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The hormone melatonin (MT) is produced by the pineal gland in animals and can also be found in plants and algae. MT regulates not only sleep, reduces the effects of jet lag, and has anti-aging properties in animals, but also eliminates intracellular reactive oxygen species and promotes vegetative growth in plants. However, research on its biological function in microalgae is still rare. The present study involved the construction of the Chlamydomonas reinhardtii expression vector pDBle-AANAT, which harbours the gene of one of the enzymes of the MT synthesis pathway, aralkylamine N-acetyltransferase (AANAT). The AANAT gene was introduced into the genome of the model microalga C. reinhardtii via glass bead transformation. The AANAT transgenic algal strain was identified by resistance screening, culture, and molecular biology methods. Changes in MT content between transformed algal strain and nontransformed algal strains were detected and the antioxidant capacity of the AANAT overexpressing algal strain was analysed under salt stress. The results showed that genetically engineered algae can increase the content of MT in algal cells by overexpressing AANAT. Under salt stress conditions, this can help to improve the enzymatic activity of the antioxidant system of algal cells and reduce membrane damage to counteract the adverse external stress. This enhances the self-protection ability of C. reinhardtii. This study reveals the effects of MT overproduction on the physiological functions of C. reinhardtii. Additionally, the potential functions of MT in the response of C. reinhardtii to stress have been elucidated, thus broadening the functional scope of MT in plants. [ABSTRACT FROM AUTHOR]

Melatonin (MT; N-acetyl-5-methoxy-tryptamine), a natural derivative of tryptophan, is a central biomolecule synthesised by almost all organisms, including animals and plants. As an antioxidant, MT is involved in many cell responses associated with the protection of plants against abiotic stress. However, there are few studies on the function of MT in microalgae. Our work focused on the relationship between exogenous MT and salt stress in the single-celled eukaryote Chlamydomonas reinhardtii, evaluating for growth, cell morphology, malondialdehyde (MDA) content (the peroxidation product of the cell membrane), activity of antioxidant enzymes and MT concentration under 100 mM NaCl treatment. Growth of Chlamydomonas was promoted at MT concentrations of 0.1 and 1.0 μM, but at 10 and 100 μM, MT was associated with negative effects on growth. MT at 1 μM maintained intact cell morphology and cellular aggregation in an unfavourable salt environment. MT concentrations of 0.1, 1 and 10 μM MT led to increased activity of the antioxidants superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) but reduced MDA. In conclusion, by increasing relative enzyme activity and maintaining a balanced redox state, exogenous MT may improve salt tolerance of C. reinhardtii to allow normal growth. A suitable concentration of exogenous MT improved the growth of C. reinhardtii in a salt environment and increased the activity of antioxidant system–related enzymes to protect cells. These results provide insight into the function of MT in a model green alga. [ABSTRACT FROM AUTHOR]