Efficient selenium bioconversion by Auxenochlorella pyrenoidosa enables production of organic selenium-enriched microalgal biomass

Selenium (Se) is an essential micronutrient for human health; however, the efficient conversion of inorganic Se into organic forms remains challenging. Microalgae have emerged as promising platforms for sustainable production of functional food ingredients due to their rapid growth, metabolic versatility, and capacity to accumulate bioactive compounds. Here, Auxenochlorella pyrenoidosa ZC-1006 was evaluated as a heterotrophic platform for producing Se-enriched biomass. The most favorable heterotrophic conditions within the tested ranges were 15 g/L glucose, 2 g/L urea, an initial pH of 6.0, and cultivation at 31 °C. Supplementation with 5 mg/L sodium selenite (Na 2 SeO 3 ) provided a favorable balance between biomass accumulation and Se enrichment, yielding 108.74 mg/kg total Se on a dry weight (DW) basis. Ion chromatography–inductively coupled plasma mass spectrometry (IC–ICP-MS) analysis showed that organic Se accounted for 95.97% of the measured Se, with selenomethionine (SeMet) and selenocystine (SeCys 2 ) as the predominant identified species. Independent fermentation experiments conducted in 5 L and 3000 L systems further supported the technical feasibility of producing Se-enriched ZC-1006 biomass under the tested conditions. Transcriptomic analysis identified Se-associated expression changes in pathways related to sulfur-containing amino acid metabolism, glutathione metabolism, and antioxidant defense, while quantitative real-time polymerase chain reaction (qRT-PCR) supported selected transcript-level trends. These findings support A. pyrenoidosa ZC-1006 as a candidate platform for producing Se-enriched biomass, and the resulting biomass as a promising food-oriented ingredient for future food product development.