Xenohormesis demystified: consuming heat-acclimated algae improves heat tolerance and longevity of Daphnia magna

The xenohormesis hypothesis states that consuming stress-exposed food organisms can increase the consumer's tolerance to similar stresses. We tested this hypothesis using the freshwater model herbivore Daphnia magna feeding on the microalga Nannochloropsis limnetica grown at either 25°C (heat stressed), 20°C (presumed optimum) or 12°C (cold stressed). Daphnia magna maintained at either 20°C or 28°C and consuming heat-stressed algae had a significantly higher short-term heat tolerance. Furthermore, D. magna kept at either 12°C or 28°C (but not at 20°C) achieved a significantly higher lifespan when feeding on heat-stressed algae compared with D. magna feeding on non-stressed or cold-stressed algae, with fecundity being higher on cold-stressed food. This was accompanied by higher antioxidant capacity in D. magna fed heat-stressed algae, but no differences in lipid peroxidation were observed. Fatty acid analysis revealed that D. magna fed heat-stressed algae contained significantly lower levels of the essential polyunsaturated fatty acid (PUFA) eicosapentaenoic acid (EPA), which is thought to be particularly susceptible to lipid peroxidation, than their counterparts feeding on algae grown at 20°C. We conclude that the lower EPA level in D. magna fed heat-stressed algae has caused the higher heat tolerance in D. magna by altering PUFA-mediated membrane properties, i.e. increasing membrane rigidity. Our findings suggest that the consumption of heat-stressed algae can provide heat tolerance and lifespan benefits in zooplankton, but at the cost of reduced fecundity. Further studies are needed to better understand the influence of heat waves on trophic interaction at the phytoplankton-zooplankton interface.