Metallomic study of selenium biomolecules metabolized by the microalgae Chlorella sorkiniana in the biotechnological production of functional foods enriched in selenium*

The optimization of culture conditions for selenium (Se) bioaccumulation in the microalgae Chlorella sorokiniana growth in a Na2SeO4-enriched medium was explored, in order to obtain a suitable approach for the biotechnological production of a Se-enriched food. Se concentration (as Na2SeO4) in the culture medium until 100 μg ml –1 allows the growth of algae colonies during long periods of time, until 300 h, but higher concentrations cause the collapse of the colony. The bioaccumulation process causes a concentration of Se in algae in the order of 3 μg g–1 in about 100 h. A metallomic analytical approach based in the coupling high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS), which uses two chromatographic switched columns (C-18 and chiral columns) with ICP-MS detection, has been applied to characterize the different Se metabolites, including chiral forms, in both the algae and culture medium. The results reveal that selenate present in the culture is biotransformed in selenocystine (SeCys2), selenomethyl selenocysteine (SeMeSeCys), and mainly selenomethionine (SeMet) by the algae, although appreciable concentration of Se(VI) is also present in cells. When algae are cultured under sulfur (S) deficiency conditions, the accumulation of Se in the cell is enhanced owing to the great chemical similarity between S and Se that promotes the substitution of S by Se in the cell metabolism, therefore, SeMet concentration in the algae increases from about 7 to 15 μg g–1 and Se(VI) from 15 to 25 μg g–1, after 500 h of exposure.