It converts iron from Fe++ to Fe+++, which is much less soluble, at least when not chelated to organics. Which form is more bioavailable at the same concentration is open to debate by scientists .
The bioavailability of iron to microorganisms and its underlying mechanisms have far reaching repercussions to many natural systems and diverse fields of research, including ocean biogeochemistry, carbon cycling and climate, harmful algal blooms, ...
www.ncbi.nlm.nih.gov
"Due to the greater solubility of Fe(II) compared to Fe(III), the occurrence of reductive processes and/or measurable Fe(II) concentrations is often equated with increased bioavailability."
It converts Cu++ to Cu+. What is more bioavailable may depend on the organism:
The concentrations of dissolved copper (Cud), copper-binding ligands, thiourea-type thiols and humic substances (HSCu) were measured in estuarine waters adja...
www.frontiersin.org
"Cu uptake has been shown to be controlled by the oxidation state of the metal and by the metal:ligand ratio, rather than by the concentration of inorganic species of Cu in solution (
Semeniuk et al., 2009). The speciation of Cu will affect its availability to microorganisms depending on whether uptake is as Cu(I) or Cu(II), as the uptake may involve a change in oxidation state with associated reaction rates. It is unclear whether Thaumarchaeota are acquiring the thiol-bound copper [as Cu(I)], or the humic-bound copper [as Cu(II)]."