Transcriptomic and microbiota responses in Mytilus galloprovincialis exposed to environmental concentrations of caffeine and cocaine

The contamination of marine ecosystems by pharmaceuticals is an increasing concern due to their persistence and widespread use. This study investigated, for the first time, the effects of cocaine (COC) and caffeine (CAF), alone and in combination (MIX), on the mussel Mytilus galloprovincialis. Mussels were exposed to environmentally relevant concentrations for 28 days, followed by a 14-day recovery. Gene expression and microbiota analyses were conducted post-exposure and post-recovery. After 28 days, CAF and MIX exposure resulted in 61 and 45 differentially expressed genes (DEGs), respectively, and led to the upregulation of immune-related pathways, while COC exposure resulted in only 2 DEGs but upregulated pathways related to translation and oxidative phosphorylation. MIX also downregulated pathways involved in cell cycle regulation. After depuration, the number of DEGs increased markedly (from 212 to 292 significant genes across the three treatments), with shared disruption of pathways related to xenobiotic metabolism, lipid metabolism, and arachidonic acid metabolism. CAF also modulated dopamine catabolism, immune responses, and apoptosis regulation, indicating conserved mechanisms. A transcriptomic hazard index indicated a "slight" hazard at day 28 to "moderate" risk after depuration in COC- and MIX-exposed mussels (THI=71.25). Microbiota analysis showed that MIX exposure reduced Simpson's Index, with a rebound after depuration. Notably, COC seemed to be the main driver of long-term changes in microbiota composition in MIX-treated mussels, while CAF exposure led to a decline in potentially beneficial bacteria, including Akkermansia, Blautia, and Faecalibacterium. Overall, this study provides novel insights into the combined and long-term effects of COC and CAF on mussel physiology and microbiota, emphasizing the need for further research into the ecological risks of pharmaceutical mixtures in marine environments.