Nova Biotechnol Chim (2022) 21(2): 954 DOI: 10.36547/nbc.954 1 Nova Biotechnologica et Chimica The specificity of changes in key performance indicators of green algae of the family Scenedesmaceae under the influence of cerium Larisa M. Cheban1, , Alexander B. Shcherbakov2, Nadiya M. Zholobak2, 3, Mikhailo M. Marchenko3 1Department of Biochemistry and Biotechnology, Chernivtsi National University named after Y. Fedkovych, Kotsiubynsky 2 Str., Chernivtsi 58012, Ukraine 2Danylo Zabolotny Institute of Microbiology and Virology of the NASU, Acad. Zabolotny 154str., Kyiv 03680, Ukraine 3Kyiv National University of Technologies and Design, Nemyrovycha-Danchenka 2 str., Kyiv 01011, Ukraine  Corresponding author: l.cheban@chnu.edu.ua Article info Article history: Received: 29th April 2021 Accepted: 7th Januar 2022 Keywords: Acutodesmus dimorphus Cerium dioxide nanoparticles Cerium ions Desmodesmus armatus Productivity Abstract The effect of various concentrations (1 μM – 100 mM) of citrate-stabilized cerium dioxide nanoparticles or cerium ions (3+) on the biomass production of two species of unicellular green algae Desmodesmus armatus (Chod.) Hegew and Acutodesmus dimorphus (Turpin) Tsarenko was studied, the amount of chlorophyll, proteins and lipids in the algae biomass was determined. It was shown that at the concentrations of 0.01 M to 0.1 M nanoparticles and cerium salt have pronounced toxic effects on the algal cultures, manifested by a sharp increase in the level of lipids in the biomass combined with the decrease in chlorophyll and protein. At lower concentrations, cerium dioxide nanoparticles stimulate algae biomass accumulation, probably due to a change in key metabolic pathways, accompanied by an increase in the accumulation of carbohydrates in the biomass. For cerium salt, these effects are less pronounced. Thus, depending on the concentration of the objects used, it is possible to obtain an increase in the food biomass production enriched with lipids or carbohydrates as appropriate to the biotechnological objectives. Introduction Of interest for the study are the group of rare earth elements, lanthanides, named from the first representative of the group. This group includes cerium, which is characterized by oxygen non- stoichiometry and relatively low toxicity (Ivanov et al. 2009; Shcherbakov et al. 2020). Cerium (III) salts are used as antineoplastic, antiemetic, antiviral, bacteriostatic and bactericidal agents (Jakupec et al. 2005; Zholobak et al. 2010; Charbgoo et al. 2017). This chemical element has attracted the attention of scientists as an inorganic antioxidant, capable of effectively protecting living systems against oxidative stress. Despite the low incidence and concentration of lanthanides in soils, these elements can have a stimulating effect on the growth and development of organisms (Chen et al. 2001; Goecke et al. 2015; Kang et al. 2020). Lanthanides are known to significantly improve light energy absorption, energy transfer, electron transfer rate and phosphorylation rate, thus stimulating the accumulation of organic matter in plant cells (Huang et al. 2008; Gong et al. 2011; Goecke et al. 2015). It is also believed that lanthanides can be used as substitutes for macronutrients (Zeng et al. 2000; Chen et al. 2001; Hong et al. 2002). mailto:l.cheban@chnu.edu.ua Nova Biotechnol Chim (2022) 21(2): e954 2 The ability of lanthanides, including cerium, to partially substitute Ca in plant cells under Ca- deficient conditions is already known (Hong et al. 2002; Chao et al. 2009). A similar compensation mechanism also works when plant cells are deprived of magnesium or manganese (Yin et al. 2009; Yuguan et al. 2009а, 2009b; Zhou et al. 2011). However, the biological behaviour of these elements is based on the principle of analogy with base metal ions: their