Enhanced bacterial decomposition with increasing addition of autochthonous to allochthonous carbon without any effect on bacterial community composition

Dissolved organic carbon (DOC) concentrations - mainly of terrestrial origin - are increasing worldwide in inland waters. Heterotrophic bacteria are the main con- sumers of DOC and thus determine DOC temporal dynam- ics and availability for higher trophic levels. Our aim was to study bacterial carbon (C) turnover with respect to DOC quantity and chemical quality using both allochthonous and autochthonous DOC sources. We incubated a natural bac- terial community with allochthonous C ( 13 C-labeled beech leachate) and increased concentrations and pulses (intermit- tent occurrence of organic matter input) of autochthonous C (phytoplankton lysate). We then determined bacterial C con- sumption, activities, and community composition together with the C flow through bacteria using stable C isotopes. The chemical analysis of single sources revealed differ- ences in aromaticity and low- and high-molecular-weight substance fractions (LMWS and HMWS, respectively) be- tween allochthonous and autochthonous C sources. Both DOC sources (allochthonous and autochthonous DOC) were metabolized at a high bacterial growth efficiency (BGE) around 50 %. In treatments with mixed sources, rising con- centrations of added autochthonous DOC resulted in a fur- ther, significant increase in bacterial DOC consumption of up to 68 % when nutrients were not limiting. This rise was accompanied by a decrease in the humic substance (HS) frac- tion and an increase in bacterial biomass. Changes in DOC concentration and consumption in mixed treatments did not affect bacterial community composition (BCC), but BCC dif- fered in single vs. mixed incubations. Our study highlights that DOC quantity affects bacterial C consumption but not BCC in nutrient-rich aquatic systems. BCC shifted when a mixture of allochthonous and autochthonous C was provided simultaneously to the bacterial community. Our results indi- cate that chemical quality rather than source of DOC per se (allochthonous vs. autochthonous) determines bacterial DOC turnover.