Effects of Selenite on Unicellular Green Microalga Chlorella pyrenoidosa: Bioaccumulation of Selenium, Enhancement of Photosynthetic Pigments, and Amino Acid Production

Synergistic effect of selenium and gibberellic acid for enhanced biomass, lipid and improved biodiesel quality from Tetradesmus obliquus through response surface methodology

Biodiesel production from microalgae presents an innovative solution for renewable energy. This study investigates biodiesel production using Tetradesmus obliquus ON506010.1 by optimizing substrates, selenium and gibberellic acid. Using 15 µg/L selenium, lipid content and biomass productivity reached 35.45 %±0.92 and 0.178 g/L/day ± 0.051. With 50 µM gibberellic acid, biomass productivity and lipid content peaked at 0.785 ± 0.101 g/L/day and 38.95 %±0.35, surpassing the control. Fatty acid composition, biodiesel properties, and mRNA expression of lipid synthesis enzymes (acetyl CoA carboxylase (ACC) and fatty acid desaturase (FAD)) correlated. Combining 10 µg/L selenium with 75 µM gibberellic acid with response surface methodology (RSM) increased lipid content (42.80 % ±0.11) and biomass productivity (0.964 g/L/day ± 0.128). ACC and FAD upregulation validated this enhancement, with a 4.4-fold increase in FAD expression. Fatty acid composition and most biodiesel properties met international standards demonstrating Tetradesmus obliquus ON506010.1's potential for sustainable biodiesel production with better cold flow property and oxidative stability.

Generation of selenium-rich wheat mutants and exploration of responsive genes for selenium accumulation

KEY MESSAGE

Salt tolerance, selenium accumulation and expression of the responsive genes were analyzed in the wheat high selenium mutants. Selenium is an essential trace element for the human body, and its deficiency can lead to various diseases such as Keshan disease and large bone disease. Wheat, being a major staple crop, plays a crucial role in providing dietary selenium supplementation to combat this deficiency. Despite progress in understanding the molecular regulation of selenium accumulation in certain crops, the molecular mechanisms governing selenium accumulation-related gene expression in wheat plants remain poorly understood. In this study, three mutant wheat lines with elevated selenium content were identified. Under the treatment of Na2SeO3 or NaCl, the selenium-rich wheat mutants exhibited decreased sensitivity to both selenium and NaCl compared to the wild type. Additionally, there was an increase in the activities of SOD and POD, while the content of MDA decreased. Through qRT-PCR analysis, the expression of selenium-related genes was affected, revealing that some of these genes not only regulate the response of wheat to salt stress, but also play a role in the process of selenium accumulation. The transcriptome results revealed that the important genes encoding glutathione S-transferases, peroxidases, superoxide dismutases, and UDP-glucosyltransferases may function in the regulation of salt tolerance and selenium accumulation in wheat. These findings significantly contribute to the current understanding of the molecular regulation of selenium accumulation in wheat crops, while also offering novel germplasm resources for cultivating selenium-rich and salt-tolerant wheat lines.

Unique high-temperature tolerance mechanisms of zoochlorellae Symbiochlorum hainanensis derived from scleractinian coral Porites lutea

