Stimulation of respiration in Ulva lactuca by high concentrations of cadmium and zinc: Evidence for an alternative respiratory pathway

The effect of zinc on respiration of Ulva lactuca (previously incubated in filtered seawater, in the dark, for 18 h) was investigated using an oxygen electrode. When exposed to zinc (as sulphate) at concentrations up to 25 mM, zinc stimulated respiration with maximum stimulation occurring at 1.5 mM, and above 5 mM respiration was inhibited. When zinc was used in conjunction with the uncoupler DNP (dinitrophenol, 50 μM), the stimulatory effects were cumulative. Oligomycin (6 mM) reduced O₂ uptake by ~42% but the addition of zinc to oligomycin-treated alga restored respiration to near normal values, indicating that zinc did not achieve its uncoupling effect via the mitochondrial ATPase. When zinc was added to cyanide-treated U, lactuca, respiration was partially restored to normal rates. These results were similar to those obtained previously, which showed that cadmium (but not uncouplers such as DNP) increased respiration in cyanide-treated U. lactuca. Cadmium-stimulated respiration was abolished only when cyanide was used in conjunction with salicylhydroxamic acid (SHAM), which indicated the presence of an alternative oxidative, cyanide-resistant pathway that has not been reported previously for U. lactuca. It is suggested that Zn (or Cd) stimulated O₂ uptake via the alternative oxidative pathway.