Gene expression of G. fossarum in response to multiple stressors. Impacts of multiple anthropogenic stressors on the transcriptional response of Gammarus fossarum in a mesocosm field experiment

Freshwaters are exposed to multiple anthropogenic stressors, leading to habitat degradation and biodiversity decline. In particular agricultural stressors are known to result in decreased abundances and community shifts towards more tolerant taxa. However, the combined effects of stressors are difficult to predict as they can interact in complex ways leading to enhanced (synergistic) or decreased (antagonistic) response patterns. Furthermore, stress responses may go unnoticed when only studying abundance changes in ecological experiments, as organisms may have physiological protective pathways to counteract stressor effects and maintain homeostasis. Therefore, we here used transcriptome-wide sequencing data to quantify single and combined effects of fine sediment deposition, increased salinity and reduced flow velocity on the gene expression of the amphipod Gammarus fossarum in a mesocosm field experiment. Stressor exposure resulted in a strong transcriptional suppression of genes involved in metabolic and energy consuming cellular processes, indicating that G. fossarum responds by directing energy to vitally essential processes. Treatments involving increased salinity induced by far the strongest transcriptional response, contrasting the observed specimen abundance data where no effect was detected. Specifically, increased salinity induced the expression of detoxification enzymes and ion transporter genes, which control the membrane permeability of sodium, potassium or chloride. Our findings show that transcriptomic data can significantly contribute to our understanding of multiple stressor effects in freshwater ecosystems.