properties are similar, but still not identical. Also, the complex mechanism of the correlation between the response of plant organisms to exposure to lanthanides and their amount in the nutrient medium is well established. The effects of even low concentrations of these elements on microalgae cells have also been shown to depend on the food security and the possible stress state of the latter (Laszló et al. 2001; Goecke et al. 2015; Řezanka et al. 2016). On the other hand, there is information on the toxic effects of CeCl3 salt (Chen et al. 2001; Tai et al. 2010; Kosak et al. 2018). The use of nanoparticles in the form of nanoacquahelates stabilized with carboxylic acids, particularly citric acid, is promising. Carboxylated nanoparticles have the ability to easily penetrate the cytoplasmic membrane and then dissociate from the ligands, which accounts for their high biological activity. The toxicity of nanoacetylates is thought to be much lower than that of the corresponding inorganic salts (Borisevich et al. 2012). Algae, as promising producers of organic raw materials for various needs, are convenient for studying the effects of nanopreparations (Jreije et al. 2020). Despite considerable interest in the prospects of using cerium on biological objects of different levels of organization, its effects on microalgae cultures remain understudied. We have shown the potential of two species of green algae, Desmodesmus armatus (Chod.) Hegew and Acutodesmus dimorphus (Turpin) Tsarenko, as feed organisms in industrial aquaculture (Cheban and Grynko 2017; Cheban et al. 2018). The biomass of genera Desmodesmus and Acutodesmus microalgae, due to their small size and rather high content of amino acids, proteins, polyunsaturated fatty acids, and carotenoids, is used as an alternative complete feed in aquaculture for feeding zooplankton and fish. At the same time, there is a need for techniques that can increase the productivity of these algal cultures both in overall biomass and in major nutrients. However, the mechanism of factors contributing to the growth of microalgae is not fully studied. Little is known about the mechanism of the positive effect of lanthanides on the cells of higher plants and whether it will be similar in the cells of algae, either. The choice of optimal concentrations that can cause a stimulating effect is also a problem. The aim of this work was to comparatively study the effect of nanocerium and cerium salt on the main indicators of growth activity and productivity of two green algae of the family Scenedesmaceae – D. armatus (Chod.) Hegew and A. dimorphus (Turpin) Tsarenko. Experimental The characteristics of the test-objects As test-objects cerium chloride salt (Sigma, USA) and citrate-stabilized sol contained cerium dioxide nanoparticles (3 – 4 nm particle size, 1 : 1 stabilizer-to-CDN ratios) were used. A citrate solution of equimolar concentration was used as a control. Citrate-stabilized aqueous sol of CeO2 was prepared by the method proposed by Ivanov et al. (2010). Briefly, 2.0 g of citric acid was mixed with 25 mL of a 4 M aqueous cerium (III) chloride solution). The resulting solution was rapidly poured, under stirring, into 100 mL of a 3 M aqueous ammonia solution, and then exposed for 2 h at ambient conditions and further boiled for 4 h. Then, the solution was cooled to room temperature and purified from precursors and by-products by sedimentation and further re-dispersion. According to TEM data, the average CeO2 particle size in this sol was about 3 – 4 nm (Fig. 1). Nova Biotechnol Chim (2022) 21(2): e954 3 Fig. 1. Photos from (A) – TEM microscopy and histogram of size distribution and (B) – high-resolution transmission electron microscopy (HRTEM) used cerium nanoparticles. Biological material and cultivation conditions The studies were performed