Global warming is a key issue that causes coral bleaching mainly because of the thermosensitivity of zooxanthellae. Compared with the well-studied zooxanthellae Symbiodiniaceae in coral holobionts, we rarely know about other coral symbiotic algae, let alone their thermal tolerance. In this study, a zoochlorellae, Symbiochlorum hainanensis, isolated from the coral Porites lutea, was proven to have a threshold temperature of 38°C. Meanwhile, unique high-temperature tolerance mechanisms were suggested by integrated transcriptomics and real-time quantitative PCR, physiological and biochemical analyses, and electron microscopy observation. Under heat stress, S. hainanensis shared some similar response strategies with zooxanthellae Effrenium sp., such as increased ascorbate peroxidase, glutathione peroxidase, superoxide dismutase activities and chlorophyll a, thiamine, and thiamine phosphate contents. In particular, more chloroplast internal layered structure, increased CAT activity, enhanced selenate reduction, and thylakoid assembly pathways were highlighted for S. hainanensis's high-temperature tolerance. Notably, it is the first time to reveal a whole selenate reduction pathway from SeO42- to Se2- and its contribution to the high-temperature tolerance of S. hainanensis. These unique mechanisms, including antioxidation and maintaining photosynthesis homeostasis, efficiently ensure the high-temperature tolerance of S. hainanensis than Effrenium sp. Compared with the thermosensitivity of coral symbiotic zooxanthellae Symbiodiniaceae, this study provides novel insights into the high-temperature tolerance mechanisms of coral symbiotic zoochlorellae S. hainanensis, which will contribute to corals' survival in the warming oceans caused by global climate change.

IMPORTANCE

The increasing ocean temperature above 31°C-32°C might trigger a breakdown of the coral-Symbiodiniaceae symbioses or coral bleaching because of the thermosensitivity of Symbiodiniaceae; therefore, the exploration of alternative coral symbiotic algae with high-temperature tolerance is important for the corals' protection under warming oceans. This study proves that zoochlorellae Symbiochlorum hainanensis can tolerate 38°C, which is the highest temperature tolerance known for coral symbiotic algae to date, with unique high-temperature tolerance mechanisms. Particularly, for the first time, an internal selenium antioxidant mechanism of coral symbiotic S. hainanensis to high temperature was suggested.

Effects of foliar selenium application on Se accumulation, elements uptake, nutrition quality, sensory quality and antioxidant response in summer-autumn tea

Summer-autumn tea is characterized by high polyphenol content and low amino acid content, resulting in bitter and astringent teast. However, these qualities often lead to low economic benefits, ultimately resulting in a wastage of tea resources. The study focused on evaluating the effects of foliar spraying of glucosamine selenium (GLN-Se) on summer-autumn tea. This foliar fertilizer was applied to tea leaves to assess its impact on plant development, nutritional quality, elemental uptake, organoleptic quality, and antioxidant responses. The results revealed that GlcN-Se enhanced photosynthesis and yield by improving the antioxidant system. Additionally, the concentration of GlcN-Se positively correlated with the total and organic selenium contents in tea. The foliar application of GlcN-Se reduced toxic heavy metal content and increased the levels of macronutrients and micronutrients, which facilitated adaptation to environmental changes and abiotic stresses. Furthermore, GlcN-Se significantly improved both non-volatile and volatile components of tea leaves, resulting in a sweet aftertaste and nectar aroma in the tea soup. To conclude, the accurate and rational application of exogenous GlcN-Se can effectively enhance the selenium content and biochemical status of tea. This improvement leads to enhanced nutritional quality and sensory characteristics, making it highly significant for the tea industry.

Effects of sodium selenite on the growth, biochemical composition and selenium biotransformation of the filamentous microalga Tribonema minus

This study aimed to investigate the physiological and biochemical responses of filamentous microalga Tribonema minus to different Na₂SeO₃ concentrations and its selenium absorption and metabolism to evaluate the potential in treating selenium-containing wastewater. The results showed that low Na₂SeO₃ concentrations promoted growth by increasing chlorophyll content and antioxidant capacity, whereas high concentrations caused oxidative damage. Although Na₂SeO₃ exposure reduced lipid accumulation compared with the control, it significantly increased carbohydrate, soluble sugar, and protein contents, with the highest carbohydrate productivity of 117.97 mg/L/d at 0.5 mg/L Na₂SeO₃. Furthermore, this alga effectively absorbed Na₂SeO₃ in the growth medium and converted most of it into volatile selenium and a small part into organic selenium (predominantly as selenocysteine), showing strong selenite removal efficacy. This is the first report on the potential of T. minus to produce valuable biomass while removing selenite, providing new insights into the economic feasibility of bioremediation of selenium-containing wastewater.

