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.

The mechanisms involved in zinc (Zn) accumulation in response to a sulfur donor (NaHS) and a sulfur acceptor (hypotaurine) were analyzed in the marine alga Ulva compressa. The alga was cultivated with 10 μM of Zn, with 10 μM of Zn and increasing concentrations of NaHS for 0 to 7 days, and with 10 μM of Zn and 500 μM of hypotaurine for 5 days. The levels of glutathione (GSH) and phytochelatins (PCs) were also analyzed as well as the levels of transcripts encoding metallothioneins (MTs). Results showed that the level of intracellular Zn mainly decreased in response to 50, 100 or 200 μM of NaHS, compared with the control treated with 10 μM of Zn, and decreased in response to hypotaurine. The level of GSH, PCs (PC2, PC3 and PC4) as well as the level of transcripts encoding MTs did not parallel the level of intracellular Zn, in response to Zn alone or Zn with a sulfur donor. Zn and GSH were released to the culture medium; their release did not increase in response to a sulfur donor, and PCs were not detected in the culture medium. Thus, zinc is probably not accumulated as Zn sulfide (ZnS) and it is co-released to the culture medium with GSH in the marine alga U. compressa.