on algological pure monocultures of green algae D. armatus (Chod.) Hegew and A. dimorphus (Turpin) Tsarenko (IBASH-A), obtained from the collection of the Kholodny Institute of Botany of the National Academy of Sciences of Ukraine, for which we express our gratitude to it. Algae were pre-cultivated under conditions of cumulative culture to the exponential growth phase in Zender and Gorham’s modification of the Fitzgerald's medium N 11 (Zolotareva et al. 2008). All manipulations related to the sowing of algae cultures were performed in a laminar box under sterile conditions. The inoculum-to-nutrient medium ratio was 1 : 10. Cultivation was performed in a climatic room with photoperiod of 16 hours provided by fluorescent lamps at a light intensity of 2,500 – 4,000 lux and temperature of 24 ± 2 ºC. To create optimum conditions for studying the effects of cerium salt and cerium nanoparticles, the algal biomass was concentrated and washed of nutrient medium residues in sterile distilled water. Algae cells were isolated from the culture medium by centrifugation at 3,500 g for 15 min. using the Heraeus Biofuge Stratos benchtop centrifuge (Heraeus Holding GmbH, Hanau, Germany). The final cell concentration was 2.6 × 105 cells.mL-1. Washed algae cells were incubated in a solution of distilled H2O with the addition of the appropriate concentration of cerium compounds. Nano-cerium sol, citrate, or cerium ions (III) were added to the hydrated cells at the concentrations of 0.001 mM, 0.01 mM, 0.1 mM, 1 mM, 10 mM and 100 mM. After 5-day exposure, the amount of algal biomass, cellular chlorophyll а, protein and lipid content were assayed. The amount of biomass was determined from culture density using an optical index at 750 nm at Agilent CaryWin UV 60 (Agilent Technologies, Inc., Santa Clara, USA). The transition from the optical density (OD) units (D750) to the value of absolutely dry biomass (ADB) was carried out through the empirical coefficient k: АDB = k × D750 (Hevorhyz et al. 2008). The coefficient k (k = g OD unit per liter) for both cultures was determined experimentally in three independent repeats. Following the cultivation, the entire biomass was separated by centrifugation at 3,500 g and 4 ºC for 15 min and, if necessary, frozen at - 20 ºC. Centrifuged microalgae biomass was disintegrated on an USDN-2T. In the hydrated cells, the total proteins were determined by the Lowry (Lowry et al. 1951) method and the total lipids were assessed (Knight et al. 1972). The pigments were extracted from the hydrated microalgae cells with chloroform-methanol 2 : 1 and centrifuged at 1,500 g until the discoloration of the extract (Macías-Sánchez et al. 2008). Pigment spectra were measured in the combined supernatant. The pigment concentrations were calculated by formulae using ODs at wavelengths corresponding to the absorption maxima (640 – 660 nm) of chlorophyll а. Data analysis Statistical treatment of data obtained was A B Nova Biotechnol Chim (2022) 21(2): e954 2 performed using a BioStat 2009 Professional 5.8.1 software in accordance with recommendations (Gtantz et al. 1998; Gubler and Genkin 1973). All values were measured in six-fold repeatability. Results are presented as the median and the interval between the first and third quartiles. A validity check for the null hypothesis was performed using non-parametric Wilcoxon-Mann-Whitney criteria. The difference between control and experimental groups was judged to be statistically significant at P < 0.05. Results Biomass accumulation is the first and indicative criterion for assessing the effect of compounds on algal cultures. The Table 1 showed the results of A. dimorphus and D. armatus biomass accumulation in the presence of CeCl3 solution, citrate or CDN in the cultivation medium. Compared to the intact culture medium, the presence