Bioaccumulation of selenium in halotolerant microalga Dunaliella salina and its impact on photosynthesis, reactive oxygen species, antioxidative enzymes, and neutral lipids

Selenium (Se) is an essential element for living systems, however, toxic at higher levels. In the present study, Dunaliella salina cells were exposed to different Se concentrations for their growth (EC₅₀ 195 mg L-¹) as well as Se accumulation. The cells exposed to 50 mg L-¹ Se showed photoautotrophic growth parallel to control and accumulated 65 μg Se g-¹ DW. A decrease in photosynthetic quantum yield, chlorophyll content, and the increase in intracellular reactive oxygen species, proline content, and lipid peroxidation accompanied by higher neutral lipid accumulation, were recorded at higher Se level. The enzymes superoxide dismutase and catalase played a pivotal role in antioxidative defense. Heterogeneity in accumulated carotenoids at varying concentrations of selenium was prevalent. The cells exposed to 200 mg L-¹ Se resulted in the disorganization of organelles. Thus, the Se enriched biomass obtained at 50 mg L-¹ may be explored for bio-fortification of food and feed.

Could Chlorella pyrenoidosa be exploited as an alternative nutrition source in aquaculture feed? A study on the nutritional values and anti-nutritional factors

This work attempted to identify if microalgal biomass can be utilized as an alternative nutrition source in aquaculture feed by analyzing its nutritional value and the anti-nutritional factors (ANFs). The results showed that Chlorella pyrenoidosa contained high-value nutrients, including essential amino acids and unsaturated fatty acids. The protein content in C. pyrenoidosa reached 52.4%, suggesting that microalgal biomass can be a good protein source for aquatic animals. We also discovered that C. pyrenoidosa contained some ANFs, including saponin, phytic acid, and tannins, which may negatively impact fish productivity. The high-molecular-weight proteins in microalgae may not be effectively digested by aquatic animals. Therefore, based on the findings of this study, proper measures should be taken to pretreat microalgal biomass to improve the nutritional value of a microalgae-based fish diet.

Influence of selenium on the mycelia of the shaggy bracket fungus, Inonotus hispidus

BACKGROUND

Selenium (Se) is a needed trace element for animals and humans. Many fungi have effective mechanisms to acquire, transform and accumulate Se in organic form. In this study, the effects of inorganic Se (sodium selenite) on the medicinal fungus Inonotus hispidus was investigated.

RESULTS

Inonotus hispidus was capable of tolerating up to 3.85 mmol L^-1 selenite, at which ~85% growth inhibition was seen, with 50% growth inhibition occurring at ~1 mmol L^-1 selenite. Growth in 0.29 mmol L^-1 Se resulted in I. hispidus mycelium with 115 times higher Se levels compared to growth in standard media, and an organic Se content of 86% to total Se content. The influence of Se accumulation on morphological features of I. hispidus were examined by microscopic and scanning electron microscopic observation. These data revealed significant shrinkage and deformations of I. hispidus hyphae with decreased branching and collapse of clamp connections under higher Se stress. However, conidial production in I. hispidus increased dramatically. The influence of Se on mycelial growth could be recovered by reinoculation in standard media. Se accumulation had only minimal impacts on the yield of the potential selenocompounds such as amino acids, proteins and polysaccharides. By contrast, Se-enriched I. hispidus mycelium was of higher quality due to reduction in crude fat and total ash contents.

CONCLUSIONS

These data provide basic and applied information on the feasibility of producing selenized I. hispidus as an enriched and better quality product. © 2021 Society of Chemical Industry.

