The effects of auxins and cytokinins at the range of concentrations 0.0001-100 µM on Acutodesmus obliquus (Chlorophyceae) cultures were studied. Microalga exhibited sensitivity to cytokinins in the following order: 0.01 µM tZ > 0.1 µM Kin > 1 µM DPU, whereas the hierarchy of auxin activity was: 0.01 µM IAA > 0.1 µM IBA > 0.1 µM PAA. Cytokinins possessed higher stimulating properties on the cell number, whereas auxins increased the size of cells. Differences in the metabolite profiles of the cultures treated with phytohormones were observed. Auxins and cytokinins had a positive effect on the photosynthetic apparatus enhancing the level of chlorophylls, carotenes, and xanthophylls. In comparison with auxins, cytokinins more effectively delayed oxidative damage by increasing the level of non-enzymatic antioxidants (ascorbate, glutathione) and the activity of enzymes scavenging reactive oxidative species (catalase, glutathione reductase, ascorbate peroxidase). On the other hand, auxins stimulated superoxide dismutase activity and provoked hydrogen peroxide generation, which may be involved in cell enlargement. All phytohormones reduced the content of abscisic acid and controlled the level of endogenous auxin and cytokinins suggesting complex interactions. Different dynamics of A. obliquus responses to auxins and cytokinins clearly demonstrated their diverse roles in algal growth and metabolism.

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Aquatic organisms are exposed to many stressors, e.g., heavy metals. Brassinosteroids, a plant hormone group, can effectively stimulate plants to defend against the negative impact of a heavy metal. The present study was conducted with an aim to find out the influence of 24-epibrassinolide (EBL) on Acutodesmus obliquus treated with 0.01 and 500 μM of lead (Pb) ions during 7 days of cultivation. Pb has a toxic effect on algal cultures because it limits both the growth and development and induces oxidative stress. Simultaneously, 1 μM EBL was involved in protecting algal cells against the toxic effect of Pb. Despite the presence of Pb, EBL significantly increased the number of algal cells and their metabolite content (e.g., proteins, monosaccharides, chlorophylls, carotenes, and xanthophylls). Cultures treated concurrently with EBL and Pb were characterized by a reduction in their content of endogenous Pb, H 2 O 2 , and malondialdehyde. Also, EBL increased the activity of catalase, ascorbate peroxidase, superoxide dismutase, glutathione reductase, and the content of ascorbate and glutathione. EBL increased the phytochelatin synthase activity, thereby enhancing the production of phytochelatins accountable for both binding and detoxification of Pb. These results indicate the influence of EBL on the inhibitory effect of Pb in A. obliquus. These findings help to clarify the role of BRs in the algal adaptation to the prevailing stressful conditions.

Phytohormones play key roles in many aspects of plant growth and development, as well as in resistance to abiotic stresses. The presence of heavy metal limits phytohormone endogenous level; however, the application of brassinosteroids (BRs) restores phytohormone homeostasis and reduces unfavorable consequences of heavy metal on plant growth. Therefore, the aim of this study was to (1) broaden previously published analyses/findings to study the effect of 24-epibrassinolide (EBL) and/or lead (Pb) on Acutodesmus obliquus phytohormone level after 5 days of cultivation using LC-QToF-MS quantification technique and (2) extend the total amount of quantified phytohormones in A. obliquus. The study confirmed that exogenous EBL increased the contents of gibberellin A3 (GA3), auxins (AXs) (indole-3-acetic acid, phenylacetic acid), BRs brassinolide, 24-epibrassinolide (EBL), 28-homobrassinolide, castasterone, 24-epicastasterone, typhasterol, cathasterone, 6-deoxotyphasterol, as well as different types of cytokinins (CKs): free bases, ribosides, and conjugates (N- and O-glucosides). On the other hand, treatment with Pb had an opposite effect on BRs, GA3, AXs, and free bases, as well as ribosides of CKs. The abscisic acid (ABA) level decreased under EBL treatment but increased in response to Pb stress. The level of N-glucosides, as well as O-glucosides of CKs, also significantly increased in response to Pb alone. Interestingly, the co-application of EBL and Pb led to an increase in BRs, GA3, AXs, and CKs content in the algae. Principal component analysis revealed that based on treatments, increase of GA3, BRs, and AXs was found to be in the following order: 1 μM EBL > 0.01 μM Pb + 1 μM EBL > 500 μM Pb + 1 μM EBL > control > 0.01 μM Pb > 500 μM Pb. Moreover, very strong significant linear relations between almost all studied AXs and BRs were noted. Summarizing, this research did not only allow to detect the occurrence of ABA, GA3, AXs, BRs, and CKs in A. obliquus for the first time but also to study the combined action of EBL and Pb, as well as compare it with treatment alone.