of CDN in the culture medium at a concentration of 0.001 – 10.0 mM resulted in a statistically significant increase of the A. dimorphus biomass by 20 – 75 %. Presence of CDN in D. armatus culture medium resulted in a 25 – 50 % increase of algae biomass in a significantly narrower concentration range of 0.01 – 1.0 mM. It should be noted that the increase in biomass in concentration 10 – 100 mM of CDN or citrate can be attributed to citrate, a stabilizer in CDN, as an organic substrate, then lower concentrations of 0.001 – 1.0 mM were not caused by the presence of citrate, but rather by CDN. It is interesting to note that the introduction of 0.001 mM citrate into the culture medium statistically significantly inhibited the accumulation of biomass (growth) of the culture compared to the culture grown on a standard medium. The use of CDN stabilized with citrate (M : M) in a similar concentration on the contrary – increased the biomass yield by 20 – 30 %. At the same concentration, cerium salt did not affect accumulation of A. dimorphus biomass (Table 1). Table 1. CDN, citrate, or CeCl3 effect on the biomass accumulation in tested green algae. The test objects concentration [mM] A. dimorphus D. armatus CDN citrate CeCl3 CDN citrate CeCl3 0 9.7 [9.3 - 10.0] 11.4 [10.7 - 11.6] 0.001 12.6* [12.1 - 12.6] 7.6** [6.4 - 8.0] 9.5 [9.3 - 9.8] 12.0** [11.9 - 12.1] 10.2 [9.6 - 10.7] 10.7 [10.6 - 10.8] 0.01 14.2* [14.0 -14.3] 9.0 [8.1 - 10.0] 11.0* [10.2 - 11.9] 14.5* [14.1 - 14.8] 10.7 [10.6 - 10.8] 10.7 [10.1 - 11.8] 0.1 15.2** [14.7 - 15.8] 10.5* [10.3 - 11.0] 12.28** [11.1 - 12.6] 15.1* [14.3 - 15.8] 11.3 [11.0 - 11.8] 12.1 [11.3 - 12.6] 1 16.9** [16.5 - 17.8] 11.0* [10.3 - 11.6] 14.5** [14.0 - 15.1] 20.0* [19.5 - 20.7] 11.5 [11.1 - 12.0] 13.9* [13.2 - 14.0] 10 12.0* [11.9 - 12.1] 13.6** [13.4 - 14.1] 11.5* [11.2 - 11.8] 12.8** [12.7 - 13.2] 12.9** [12.9 - 13.0] 11.9 [10.9 - 12.9] 100 11.1* [10.5 - 11.2] 11.0* [11.0 - 11.2] 9.9 [9.7 - 10.6] 12.0 [11.2 - 12.4] 11.0 [10.4 - 11.7] 11.0 [10.9 - 11.1] Notes: * – P ˂ 0.05; ** – P ˂ 0.01, n = 6, results presented as the median and the interval between the first and third quartiles [Q1-Q3]. The maximum quantity of biomass of both cultures was observed when CDN was applied in an amount of 1 mM. Both algocultures showed a similar reactive response to exposure to all factors, some numerical differences being due to the species specifics of the algocultures. Thus, in A. dimorphus culture, the amount of biomass increased by 60 % upon CDN exposure, while in D. armatus culture it increased by 45 %. The results obtained clearly correlate with the amount of chlorophyll a in A. dimorphus and D. armatus cells (Table 2). 4 Nova Biotechnol Chim (2022) 21(2): e954 3 Table 2. CDN, citrate, or CeCl3 effect on сhlorophyll a concentration in green algae cells. The test objects concentration [mM] A. dimorphus D. armatus CDN citrate CeCl3 CDN citrate CeCl3 0 10.5 [10.2 - 11.0] 10.3 [10.0 - 10.6] 0.001 11.0 [10.9 - 11.7] 10.1 [8.8 - 11.2] 10.1 [9.7 - 10.6] 9.9 [9.7 - 10.1] 10.0 [9.7 - 10.4] 9.9 [9.3 - 10.5] 0.01 10.4 [10.0 - 11.0] 10.3 [9.6 - 11.1] 10.4 [10.1 - 10.9] 10.3 [10.0 - 10.6] 9.9 [9.7 - 10.1] 10.6 [10.0 - 11.0] 0.1 10.0 [9.5 - 11.5] 11.1 [10.7 - 11.4] 10.6 [10.3 - 10.9] 10.7 [10.1 - 10.9] 9.9 [9.2 - 10.5] 9.9 [9.4 - 10.0] 1 10.2 [9.8 - 10.6] 10.9 [10.1 - 11.6] 10.5 [10.4 - 11.1] 9.7 [9.5 - 10.4] 9.4 [9.2 - 10.6] 10.0 [9.7 - 10.6] 10 11.0 [10.7 - 11.2] 10.6 [10.4 - 10.9] 6.7** [6.5 - 7.0] 9.9 [9.2 - 10.1] 9.0* [8.8 - 9.8] 8.7** [8.1 - 8.9] 100 10.6 [10.3 - 11.3] 10.8 [10.4 - 11.0] 6.1** [5.7 - 6.8] 10.4 [10.0 - 10.9] 9.5* [8.7 - 9.9] 7.9** [7.3 - 8.0] Notes: * – P ˂ 0.05; ** – P ˂ 0.01, n = 6, results presented as the median and the interval between the first and third quartiles [Q1-Q3]. Its amount does not significantly differ from the control values, except