Selenium in Soil-Plant-Microbe: A Review

Selenium (Se) plays an important role in geochemistry and is an essential trace element for humans and animals. This review summarizes the transformation and accumulation of Se in the plant-soil-microbe system. As one of the important reservoirs of Se, soil is an important material basis of its entry into the food chain through plants. Soil with an appropriate amount of Se is beneficial for plant growth and plays a valuable role in a stress-resistant environment. Among the many migration and transformation pathways, the transformation of Se by microorganisms is particularly important and is the main form of Se transformation in the soil environment. In this review, the role and form transformation of Se in plants, soil, and microorganisms; the role of Se in plants; the form, input, and output of Se in soil; the absorption and transformation of Se by plants; and the role of microorganisms in Se transformation are presented. In addition to describing the migration and transformation laws of Se in the environment, this review expounds on the main directions and trends of Se research in the agricultural field as well as current gaps and difficulties in Se-related research. Overall, this reviews aims to provide necessary information and theoretical references for the development of Se-rich agriculture.

Selenite as a Lipid Inductor in Marine Microalga Dunaliella tertiolecta: Comparison of One-Stage and Two-Stage Cultivation Strategies

Microalgae have emerged as one of the most promising alternative sources of biofuels due to their high lipid accumulation ability. High lipid content is of pivotal importance for biodiesel production. In order to obtain high lipid content, modifications of culture conditions and development of an efficient lipid induction method are called for. In the present study, the possibility of using selenium in a form of sodium selenite as a lipid inductor in marine microalga Dunaliella tertiolecta was investigated during one- and two-stage cultivation modes. The effects of selenite on algal growth, pigment content, oxidative stress, and neutral lipid content were determined during both cultivation modes. The results revealed that the two-stage cultivation on 10.00-40.00 mg L-1 of selenite resulted in up to twofold higher algal cell density compared to the one-stage cultivation. Selenite concentrations from 2.50 to 20.00 mg L-1 increased lipid peroxidation during both cultivation modes, emphasizing the selenite-induced oxidative stress accompanied by the increased lipid accumulation in microalgae cells. During one- and two-stage cultivation on 20.00 mg L-1 of selenite, lipid content increased 2.39- and 5.73-fold at days 9 and 14 of cultivation, respectively. Moreover, the highest obtained neutral lipid content during the two-stage cultivation was 5.40-fold higher than lipid content obtained during the one-stage cultivation. Collectively, these results suggest that the two-stage cultivation strategy, initiated with optimal culture conditions for biomass production and followed by the addition of selenite as a stress inductor, can be successfully deployed to enhance the lipid content in D. tertiolecta.

Production of selenium-enriched microalgae as potential feed supplement in high-rate algae ponds treating domestic wastewater

This study assessed the selenium (Se) removal efficiency of two pilot-scale high-rate algae ponds (HRAPs) treating domestic wastewater and investigated the production of Se-enriched microalgae as potential feed supplement. The HRAP-Se had an average Se, NH₄⁺-N, total phosphorus and COD removal efficiency of, respectively, 43%, 93%, 77%, and 70%. Inorganic Se taken up by the microalgae was mainly (91%) transformed to selenoamino acids, and 49-63% of Se in the Se-enriched microalgae was bioaccessible for animals. The crude protein content (48%) of the microalgae was higher than that of soybeans, whereas the essential amino acid content was comparable. Selenium may induce the production of the polyunsaturated fatty acids omega-3 and omega-6 in microalgae. Overall, the production of Se-enriched microalgae in HRAPs may offer a promising alternative for upgrading low-value resources into high-value feed supplements, supporting the drive to a circular economy.

Selenium Fortification Alters the Growth, Antioxidant Characteristics and Secondary Metabolite Profiles of Cauliflower (Brassica oleracea var. botrytis) Cultivars in Hydroponic Culture

Nowadays the importance of selenium for human health is widely known, but most of the plants are poor in terms of selenium storage and accumulation because of the low selenium mineralization potential of the soil. For this purpose, foliar application of different sodium selenate concentrations (0, 5, 10, 15, 20 mg/L) was used to treat the cauliflower cultivars "Clapton" and "Graffiti". Higher yields and other related vegetative attributes were improved at 10 and 15 mg/L sodium selenate application. At a concentration of 10 mg/L sodium selenate, photosynthetic pigments, total phenolic compounds and antioxidant capacity were enhanced in both cultivars, but the "Graffiti" cultivar responded stronger than the "Clapton" cultivar. The glucosinolates were accumulated in response to selenium fortification and the highest amounts were found in the "Graffiti" cultivar at 10 mg/L. Selenium accumulated concentration-dependently and rose with higher fertilization levels. In general, foliar application of selenium at 10 mg/L led to an accumulation of secondary metabolites and also positively affected the growth and yield of florets.

