| Summary: | Abstract Background Hypercapnia and elevated temperatures resulting from climate change may have adverse consequences for many marine organisms. While diverse physiological and ecological effects have been identified, changes in those molecular mechanisms, which shape the physiological phenotype of a species and limit its capacity to compensate, remain poorly understood. Here, we use global gene expression profiling through RNA-Sequencing to study the transcriptional responses to ocean acidification and warming in gills of the boreal spider crab Hyas araneus exposed medium-term (10 weeks) to intermediate (1,120 μ atm) and high (1,960 μ atm) P CO 2 at different temperatures (5°C and 10°C). Results The analyses reveal shifts in steady state gene expression from control to intermediate and from intermediate to high CO 2 exposures. At 5°C acid–base, energy metabolism and stress response related genes were upregulated at intermediate P CO 2 , whereas high P CO 2 induced a relative reduction in expression to levels closer to controls. A similar pattern was found at elevated temperature (10°C). There was a strong coordination between acid–base, metabolic and stress-related processes. Hemolymph parameters at intermediate P CO 2 indicate enhanced capacity in acid–base compensation potentially supported by upregulation of a V-ATPase. The likely enhanced energy demand might be met by the upregulation of the electron transport system (ETS), but may lead to increased oxidative stress reflected in upregulated antioxidant defense transcripts. These mechanisms were attenuated by high P CO 2 , possibly as a result of limited acid–base compensation and metabolic down-regulation. Conclusion Our findings indicate a P CO 2 dependent threshold beyond which compensation by acclimation fails progressively. They also indicate a limited ability of this stenoecious crustacean to compensate for the effects of ocean acidification with and without concomitant warming.
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