for the concentration variants in which the growth activity of both algocultures is inhibited (10 – 100 mM). But even under these conditions, a decrease in the amount of chlorophyll a was noted when exposed to CeCl3, while in the presence of CDN, on the contrary, the negative effect of high concentrations of cerium ions on algocultures is compensated. Despite the positive effect of cerium both in ionic form and in the form of nanoparticles on the growth activity of A. dimorphus and D. armatus, a biochemical analysis showed some negative effect of CeCl3 on the process of protein accumulation in the algal biomass (Table 3). Table 3. The effect of CDN, citrate or CeCl3 on the protein content (% of biomass) in the biomass of tested green algae. The test objects concentration [mM] A. dimorphus D. armatus CDN citrate CeCl3 CDN citrate CeCl3 0 49.9 [48.6 - 50.9] 56.7 [56.0 - 57.5] 0.001 50.0 [46.1 - 51.2] 50.9 [49.5 - 51.5] 33.9* [33.3 - 34.8] 53.5* [53.3 - 54.6] 52.4* [51.9 - 53.6] 46.0** [45.5 - 46.8] 0.01 50.9 [49.3 - 51.4] 50.7 [49.6 - 51.8] 34.1* [33.1 - 35.0] 54.5* [53.4 - 55.7] 54.6* [53.5 - 55.2] 46.5** [46.0 - 47.5] 0.1 49.6 [49.2 - 50.7] 50.0 [48.3 - 51.0] 33.5* [32.8 - 34.7] 58.0* [57.8 - 58.1] 54.4* [53.5 - 55.3] 44.3** [43.9 - 44.8] 1 44.5** [42.7 - 45.7] 47.3 [47.0 - 48.6] 32.5* [31.3 - 33.5] 54.5* [52.5 - 55.3] 53.6* [52.4 - 54.7] 43.5** [42.3 - 44.0] 10 37.0** [35.1 - 38.0] 45.0* [44.1 - 45.8] 25.3* [24.1 - 26.0] 52.8** [52.2 - 53.1] 50.5** [50.0 - 51.6] 40.0** [39.9 - 42.1] 100 28.5* [27.9 - 30.5] 43.6* [43.1 - 44.7] 18.1* [17.5 - 22.1] 50.5** [49.6 - 51.7] 50.9** [50.3 - 51.1] 40.0** [38.9 - 41.6] Notes: * – P ˂ 0.05; ** – P ˂ 0.01, n = 6, results presented as the median and the interval between the first and third quartiles [Q1-Q3]. Thus, a rather low content of total protein in cells was noted in the presence of CDN ≥ 1 mM. The use of nanocerium in a smaller amount does not reliably reduce the protein content in algal cells. The use of CeCl3 in all cases is accompanied by a decrease in the amount of protein in A. dimorphus cells by 25 – 55 %, and in D. armatus cells by 10 – 25 %. While the amount of protein tended to 5 Nova Biotechnol Chim (2022) 21(2): e954 2 decline, there was an increase in the accumulation of lipids in the biomass of both cultures (Table 4). Table 4. The effect of CDN, citrate or CeCl3 on the lipid content (% of biomass) in the biomass of tested green algae. The test objects concentration, mM A. dimorphus D. armatus CDN citrate CeCl3 CDN citrate CeCl3 0 15.2 [14.6 - 15.6] 16.1 [15.8-16.7] 0.001 15.8 [15.5 - 16.1] 15.3 [14.7 - 15.9] 16.6* [16.0 - 17.4] 19.5** [18.7 - 20.1] 19.6** [19.1 - 20.7] 29.0** [28.1 - 29.7] 0.01 16.2 [14.8 - 16.8] 16.3* [15.9 - 17.2] 19.0** [18.9 - 19.5] 18.4* [18.0 - 18.9] 17.9* [17.2 - 18.6] 29.8** [29.0 - 30.0] 0.1 16.1 [15.2 - 16.3] 16.0* [15.8 - 16.6] 22.9** [22.8 - 23.6] 18.2* [17.9 - 18.7] 17.8 [16.5 - 18.7] 21.5** [21.0 - 22.0] 1 18.2* [18.0 - 18.9] 17.1* [16.8 - 17.3] 28.4** [27.7 - 29.5] 16.1 [15.2 - 16.3] 16.9 [16.3 - 17.3] 18.9** [18.2 - 20.0] 10 27.8** [25.7 - 28.7] 16.4* [15.9 - 17.0] 31.1** [30.8 - 31.6] 16.2 [14.8 - 16.8] 16.0 [15.7 - 16.6] 16.9* [16.8 - 17.3] 100 29.8** [28.1 - 31.3] 17.0* [16.7 - 17.6] 31.3** [30.2 - 32.5] 15.8 [15.5 - 16.1] 15.6 [15.1 - 16.0] 16.4 [16.0 - 16.9] Notes: * – P ˂ 0.05; ** – P ˂ 0.01, n = 6, results presented as the median and the interval between the first and third quartiles [Q1-Q3]. There is a clear correlation between the amount of CeCl3 in the medium and the increase in the number of total lipids in A. dimorphus and D. armatus cells. It should be noted that judging by changes in the number of total lipids in algal biomass in response to the presence of cerium ions. A. dimorphus culture is more sensitive than that of D. armatus. Thus, in the presence of 0.01 