Microorganisms are the main drivers of key processes in global biogeochemical cycles, including the degradation of organic matter. An important question raised is if the diversity and composition of microbial communities influence their ability to degrade organic matter. Results from previous studies are ambiguous. The development of new techniques for characterization of organic matter as well as microbial structures have, however, opened up for more sophisticated analyses of this relationship. The aim of this study was to investigate if bacterial communities can change their ability to use a carbon source over time. In a transplant experiment we inoculated bacteria from two lakes with organic matter from their own and a foreign source and let them grow there for 42 days. We expected the indigenous community to utilize the carbon source more efficiently than the foreign community. We further expected the efficiency of the two communities to converge over time. Bacterial communities were characterized by Illumina sequencing of the 16S rRNA gene and the organic matter was characterized by fluorescence spectroscopy.
The composition of the bacterial communities depended both on the inoculum and the organic matter provided, as well as time, with the strongest effect of the carbon source. The bacterial carbon processing ability (maximal abundance/consumed carbon), however, did not depend on the inoculum but only on the carbon source, already at the initial measurement after 6 days of growth. The inoculum source did also not affect the processing of DOM, neither in the quantity consumed nor in the qualitative change. Thus, the relationship between bacterial community composition and the processing of dissolved organic matter can be characterized as a diversity response rather than diversity effect, meaning that organic matter influences bacterial communities whereas the opposite effect is more uncertain. Our results, thus, suggests that bacterial community structure is of little importance for this important step in carbon cycling.