Selenium Biotransformation by Food-Relevant Microorganisms: Bacterial and Fungal Differences in Diversified Uptake, Metabolic Partitioning, and Terminal Fate

Selenium (Se) biotransformation by microorganisms determines how inorganic selenium is redistributed into organic selenium, selenium nanoparticles (SeNPs), methylated species, and other intracellular or extracellular pools. Although microbial selenium metabolism has been extensively studied, evidence supporting individual mechanisms remains uneven, and bacterial and fungal observations are often compared without sufficient consideration of energetic context, cellular architecture, or evidence strength. This review reorganizes the field around an evidence-graded mechanistic framework. We distinguish direct functional validation from convergent pathway evidence and association-level observations, compare bacterial and fungal selenium handling across uptake, redox metabolism, sulfur assimilation, methylation, compartmentalization, and terminal localization, and introduce cellular selenium loading as a conceptual bridge between uptake and downstream metabolic partitioning. We further emphasize that methylation is not equivalent to volatilization and that terminal selenium fate is best resolved by combining chemical speciation with spatial localization. These mechanisms are integrated into strain-specific selenium-transformation profiles and linked to representative applications and evidence-guided microbial engineering. This synthesis aims to shift microbial selenium research from empirical total-selenium accumulation toward mechanism-resolved, testable, and engineerable selenium biotransformation.