Speciation-dependent intracellular reduction and nanostructure formation of selenium and tellurium in unicellular algae

Understanding how chalcogen elements taken up by biological systems change their chemical speciation is essential for elucidating their intracellular behaviour. However, knowledge of the uptake and transformation of selenium and tellurium oxyanions in unicellular algae remains limited. In this study, selenium and tellurium oxyanions in different oxidation states (VI or IV) were added to two unicellular algae, Chlamydomonas reinhardtii and Pseudococcomyxa simplex, and their intracellular accumulation and chemical speciation were systematically investigated. X-ray absorption fine structure (XAFS) analysis revealed that both selenium and tellurium underwent intracellular reduction irrespective of their initial oxidation states. However, the extent of reduction, accumulation efficiency, and final chemical speciation differed markedly depending on both the oxidation state of the added oxyanion and the algal species, indicating shared yet species-dependent intracellular transformation patterns. In particular, tetravalent oxyanions (selenite and tellurite) underwent more rapid reduction and exhibited higher cellular accumulation than the corresponding hexavalent species in both algae, although the extent and kinetics of reduction differed markedly between species. Scanning and transmission electron microscopy demonstrated that selenium was immobilised as spherical elemental nanoparticles, whereas tellurium formed needle-like metallic nanorods within algal cells. Under hexavalent selenate exposure, higher-valent and organoselenium species remained detectable, and the formation of elemental selenium nanoparticles was limited. These results demonstrated speciation- and species-dependent intracellular transformation and accumulation of selenium and tellurium in unicellular algae, providing chemical speciation-based insights into algal chalcogen metabolism and detoxification processes.