Effects of Sodium Selenite on the Growth and Photosystem II Activity of Arthrospira platensis Gom.

Arthrospira platensis (A. platensis) is a species of cyanobacteria with high economic value; the species is commercially well known as Spirulina platensis, and A. platensis was used in this paper. Its high adaptability, high photosynthetic efficiency, and fast growth rate make it one of the few cyanobacteria that can be cultivated on a large scale. Therefore, using the selenium enrichment property of A. platensis to cultivate selenium-enriched A. platensis will not only enhance the physiological efficacy of A. platensis but also increase its economic value significantly. In this study, we investigated the effects of sodium selenite on the growth and photosynthetic performance of A. platensis selenium by setting different amounts and methods of sodium selenite addition, and we explored the optimal culture conditions of the best dosage and method of sodium selenite addition. The results showed that the experimental group treated with sodium selenite at 700 μmol/L had the fastest growth, and the contents of soluble protein, phycocyanin C, and chlorophyll a increased by approximately 67.9%, 1.44 times, and 38.8% compared to the control group, respectively. Superoxide dismutase (SOD) and catalase (CAT) activity increased by 1.88-fold and 65%, respectively, and malondialdehyde (MDA) levels were reduced by 62% compared to the control group. The results of the OJIP assay showed that the J and I points were significantly higher at the batch addition and treatment concentration of 700 μmol/L, with the rate of QA being reduced and the proportion of the slowly reduced PQ pool being increased. The values of the maximum light energy conversion efficiency (Fv/Fm) per unit of reaction center were higher in both sodium selenite treatment groups than in the control group, indicating that the light energy conversion efficiency of A. platensis was promoted under all concentration treatment conditions. The batch addition of sodium selenite at concentrations less than 700 μmol/L resulted in significantly higher ABS/RC values than the control, and they were far superior to the one-time addition method. The reason for this may have been that the batch addition of sodium selenite at low concentrations increased the light absorption capacity of the unit reaction center of PSII, resulting in a rise in captured light energy, a rise in the energy captured by the reaction center for electron transfer (ETo/RC), a decrease in the energy dissipated in the absorption of light energy by the reaction center (DIo/RC), and an increase in the photosynthetic performance index (PI abs).