Metabolic responses of Ulva compressa to single and combined heavy metals

Accumulation of metals and metabolic responses were studied for two Cd and Cu concentrations (1 and 10 mM) either alone or as a combination in marine macroalga after 7 days of exposure. Cd accumulated more at a low dose (115 mg of Cd/g DW) but Cu at a high dose (378 mg of Cu/g DW); Cu suppressed Cd accumulation (by 57%). Na and Zn levels were unaffected, but higher metal doses depleted K and Ca levels. Higher metal concentrations strongly stimulated reactive oxygen species and depleted nitric oxide (NO) formation, but differences between the action of Cd and Cu were not extensive. Higher metal doses increased cell wall thickness with a potential relation to NO signal that is visible mainly in the apoplast in those treatments. A higher Cu dose depleted proline, ascorbic acid, and phenol levels more than Cd, whereas Cd elevated nonprotein thiols and ascorbic acid in combined treatments. An eventual role of malic or citric acid in metal chelation was not evident: malic acid level decreased in all treatments. The total content of fatty acids reached 16.7 mg/g DW in control with the quantitative order of PUFAs > SFAs > MUFAs; palmitic, vaccenic, linoleic, and a-linolenic acids were the major compounds. Cu was more toxic for fatty acids than Cd (even at 1 mM); mainly, PUFA levels strongly decreased (from 43% of total acids in control to 28.9% and 5.4% at 1 and 10 mM Cu treatment, respectively). Results are precisely and critically discussed in relation to limited literature focused on macroalgae, and a comparison with microalgae is also provided.