Cadmium Is Accumulated as Electron-Dense Nanoparticles, Not Bound to Glutathione (GSH), Phytochelatins or Metallothioneins, and Extruded to the Culture Medium with GSH in the Marine Alga Ulva compressa

The mechanism of cadmium (Cd) accumulation was analyzed in the marine alga Ulva compressa. The alga was cultivated with 10 µM Cd, with 10 µM of Cd and increasing concentrations of a sulfide donor (NaHS), or with a sulfide acceptor (hypotaurine), and intracellular Cd levels were monitored for 7 d. Glutathione (GSH) and phytochelatins (PCs) levels, and metallothioneins (MTs) transcript levels were also quantified, along with the extrusion of Cd, GSH, and PCs to the culture medium. The results showed that the sulfide donor increased intracellular Cd levels, whereas the sulfide acceptor decreased them. GSH, PCs, and MTs levels did not correlate with intracellular Cd contents. Both Cd and GSH were extruded to the culture medium, along with lower amounts of PCs. TEM-EDXS analysis revealed electron-dense nanoparticles containing Cd and O, likely CdO or Cd bound to fatty acids; in the presence of NaHS, nanoparticles containing Cd and S (likely CdS) or Cd, S, and N (likely Cd bound to GSH) were also observed. In conclusion, Cd accumulates as insoluble nanoparticles—probably not bound to GSH, PCs, or MTs—and is extruded to the culture medium together with GSH in the marine alga U. compressa.