Abstract： In supergene environments, nanosized iron (hydr)oxides and phosphate minerals are widely distributed and exhibit significant sequestration effects on phosphorus (P) and rare earth elements (REEs). Although previous studies have found that both forms of P can be utilized by microorganisms, how microbial activities constrain the geochemical behavior of mineral-bound REEs during P utilization has received little attention thus far. This study investigated the utilization of Nano-mineral bound P by Microcystis aeruginosa and the associated REEs fractionation under weakly alkaline and high CO23- conditions, using ferrihydrite (Fh) and apatite (Ap) loaded with P and REEs as P sources through dialysis methods (isolating cells from minerals). Results demonstrated that M. aeruginosa utilized Nano-mineral bound P at low efficiency, while both dissolved and Nano-mineral bound REEs exhibited moderate toxicity to the cyanobacterium. In REE experiments, all solutions were enriched in heavy REEs (HREEs) after 17 day cultivation. For algal cells and extracellular polymeric substances (EPS), REE fractionation was observed only in experiment with highly dissolved REE concentration (enriched in light REEs, LREEs) and experiment of ferrihydrite + dialysis (enriched in HREEs). Filamentous EPS preferentially accumulated middle REEs (MREEs, particularly Sm, Eu, and Gd), while secondary calcium phosphates and iron (hydr)oxides sequestered MREEs −HREEs. It is considered that: ①The selective adsorption of REE³ ⁺ under weak alkaline conditions by cells and EPS (C&E) consistently enriches HREEs in solutions; ②Cells and EPS enrichment in LREEs occurs when the REE/anion (especially CO₂^3-) ratio in solution is elevated; ③No REE fractionation in cells and EPS when REEs originate from mineral phases; ④ EPSmediated selective REE complexation from minerals may drive HREE enrichment in cells and EPS; ⑤ The preferential accumulation of MREEs−HREEs in EPS and secondary solid phases may be the underlying cause of positive anomalies of these elements in eutrophic waters. Thus, anomalies of MREEs (e. g., Eu) may serve as effective proxies for assessing the degree of aquatic eutrophication.