Differential responses to cadmium induced oxidative stress in marine macroalga Ulva lactuca (Ulvales, Chlorophyta)

This study describes various biochemical processes involved in the mitigation of cadmiumtoxicity in green alga Ulva lactuca. The plants whenexposed to 0.4 mM CdCl₂ for 4 days showed twofold increase in lipoperoxides and H₂O₂ content that collectively decreased the growth and photosynthetic pigments by almost 30% over the control. The activities of antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and glutathione peroxidase (GPX) enhanced by twofold to threefold and that of catalase (CAT) diminished. Further, the isoforms of these enzymes, namely, Mn-SOD (̃85 kDa), GR (̃180 kDa) and GPX (̃50 kDa) responded specifically to Cd²⁺ exposure. Moreover, the contents of reduced glutathione (3.01 fold) and ascorbate (1.85 fold) also increased substantially. Lipoxygenase (LOX) activity increased by two fold coupled with the induction of two new isoforms upon Cd²⁺ exposure. Among the polyunsaturated fatty acids, although n - 3 PUFAs and n - 6 PUFAs (18:3n - 6 and C18:2n - 6) showed relatively higher contents than control, the latter ones showed threefold increase indicating their prominence in controlling the cadmium stress. Both free and bound soluble putrescine increased noticeably without any change in spermidine. In contrast, spermine content reduced to half over control. Among the macronutrients analysed in exposed thalli, the decreased K content was accompanied by higher Na and Mn with no appreciable change in Ca, Mg, Fe and Zn. Induction of antioxidant enzymes and LOX isoforms together with storage of putrescine and n - 6 PUFAs in cadmium exposed thallus in the present study reveal their potential role in Cd ²⁺ induced oxidative stress in U. lactuca.