Strategy for Managing Industrial Anaerobic Sludge through the Heterotrophic Cultivation of Chlorella sorokiniana: Effect of Iron Addition on Biomass and Lipid Production

Microalgae provides an alternative for the valorization of industrial by-products, in which the nutritional content varies substantially and directly affects microalgae system performance. Herein, the heterotrophic cultivation of Chlorella sorokiniana was systematically studied, allowing us to detect a nutritional deficiency other than the carbon source through assessing the oxygen transfer rate for glucose or acetate fermentation. Consequently, a mathematical model of the iron co-limiting effect on heterotrophic microalgae was developed by exploring its ability to regulate the specific growth rate and yield. For instance, higher values of the specific growth rate (0.17 h^-1) compared with those reported for the heterotrophic culture of Chlorella were obtained due to iron supplementation. Therefore, anaerobic sludge from an industrial wastewater treatment plant (a baker's yeast company) was pretreated to obtain an extract as a media supplement for C. sorokiniana. According to the proposed model, the sludge extract allowed us to supplement iron values close to the growth activation concentration (K_Fe ~12 mg L^-1). Therefore, a fed-batch strategy was evaluated on nitrogen-deprived cultures supplemented with the sludge extract to promote biomass formation and fatty acid synthesis. Our findings reveal that nitrogen and iron in sludge extract can supplement heterotrophic cultures of Chlorella and provide an alternative for the valorization of industrial anaerobic sludge.

Raman Microspectroscopic Analysis of Selenium Bioaccumulation by Green Alga Chlorella vulgaris

Selenium (Se) is an element with many commercial applications as well as an essential micronutrient. Dietary Se has antioxidant properties and it is known to play a role in cancer prevention. However, the general population often suffers from Se deficiency. Green algae, such as Chlorella vulgaris, cultivated in Se-enriched environment may be used as a food supplement to provide adequate levels of Se. We used Raman microspectroscopy (RS) for fast, reliable, and non-destructive measurement of Se concentration in living algal cells. We employed inductively coupled plasma-mass spectrometry as a reference method to RS and we found a substantial correlation between the Raman signal intensity at 252 cm⁻¹ and total Se concentration in the studied cells. We used RS to assess the uptake of Se by living and inactivated algae and demonstrated the necessity of active cellular transport for Se accumulation. Additionally, we observed the intracellular Se being transformed into an insoluble elemental form, which we further supported by the energy-dispersive X-ray spectroscopy imaging.

Selenium enrichment improves anti-proliferative effect of oolong tea extract on human hepatoma HuH-7 cells

Selenium (Se)-enriched tea is attracting increasing interests due to its significantly improved health benefits. This study was to investigate the anti-proliferative effects of Se-enriched oolong tea against human hepatoma HuH-7 cells. Compared with regular oolong tea extract (TE, 0.04 μg selenium/g), Se-enriched oolong tea extract (Se-TE, 0.51 μg selenium/g) exhibited more prominent anti-proliferative effect against HuH-7 cells with an IC₅₀ of 203.1 μg/mL, mainly due to the synergistic effects of organic selenium and tea polyphenols. Our results found that Se-TE increased intracellular ROS production, arrested the cell cycle at G2/M phase, and thus induced cell apoptosis. In addition, western blotting assay revealed the increased expressions of the p53, Bax, caspase 3, and a reduction of Bcl-2 and CDK2, resulting in Se-TE-induced apoptosis. The improved anti-proliferative effect makes Se-enriched oolong tea extract a promising health-promoting ingredient in food industry.

