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 .
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3328120/
"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:
https://www.frontiersin.org/articles/10.3389/fmars.2017.00178/full
"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)]."