What caused dissolved iron to rust and deposit in banded iron formations?

Dissolved iron in ancient oceans was primarily oxidized by photosynthetic microorganisms, which produced oxygen as a byproduct. This increase in oxygen levels led to the formation of insoluble iron oxides, causing the dissolved iron to precipitate and form layers. These processes contributed to the deposition of banded iron formations (BIFs), characterized by alternating layers of iron-rich minerals and silica or chert, reflecting the redox conditions of the early Earth's environment.