Effect of microplastics PAN polymer and/or Cu2+ pollution on the growth of Chlorella pyrenoidosa

Polyacrylonitrile polymer (PAN), a common representative textile material and a microplastic, has significant influence on phytoplankton algae, especially with co-exposure with other pollutants, e.g. Cu²⁺. In the present study, we carried out experiments to reveal the population size variation trends of Chlorella pyrenoidosa over time (during a whole growth cycle of 6 days) under PAN and/or Cu²⁺. The levels of pigments (chlorophyll a, b, total chlorophyll and carotenoids), chlorophyll a fluorescence parameters, and other physiological and biochemical indices, containing total protein measurements of H₂O₂, catalase (CAT), and malondialdehyde (MDA) under different treatment groups were measured to explain the physio-ecological mechanism of the effect of PAN and/or Cu²⁺ on the growth of C. pyrenoidosa. The results showed that PAN, Cu²⁺ and the combination of PAN and Cu²⁺ inhibited the growth of C. pyrenoidosa. Chlorophyll a and b decreased significantly with increasing levels of pollutants (PAN and/or Cu²⁺); however, the carotenoid levels increased with increasing levels of pollutants (PAN and/or Cu²⁺) for the first three cultivation days. The oxygen-evolving complexes (OECs) of C. pyrenoidosa had been damaged under Cu²⁺ pollution. The results also showed that CAT activity, MDA content and H₂O₂ activity of C. pyrenoidosa increased with increasing levels of pollutants (PAN and/or Cu²⁺); however, total protein content decreased with increasing levels of pollutants (PAN and/or Cu²⁺) at the first cultivation day. These results indicate that pollutants (PAN and/or Cu²⁺) are harmful to the growth of the C. pyrenoidosa population and negatively affect the levels and function of the pigments in C. pyrenoidosa by decreasing chlorophyll a and b levels, increasing carotenoid levels, and increasing antioxidant enzyme activity.

Reactive oxygen species and their applications toward enhanced lipid accumulation in oleaginous microorganisms

Oleaginous microorganisms are among the most promising alternative sources of lipids for oleochemicals and biofuels. However, in the course of lipid production, reactive oxygen species (ROS) are generated inevitably as byproducts of aerobic metabolisms. Although excessive accumulation of ROS leads to lipid peroxidation, DNA damage, and protein denaturation, ROS accumulation has been suggested to enhance lipid synthesis in these microorganisms. There are many unresolved questions concerning this dichotomous view of ROS influence on lipid accumulation. These include what level of ROS triggers lipid overproduction, what mechanisms and targets are vital and whether ROS act as toxic byproducts or cellular messengers in these microorganisms? Here we review the current state of knowledge on ROS generation, antioxidative defense system, the dual effects of ROS on microbial lipid production, and ROS-induced lipid peroxidation and accumulation mechanisms. Toward the end, the review summarizes strategies that enhance lipid production based on ROS manipulation.

Biofortification with selenium and iodine changes morphological properties of Brassica oleracea L. var. gongylodes) and increases their contents in tubers

Kohlrabi (Brassica oleracea L. var. gongylodes L.) was biofortified with selenium (Se), as selenite and selenate, and iodine (I), as iodide and iodate, and their combinations through foliar spraying, to study absorption of these elements by the plants, separately and in combination. The effects on selected physiological and morphological traits and optical characteristics were monitored. Treatments with Se positively affected total chlorophylls and carotenoids, and leaf stomata dimensions. Addition of I decreased total chlorophylls and increased anthocyanins. In reflectance spectra of the leaves, specific colour regions differed significantly due to the different treatments. Reflectance in the UV correlated positively with Se and I contents of the leaves, which indicated lower demand for production of phenolic compounds. Differences in reflectance in UV part of the spectra could be a consequence of changes in the cuticle. The Se and I levels increased markedly in leaves and tubers, without loss of biomass or yield. Se had antagonistic effects on accumulation of I in leaves. The similar levels of Se and I in the leaves and tubers suggest that the transport of both elements in these plants occurs from the leaves to the tubers through the phloem. According to the Se and I contents in the kohlrabi tubers, biofortification with both elements simultaneously is feasible for human nutrition.

