Diana Walstad, in her book, "Ecology of the Planted Aquarium", proposes that algae is limited in its growth by the limited availability of iron in the water. She says that while plants can get iron from the substrate, through their roots, algae doesn't have that ability, and has to depend on the iron in the water. Furthermore, that iron in the water has to be bio-available, in the form of soluble ions, Fe2+ and Fe3+. But, those ions have a very brief "life" in water because it easily bonds to various water soluble organic carbon compounds. As a result, algae has a difficult time reproducing unless there is a reliable source of Fe2+ and/or Fe3+ in the water.
One way those iron ions can be reliably available is if the iron that is bound to soluble organic carbon compounds is exposed to lots of light. When that bound iron is exposed to light of adequate intensity and spectrum, the light can cause the iron ions to be released into the water, by a process called photo reduction.
I find this extremely interesting, and one of the most compelling arguments about "why does algae take over my aquarium"? So, I am going to do a simple experiment to try to demonstrate that process.
I have a 10 gallon tank, modified with a divider into two adjacent 5 gallon tanks. The same light fixture lights both halves of the tank. See https://www.aquaticplantcentral.com/forumapc/diy-aquarium-projects/143379-tank-divider.html
If I dose each 5 gallon half of the tank with the same amount of iron (from Flourish Iron), but add the iron as the light comes on for one half, and after the light goes off for the other half, there should be a significant difference in availability of Fe2+/Fe3+ in the two half tanks. I should see more algae growth in the half with the most available iron.
I'm about 2 weeks or so from being ready to start my experiment.