The heart's energy secret: impact of DHA deficiency on cardiac mitochondrial efficiency and thermal performance in sardine ( <i>Sardina pilchardus</i> )

Climate-induced changes in marine ecosystems are predicted to reduce the production of omega-3 long-chain polyunsaturated fatty acids (n-3 LC PUFA), such as docosahexaenoic acid (DHA), in primary producers. This decline can spread through food webs, reducing DHA availability in small pelagic fish such as sardines. DHA is essential for mitochondrial membrane integrity and ATP production. We tested whether dietary DHA deficiency impairs cardiac mitochondrial function in sardines, limiting their ability to cope with rapid temperature changes. Wild adult sardines reared under laboratory conditions were fed either a control or a DHA-deficient diet at natural fluctuating temperature. After 6 months, we measured in vitro cardiac mitochondria performance from nine individuals per group at 12, 15, 18 and 21°C. DHA-deficient sardines had lower DHA in cardiac cell membranes, leading to reduced mitochondrial efficiency, with increased proton leakage, reduced oxidative phosphorylation and ATP production. High inter-individual variability suggests DHA alone does not fully explain mitochondrial efficiency changes, with additional metabolic mechanisms involved. While mitochondria maintained respiratory capacity up to 21°C, ATP synthesis efficiency declined with temperature, suggesting DHA-deficient sardines may face greater energetic constraints under warming. These findings highlight how dietary changes alter the metabolic resilience in marine ectotherms facing rapid environmental fluctuations, although responses under more extreme temperatures remain to be explored.