Species persistence in the Anthropocene is dramatically threatened by global climate
change. Large emissions of carbon dioxide (CO2) from human activities are driving
increases in mean temperature, intensity of heatwaves, and acidification of oceans
and freshwater bodies. Ectotherms are particularly sensitive to CO2-induced stressors,
because the rate of their metabolic reactions, as well as their immunological performance,
are affected by environmental tem-peratures and water pH. We reviewed and performed
a meta-analysis of 56 studies, involving 1259 effect sizes, that compared oxidative
status or immune function metrics between 42 species of ectothermic vertebrates exposed
to long-term increased temperatures or water acidification (>= 48 h), and those exposed
to control parameters resembling natural conditions. We found that CO2-induced stressors
enhance levels of molecular oxidative damages in ectotherms, while the activity of
antioxidant enzymes was upregulated only at higher temperatures, possibly due to an
increased rate of biochemical reactions dependent on the higher ambient temperature.
Differently, both temperature and water acidification showed weak impacts on immune
function, indicating different direction (increase or decrease) of responses among
immune traits. Further, we found that the intensity of temperature treatments (Delta
degrees C) and their duration, enhance the physiological response of ectotherms, pointing
to stronger effects of prolonged extreme warming events (i.e., heatwaves) on the oxidative
status. Finally, adult individuals showed weaker antioxidant enzy-matic responses
to an increase in water temperature compared to early life stages, suggesting lower
acclimation capac-ity. Antarctic species showed weaker antioxidant response compared
to temperate and tropical species, but level of uncertainty in the antioxidant enzymatic
response of Antarctic species was high, thus pairwise comparisons were statistically
non-significant. Overall, the results of this meta-analysis indicate that the regulation
of oxidative status might be one key mechanism underlying thermal plasticity in aquatic
ectothermic vertebrates.
