Regulatory control of exogenous phytohormone on multiple bioproducts using triple algal co-culture technique

Although the benefits of melatonin as a phytohormone in monoculture systems are well documented, its regulatory function in complex algal co-cultures remains largely unknown. This study explored the effects of melatonin at 0.5 and 1 mu M concentrations on biomass production and the accumulation of key metabolites, including astaxanthin, chlorophyll, phycobiliproteins, and lipids, in a triple algal consortium comprising Haematococcus pluvialis, Chlorella vulgaris, and Spirulina platensis, compared with a control group without melatonin supplementation under abiotic stress (high illumination and nitrogen deficiency). In the triple algal consortium, 1 mu M melatonin significantly enhanced the productivities of biomass, protein, and phycocyanin by 57 %, 40 %, and 23 %, respectively, compared to that of the control. Meanwhile, the 0.5 mu M melatonin treatment notably increased the productivity of astaxanthin (22 %), total carotenoid (37 %), and lipid (67 %) compared to that of the control. These results demonstrate that the accurate control of melatonin concentration is essential for maximizing bioprocess efficiency. Moreover, melatonin treatment enhanced superoxide dismutase enzyme activity and decreased reactive oxygen species accumulation under stress conditions, thereby activating the antioxidant defense system. Alterations in fatty acid fractions demonstrated that melatonin treatment led to an increase in monounsaturated and polyunsaturated fatty acid levels, accompanied by a reduction in saturated fatty acid content. These changes enhanced the biodiesel quality by improving the cetane number and cold flow properties and offered nutritional benefits. Therefore, the improved fatty acid profile supports the use of melatonin-treated algal consortia in both the fuel and functional food industries.