Production of selenium- and zinc-enriched Lemna and Azolla as potential micronutrient-enriched bioproducts

Selenium (Se) and zinc (Zn) are essential micronutrients that are often lacking in the diet of humans and animals, leading to deficiency diseases. Lemna and Azolla are two aquatic plants with a substantial protein content, which offer the possibility of utilizing them to remove Se and Zn from (waste)water while producing micronutrient-enriched dietary proteins and fertilizers. In this study, we explored interaction effects occurring between Se and Zn when these micronutrients are taken up by Azolla and Lemna. The two aquatic plants were grown on hydroponic cultures containing 0-5.0 mg/L of Se (Se(IV) or Se(VI)) and Zn. The Se and Zn content of the plants, growth indicators, bioconcentration factor (BCF) and Se/Zn removal efficiency from the water phase were evaluated. The results demonstrated that Se(IV) is more toxic than Se(VI) for both plant species, as evidenced by the remarkable decrease of biomass content and root length when exposed to Se(IV). Both aquatic plants took up around 10 times more Se(IV) than Se(VI) from the medium. Moreover, the Se accumulation and removal efficiency increased by 66-99% for Se(IV) and by 34-59% for Se(VI) in Lemna when increasing Zn dosage from 0 to 5.0 mg/L in the medium, whereas it declined by 13-26% for Se(IV) and 21-35% for Se(VI) in Azolla, suggesting a synergetic effect in Lemna, but an antagonistic effect in Azolla. The maximum BCF of Se in Lemna and Azolla were 507 and 667, respectively. The protein content in freeze-dried Lemna and Azolla was approximately 17%. The high tolerance and accumulation of Se and Zn in Lemna and Azolla, combined with their rapid growth, high protein content and transformation of inorganic to organic Se species upon Se(IV) exposure make Lemna and Azolla potential candidates for the production of Se(IV)- and Zn-enriched biomass that can be used as crop fertilizers or protein-rich food/feed supplements or ingredients. Accordingly, by growing the Azolla and Lemna on wastewater, a high-value product can be produced from wastewater while recovering resources.