Selenium Incorporation to Amino Acids in Chlorella Cultures Grown in Phototrophic and Heterotrophic Regimes

Microalgae accumulate bioavailable selenium-containing amino acids (Se-AAs), and these are useful as a food supplement. While this accumulation has been studied in phototrophic algal cultures, little data exists for heterotrophic cultures. We have determined the Se-AAs content, selenium/sulfur (Se/S) substitution rates, and overall Se accumulation balance in photo- and heterotrophic Chlorella cultures. Laboratory trials revealed that heterotrophic cultures tolerate Se doses ∼8-fold higher compared to phototrophic cultures, resulting in a ∼2-3-fold higher Se-AAs content. In large-scale experiments, both cultivation regimes provided comparable Se-AAs content. Outdoor phototrophic cultures accumulated up to 400 μg g^-1 of total Se-AAs and exhibited a high level of Se/S substitution (5-10%) with 30-60% organic/total Se embedded in the biomass. A slightly higher content of Se-AAs and ratio of Se/S substitution was obtained for a heterotrophic culture in pilot-scale fermentors. The data presented here shows that heterotrophic Chlorella cultures provide an alternative for Se-enriched biomass production and provides information on Se-AAs content and speciation in different cultivation regimes.

Effect of betaine and selenium on the growth and photosynthetic pigment production in Dunaliella salina as biostimulants

The aim of our study was to establish the effect of selenium and betaine on the growth of D. salina, accumulation of photosynthetic pigments and antioxidant activity of the hydrophobic fraction. This approach was an attempt to demonstrate 'microalgae biostimulant' effects, similar to 'plant biostimulant' effects, i.e. increased tolerance to abiotic stress and enhanced accumulation of bioactive compounds. A high-throughput assay was done in 24-well microplates, at 15% NaCl and different concentrations of sodium selenite (0, 0.5, 2 and 8 µM) or betaine (0, 5, 50 and 500 µM). Both selenium and betaine induced a slight delay in algae growth during the actively growing stage but the final density reached similar values to the control. Betaine significantly enhanced (50%-100%) carotenoids and chlorophyll a accumulation, in a concentration depending manner. Antioxidant activity increased almost 3-fold in extracts of algae treated with 50 µM betaine. Selenium had a much more discrete effect than betaine on pigments biosynthesis. The antioxidant activity of the extracts increased 2-fold in the presence of Se compared to the control. Our work proves that it is possible to enhance production and activity of bioactive compounds from microalgae by using ingredients, which already proved to act as plant biostimulants.

Selenium cytotoxicity in Tetrahymena thermophila: New clues about its biological effects and cellular resistance mechanisms

Selenium is an essential micronutrient but at high concentrations can produce severe cytotoxicity and genomic damage. We have evaluated the cytotoxicity, ultrastructural and mitochondrial alterations of the two main selenium inorganic species; selenite and selenate, in the eukaryotic microorganism Tetrahymena thermophila. In this ciliate, selenite is more toxic than selenate. Their LC₅₀ values were calculated as 27.65 μM for Se(IV) and 56.88 mM for Se(VI). Significant levels of peroxides/hydroperoxides are induced under low-moderate selenite or selenate concentrations. Se(VI) exposures induce an immediate mitochondrial membrane depolarization. Selenium treated cells show an intense vacuolization and some of them present numerous discrete and small electrondense particles, probably selenium deposits. Mitochondrial fusion, an intense swelling in peripheral mitochondria and mitophagy are detected in selenium treated cells, especially in those exposed to Se (IV). qRT-PCR analysis of diverse genes, encoding relevant antioxidant enzymes or other proteins, like metallothioneins, involved in an environmental general stress response, have shown that they may be crucial against Se(IV) and/or Se (VI) cytotoxicity.