Effects of ocean acidification increase embryonic sensitivity to thermal extremes in Atlantic cod, Gadus morhua

Thermal tolerance windows serve as a powerful tool for estimating the vulnerability of marine species and their life stages to increasing temperature means and extremes. However, it remains uncertain to which extent additional drivers, such as ocean acidification, modify organismal responses to temperature. This study investigated the effects of CO₂ -driven ocean acidification on embryonic thermal sensitivity and performance in Atlantic cod, Gadus morhua, from the Kattegat. Fertilized eggs were exposed to factorial combinations of two PCO₂ conditions (400 μatm vs. 1100 μatm) and five temperature treatments (0, 3, 6, 9 and 12 °C), which allow identifying both lower and upper thermal tolerance thresholds. We quantified hatching success, oxygen consumption (MO₂ ) and mitochondrial functioning of embryos as well as larval morphometrics at hatch and the abundance of acid-base-relevant ionocytes on the yolk sac epithelium of newly hatched larvae. Hatching success was high under ambient spawning conditions (3-6 °C), but decreased towards both cold and warm temperature extremes. Elevated PCO₂ caused a significant decrease in hatching success, particularly at cold (3 and 0 °C) and warm (12 °C) temperatures. Warming imposed limitations to MO₂ and mitochondrial capacities. Elevated PCO₂ stimulated MO₂ at cold and intermediate temperatures, but exacerbated warming-induced constraints on MO₂ , indicating a synergistic interaction with temperature. Mitochondrial functioning was not affected by PCO₂ . Increased MO₂ in response to elevated PCO₂ was paralleled by reduced larval size at hatch. Finally, ionocyte abundance decreased with increasing temperature, but did not differ between PCO₂ treatments. Our results demonstrate increased thermal sensitivity of cod embryos under future PCO₂ conditions and suggest that acclimation to elevated PCO₂ requires reallocation of limited resources at the expense of embryonic growth. We conclude that ocean acidification constrains the thermal performance window of embryos, which has important implication for the susceptibility of cod to projected climate change.