INTRASPECIFIC VARIATION IN STRESS‐INDUCED HYDROGEN PEROXIDE SCAVENGING BY THE ULVOID MACROALGA ULVA LACTUCA1

We investigated the presence and kinetics of the oxidative stress response in intertidal and subtidal individuals of the ulvoid macroalga Ulva lactuca L. Stress responses, as measured with both enzymatic and fluorescent‐based antioxidant assays, differed between individuals collected from a subtidal and an intertidal habitat. Subtidal individuals secreted significantly more hydrogen peroxide (H₂O₂) than intertidal individuals when subjected to osmotic stress or desiccation. The activity of reactive‐oxygen‐scavenging enzymes and the ability to scavenge exogenous H₂O₂ were lower in subtidal than in intertidal individuals, suggesting that subtidal individuals are less stress tolerant. In vitro experimentation demonstrated that millimolar concentrations of dimethylsulfoniopropionate (DMSP) and its breakdown products could efficiently scavenge H₂O₂, with DMSP being a less‐effective scavenger than dimethyl sulfide (DMS), acrylic acid, and acrylate. The addition of H₂O₂ at concentrations of 2.5 mM or greater induced the cleavage of DMSP into DMS and acrylic acid in subtidal individuals. Intertidal individuals were affected in the same manner with the addition of 5 mM H₂O₂. There were no differences in the amounts of DMSP cleavage in subtidal and intertidal algae when the algae were subjected to hyposaline conditions. Our data suggest that the oxidative‐stress‐induced cleavage of DMSP affords products with efficient H₂O₂‐scavenging abilities. In addition, U. lactuca individuals growing in intertidal habitats are better acclimatized to changing environments and thus have a higher threshold for oxidative stress than conspecifics in subtidal habitats.