mM CeCl3 in the culture medium the amount of total lipids in A. dimorphus cells increases by 20 % while in the presence of ≥1 mM of CeCl3 this indicator increases by almost 100 %. CDN does not have such a strong impact on lipid accumulation by algae cells, which can be considered as an indirect sign of its lower toxicity. In the presence of 10 – 100 mM of CDN in the culture medium an increase of 80 – 90 % in the number of lipids in A. dimorphus biomass compared to control samples biomass was recorded. Thus, the results obtained allowed us to estimate the effect of CDN on cells of two green algae. A. dimorphus and D. armatus. Having compared all indicators. We determined the optimal CDN concentrations which when applied to bring an increase of the biomass in both algocultures and optimize this biomass by productivity outputs (Fig. 2). A 6 Nova Biotechnol Chim (2022) 21(2): e954 3 Fig. 2. The effect of CDN (А) or CeCl3 (B) on key indicators of the green algae viability. The data on the content of biomass, chlorophyll a, proteins, and lipids in percent are given, calculated relative to their amount in the control samples of biomass grown on a standard medium. Notes: on the abscissa – the concentration of cerium; on the y-axis – the percentage of content of biomass, chlorophyll a, proteins, and lipids in percent, calculated relative to their amount in the control samples of biomass grown on a standard medium (taken as 100 %), n = 6, error bars indicate [Q1-Q3] of the mediane. It was determined that 0.1 mM of CDN is the optimal amount of the preparation that causes positive changes in cells A. dimorphus and D. armatus. Under such conditions, it is possible not only to increase the yield of biomass of algal crops, but also to achieve balanced levels of chlorophyll a, proteins, and lipids. The concentration of 1 mM of CDN can be considered as suboptimal, recommended for use in cases of an urgent need for rapid mass growth of cell mass of green algae A. dimorphus and D. armatus. The use of CeCl3 in all analyzed concentrations leads to increased growth activity of algocultures, but also in all cases there is negatively affects the processes of proteins and lipids accumulation. The generalized results are presented in the form of a summary table, which allows to clearly determine, depending on the goal, the dose of CDN or CeCl3 (Table 5). Table 5. The nontoxic (CC0. mM) concentration of CDN and CeCl3. Criteria Acutodesmus dimorphus Desmodesmus armatus CDN citrate CeCl3 CDN citrate CeCl3 Chlorophyll a 100.0 100.0 1.0 100.0 1.0 1.0 Total proteins 0.1 0.5 ˂0.001 ˂0.001 ˂0.001 ˂0.001 Total lipids 0.1 0.001 ˂0.001 ˂0.001 ˂0.001 ˂0.001 Biomass increasing (> 20 %) ˂0.001 – 1.0 – 0.1 – 1.0 0.01 – 1.0 – 1.0 Discussion Like most trace elements, Ce depending on the concentration can have both positive and negative effects on the growth and development of algae. It is known that in the aqueous medium NP CeO2 undergo redox modifications, form polydisperse aggregates, which lead to a change in their catalytic activity (Santschi et al. 2017; Gagnon et al. 2018; Rozhin et al. 2021). First of all, it should be noted that the sharp increase in the amount of lipids found in D. armatus and A. dimorphus cells is a well-known response of algae cells to stress, in our case to the presence of high concentrations of cerium in the growth medium. A similar increase in the amount B 7 Nova Biotechnol Chim (2022) 21(2): e954 2 of lipids in response to the presence of alkaline earth metals has been described for Chlorella vulgaris and Scenedesmus obliquus (Gorain et al. 2013). Dunaliella tertiolecta, and Tetraselmis suecica (Ghafari et al. 2016). Stress activation of metabolic pathways of lipid synthesis by Ce3+ ions may be due to inhibition of further conversion of the intermediate product of glycolysis – glyceraldehyde-3-phosphate. In this case, instead of the synthesis of carbohydrates, the synthesis of triacylglycerols (Wang et al. 2009; Li et al. 2010) will take place, which will be accompanied by an increase in the accumulation of lipids in the cells of microalgae. As a consequence of such processes, the redistribution of the relation between the major macromolecules in the cells of algae, which we found. It is known that high concentrations (≈ 30 µM.L-1) of lanthanides lead to a sharp slowdown in the growth processes of unicellular algae (Tai et al. 2010), while low concentrations stimulate these processes (Laszló et al. 2001; Goecke et al. 2015; Řezanka et al. 2016). In our studies it was shown that in the presence of ≤ 1 mM cerium concentration in the medium, there is an intensification of growth processes in cultures D. armatus and A. dimorphus and, as a consequence an increase in the amount of biomass by more than 25 %. However, the effect of any chemical element depends also on the form of its introduction into the incubation medium (Chen et al. 2000; Tai et al. 2010). CeCl3 has a more pronounced toxic effect on algae cultures than CDN. A similar effect was shown by zirconium on the cells of the alga Chlorella pyrenoidosa (Laszló et al. 2001). Lanthanides can also replace certain metals in some physiological processes, as has been demonstrated, in particular, in plants with Ca2+ or Mg2+ deficiency (Huang et al. 2008а; Chao et al. 2008; Gong et al. 2011) The primary cause of this phenomenon may be the property of lanthanides, whose combinatorial ability is much higher than that of other divalent cations, to interact with a large number of biological macromolecules to form stable complexes (Goecke et al. 2015). It is believed that the ability of lanthanides, including cerium, to mimic some biological functions of Ca2+, primarily affects the processes of photosynthesis and ion transport in plant cells (Wei and Zhou 2000; Gong et al. 2011). Obviously, this property of cerium was decisive in the manifestation of the stimulating effect on the growth processes of green algae cultures D. armatus and A. dimorphus, studied by us. The stimulating effect of Ce on photosynthetic organisms is associated with the ability of the latter to accumulate in chloroplasts as well at low concentrations of Mg2+.to replace it in the chlorophyll molecule (Zhou et al. 2011). The effects we found are most likely related to an increase in carbohydrate production. The mechanism of this process is due to the influence of Ce ions on intracellular anabolic pathways in particular, transmebranic mitochondrial transport of pyruvate and oxaloacetate production. It is known that rare earth metals in general, and cerium in particular, significantly improve the process of light energy absorption, energy transfer, electron transfer rate and phosphorylation rate, thus stimulating the accumulation of organic matter in cells (Zeng et al. 2000; Gong et al. 2011). Conclusion Compared to CDN, CeCl3 solution is more toxic to green algae cells. CDN is 100 – 1,000 times less toxic than CeCl3 solution. It is shown that the use of high concentrations of Ce is accompanied by a well-known response of algae cells to the action of a stress factor – an increase in total lipids and a decrease in the synthesis of proteins and chlorophyll a. The introduction of CDN into the culture of algae in non-toxic concentrations (CC0 and below) provides a significant increase (25 – 75 %) in the total amount of biomass, whereas with the use of CeCl3 solution such an effect is absent. The obtained results allow assessing the mechanism of realization of biological effects of cerium dioxide nanoparticles. Conflict of Interest The authors declare that they have no conflict of interest. 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