Rearing temperature and fatty acid supplementation jointly affect membrane fluidity and heat tolerance in Daphnia

The homeoviscous adaptation hypothesis states that the relative abundance of polyunsaturated fatty acids (PUFA) decreases in the membrane phospholipids of ectothermic organisms at higher temperatures to maintain vital membrane properties. We hypothesized that the well-documented reduced heat tolerance of cold-reared Daphnia is due to the accumulation of PUFA in their body tissues and that heat-reared Daphnia contain reduced amounts of PUFA even when receiving a high dietary supply of PUFA. In Daphnia reared at 15{degrees}C, supplementation of a PUFA-deficient food with the long-chain PUFA eicosapentaenoic acid (EPA) resulted in an increase in the relative abundance of EPA in body tissues and a decrease in heat tolerance. However, the same was observed in Daphnia reared at 25{degrees}C, indicating that the ability of heat-acclimated Daphnia to adjust EPA body concentrations is limited when exposed to high dietary EPA concentrations. Daphnia reared at 25{degrees}C showed the lowest change in membrane fluidity, measured as fluorescence polarization. For Daphnia reared at three different temperatures, thermal tolerance (time to immobility at a lethally high temperature) and increasing dietary EPA concentrations correlated with fluorescence polarization and the degree of fatty acid unsaturation. Overall, our results support the homeoviscous adaptation hypothesis by showing that cold-reared Daphnia, which accumulate PUFA within their tissues, are more susceptible to heat than hot-reared Daphnia, which contain less PUFA.