BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-11-1479-2014 Enhanced bacterial decomposition with increasing addition of autochthonous to allochthonous carbon without any effect on bacterial community composition Attermeyer K. 1 Hornick T. 1 Kayler Z. E. 2 Bahr A. 3 Zwirnmann E. 4 Grossart H.-P. https://orcid.org/0000-0002-9141-0325 1 5 Premke K. 2 4 Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Experimental Limnology, 16775 Stechlin, Germany Leibniz Centre for Agricultural Landscape Research (ZALF), 15374 Müncheberg, Germany UFZ – Helmholtz Centre for Environmental Research – Department of Isotope Biogeochemistry, 04318 Leipzig, Germany Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), 12587 Berlin, Germany Potsdam University, Institute for Biochemistry and Biology, 14469 Potsdam, Germany 20 03 2014 11 6 1479 1489 Copyright: © 2014 K. Attermeyer et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/11/1479/2014/bg-11-1479-2014.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/11/1479/2014/bg-11-1479-2014.pdf

Dissolved organic carbon (DOC) concentrations – mainly of terrestrial origin – are increasing worldwide in inland waters. Heterotrophic bacteria are the main consumers of DOC and thus determine DOC temporal dynamics 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 bacterial community with allochthonous C (<sup>13</sup>C-labeled beech leachate) and increased concentrations and pulses (intermittent occurrence of organic matter input) of autochthonous C (phytoplankton lysate). We then determined bacterial C consumption, activities, and community composition together with the C flow through bacteria using stable C isotopes. The chemical analysis of single sources revealed differences in aromaticity and low- and high-molecular-weight substance fractions (LMWS and HMWS, respectively) between 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 concentrations of added autochthonous DOC resulted in a further, 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) fraction and an increase in bacterial biomass. Changes in DOC concentration and consumption in mixed treatments did not affect bacterial community composition (BCC), but BCC differed 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 indicate that chemical quality rather than source of DOC per se (allochthonous vs. autochthonous) determines bacterial DOC turnover.

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