Acclimatory gene expression of primed clams enhances robustness to elevated pCO2

Sub-lethal exposure to environmental challenges may enhance ability to cope with chronic or repeated environmental change, a process known as priming. In a previous study, post-larval pre-exposure to seawater enriched with elevated pCO2 improved growth and oxidative status of juvenile Pacific geoduck Panopea generosa under repeated encounters (Gurr et al. 2021), indicating the potential for transcriptional shifts to drive modified organismal and cellular phenotypes. Juvenile clams were sampled and TagSeq gene expression data analyzed after 1) acclimation at the pediveliger stage under ambient (921 µatm, naïve) and moderately-elevated pCO2 (2870 µatm, pre-exposed) for 110 days (40-150 days post-fertilization; day 0); then 2) following a second exposure in a reciprocal fashion to three pCO2 treatments (ambient: 754 µatm; moderately-elevated: 2750 µatm; severely-elevated: 4940 µatm, day 7), a return to ambient pCO2 (day 14), and a third exposure in a reciprocal fashion to two pCO2 treatments (ambient: 967 µatm; moderately-elevated: 3030 µatm, day 21). Pre-exposed geoducks (as quantified at day 0) showed frontloading of genes for stress and apoptosis/innate immune response, homeostatic processes, protein degradation, and transcriptional modifiers under elevated pCO2. Across timepoints, pre-exposed geoducks had sets of genes with higher expression relative to naïve animals which were also responsive to subsequent encounters. These genes were enriched for functions including quality control of mitochondria and immune defense under elevated pCO2 and energy metabolism and biosynthesis under ambient recovery. In contrast, gene sets with higher expression in naïve clams were consistently larger and were enriched for fatty-acid degradation and glutathione components at all timepoints, suggesting naïve clams could be depleting endogenous fuels, with unsustainable energetic requirements if changes in carbonate chemistry persist. Collectively, our transcriptomic data indicates low pH/high pCO2 priming of geoduck clams in post-larval periods could, via gene expression regulation, enhance robustness to environmental change. Such priming approaches may be beneficial for aquaculture husbandry practices, as global seafood demand intensifies concurrent with increasing climate change in marine systems.