (Kirchman 2002). Recent studies (Kirchman et al. 2004) have focused on assessing changes in the structure and activity of the bacterial community in relation to DOM or examined the ability of different bacterial groups to use various compounds of the DOM pool (Cottrell and Kirchman 2000; Elifantz et al. 2005; Malmstrom et al. 2005). From these studies, it seems that bacteria differ in their capacity to use specific dissolved organic compounds, even when considering large taxonomic groups. Therefore, qualitative changes in the DOM source might trigger a shift in bacterial community structure, activity, or both.
Alpine lakes, located by definition above the tree line, have generally small catchment areas with poorly developed soils and sparse terrestrial vegetation. Consequently, dissolved organic carbon (DOC) concentrations are generally low (,1.5 mg L21), and DOM is mainly autochthonous (Sommaruga et al. 1999; Sommaruga and Augustin 2006). It is generally accepted that the location of the tree line is principally a function of air temperature and that climate warming promotes its upward migration (Hauer et al. 1997), as well as that of alpine flora (Grabherr et al. 1994). In fact, mean air temperatures in the Alps have increased by 1uC since 1980 (Beniston 2000). If present trends in mean air temperature continue, two expected effects on a time scale of 50–100 years are a strong development of the vegetation cover in the catchment and an increase in the export of allochthonous DOM to these lakes (Hauer et al. 1997; Sommaruga et al. 1999; Sommaruga 2001). On the other hand, it is known that the disappearance of the ice cover improves the exchange of gases and nutrients, wind-driven circulation, and light conditions in lakes (Pechlaner, 1971). Hence, shorter periods of ice cover caused by higher air temperatures may have a large stimulatory effect on phytoplankton growth and production (Pechlaner 1971) and on autochthonous DOM production.
To understand how a shift in the dominant DOM source (autochthonous vs. allochthonous) could influence the bacterial community of alpine lakes, we ran an experiment where the natural bacterioplankton assemblage was supplied with DOM derived either from an algal lysate or from a soil extract. A treatment without DOM addition served as a control of the experiment. In addition, lake samples were collected to obtain background information about natural changes in the bacterial community during the course of the experiment.