Multiple stressor effects on organic carbon degradation and microbial community composition in urban river sediments in a mesocosm experiment

Microorganisms in river sediments are the primarily responsible organisms for the turnover of dissolved organic carbon (DOC) in these systems and therefore are key players for river ecosystem functioning. Rivers are increasingly threatened by multiple stressors such as salinization and temperature rise, but little is known about how microbial DOC-degradation responds to these stressors and whether this function recovers after stressor release. Here, we investigated the direct and indirect effects of salinity and temperature increase and decrease on microbial communities and their ability to degrade DOC in river sediments using the outdoor experimental mesocosm system ExStream. Composition of sediment microbial communities was determined at the end of acclimatization, stressor, and recovery phase using 16S rRNA gene sequencing. At the same time points, DOC degradation rates were quantified in additional microcosm incubations based on isotopic changes of CO2 with the help of reverse stable isotope labelling. Our results showed that raising the salinity by 154.1 mg Cl- L-1 and temperature by 3.5 {degrees}C did not affect DOC degradation during the stressor phase but significantly increased DOC degradation in the recovery phase after stressors were released. Likewise, microbial community composition stayed constant during acclimation and stressor phase, but became more diverse in the recovery phase. The results indicate that microbial community composition and functioning were resistant towards both stressors, but responded to stressor release due to indirect effects of stressor increase and release on the riverine food web. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=113 SRC="FIGDIR/small/602289v1_ufig1.gif" ALT="Figure 1"> View larger version (36K): org.highwire.dtl.DTLVardef@1074eccorg.highwire.dtl.DTLVardef@c634a1org.highwire.dtl.DTLVardef@a96182org.highwire.dtl.DTLVardef@40ae8e_HPS_FORMAT_FIGEXP M_FIG C_FIG