Toxic effect and bioaccumulation of selenium in green alga Chlorella pyrenoidosa

Se is required for the growth and metabolism of many species of microalgae. However, high concentrations of Se show toxicity to microalgal cells. In the study, the biomass, growth rate, total Se content, organic Se content, SOD, CAT, and LPO of Chlorella pyrenoidosa in an environment containing Se were determined to explore the effects of selenite on the growth of C. pyrenoidosa in different growth phases (exponential phase, late exponential phase, and early stationary phase). Bioaccumulation and conversion of selenium in C. pyrenoidosa and toxic effect of selenium were further explored. A low dosage of selenite could promote the growth of C. pyrenoidosa, but the higher Se concentration strongly inhibited growth and led to the low biomass. Algae cells in exponential phase could be exposed to high doses of selenium (≤ 5 mg L−1), which was higher than that for the cells in late exponential phase (≤ 2 mg L−1) and the cells in early stationary phase (≤ 2 mg L−1). The optimal growth conditions for enriching selenium in C. pyrenoidosa in exponential phase were determined as 2 mg L−1 selenite and 5-day cultivation. The highest transformation mass was 78 ± 0.5% and corresponding intracellular accumulation of organic Se was 72 ± 0.1 μg g−1 dry weight. The lower Se concentration induced the activities of oxidases, which was conducive to the growth of microalgal cells. The higher concentration of Se significantly induced the activities of oxidases, indicating that Se accumulation had a significant toxic effect on C. pyrenoidosa. In this way, a great quantity of Se-enriched biomass can be produced for nutritional purposes.