Dissolved organic matter specialization drives temporal dynamics of simplified bacterial communities in a microcosm experiment

Microorganisms form the base of aquatic food webs and play a key role in the global carbon cycle by decomposing dissolved organic matter (DOM). Climate change is predicted to shift the composition of DOM in northern freshwaters from predominantly small, low molecular weight compounds to aromatic, high molecular weight compounds. However, the consequences of these changes for bacterial communities and their role in wider ecosystem processes are poorly understood. Here, we used a 14-day incubation experiment to test how the same bacterial community responded to diverse DOM sources that varied in their bioavailability and were representative of predicted compositional changes in northern waters. Using full-length 16S amplicon sequencing, we found that bacterial communities differed in their composition across sources within 24 h of exposure to novel DOM, but changed similarly over time thereafter, primarily driven by consistent increases in the relative abundance of one generalist species. Microbial reworking of DOM, characterized using ultra-high-resolution mass spectrometry, led to an increase in the relative abundance of less bioavailable compounds on sources with higher initial bioavailability. Our study advances previous work by suggesting that interactions between bacteria and DOM under novel environmental conditions depend on their level of specialization and that any losses in resource specialization may have consequences for the persistence and trophic transfer of carbon in aquatic food webs.