Alterations to cellular metabolism are linked to multiple natural parasite infections across populations of a freshwater fish

Exposure to environmental stressors can induce physiological responses in organisms, which can lead to population-level differences in physiological traits. However, the mechanisms underlying these responses both within individuals and among populations are often unknown. Parasite infections, in particular, are important biotic stressors that can induce a range of effects in hosts, including altered whole-organism metabolism. However, few studies have explored sub-cellular alterations to metabolic performance across both an individual infection gradient and population-level differences in infection prevalence. We compared mitochondrial enzyme activities across five distinct populations of pumpkinseed sunfish (Lepomis gibbosus) differing in the prevalence of cestode and trematode parasites. Overall, we found that enzymes from the OXPHOS pathway (cytochrome c oxidase, NADH dehydrogenase and coenzyme Q : cytochrome c oxidoreductase), the anaerobic pathway (lactate dehydrogenase), the TCA cycle (citrate synthase) and lipid metabolism (carnitine palmitoyl transferase) were positively correlated with parasite density with trends depending on the organ, parasite species and host population studied. Our results suggest complex and nuanced relationships among infections and host physiological performance and provide strong evidence of parasite-induced alterations to mitochondrial metabolism which may explain previously reported whole-organism responses to infections.