Reductive transformation of iron (hydr)oxides resulting from dissimilatory iron reduction plays a significant role in governing the fate and transport of nutrients such as phosphate within soils. Under aerobic conditions, phosphate partitions strongly on iron oxides. However, under anaerobic conditions, phosphate and other nutrients/contaminants retained on iron oxides are often subject to release into the aqueous phase. Ferrihydrite, for example, undergoes rapid conversion to secondary iron phases due to (a)biotic mediated transformations. Here we explore changes in phosphate retention upon reductive transformation of ferrihydrite. In addition, the impact of phosphate on the biomineralization pathway of iron oxides is elucidated. We examined microbially (Shewanella putrefaciens) and abiotically (FeIISO4) induced iron transformations in artificial groundwater with lactate as electron donor at pH 7. Phosphate induced non-linear effects on the extent of ferrihydrite biomineralization. At low surface coverage little impact is noted while at high coverage the extent of bioreduction and remineralization is greatly impeded. Rather than being displaced, phosphate is incorporated into secondary phases of green rust and vivianite, products specific to the presence of phosphate in iron biomineralization.