University of Guilan Caspian Journal of Environmental Sciences 1735-3033 23 5 2025 12 01 Effects of Haematococcus pluvialis green alga extract on the expression of p53 and Bcl-2 genes in the MDA-MB-231 human breast cancer cell line 1303 1311 9260 10.22124/cjes.2025.9260 EN Pouya Ghaleb Department of Biology, Faculty of Basic Sciences, University of Guilan, Rasht, Iran Fatemeh Moradi Department of Biology, Faculty of Basic Sciences, University of Guilan, Rasht, Iran Mahmoudreza Aghamaali Department of Biology, Faculty of Basic Sciences, University of Guilan, Rasht, Iran Journal Article 2025 12 05 Seaweeds are a natural source of a wide range of secondary metabolites with different biological activities, including anticancer activity. The present study aimed to investigate the toxicity of aqueous and organic extracts of the <em>Haematococcus pluvialis</em> green microalgae to varying concentrations on the MDA-MB-231 breast cancer cell line and to investigate the expression of genes involved in the apoptosis pathway, Bcl-2, and p53. The toxic effect of different concentrations (50, 100, 150, and 200 μg mL^-1) of aqueous, methanolic, hexane, and chloroform extracts of the microalgae was measured by the MTT method, and then the expressions of p53 and Bcl-2 genes were evaluated by the RT-qPCR method. The percentage of cancer cell viability was inversely related to the concentration of the extracts and the treatment time. Among the extracts, the hexane extract of microalgae at a concentration of 200 µg mL^-1showed the highest cytotoxic effect (18%) compared to other extracts. Also, the expression of Bcl-2 and p53 genes in the treatment with hexane extract significantly decreased and elevated compared to the control and aqueous extract, respectively. As a result, the toxicity effect of the hexane extract of <em>H. pluvialis</em> against MDA-MB-231 breast cancer cells was efficient, so it can be a basis for further studies to introduce this green microalga as a suitable candidate for a biological compound with anticancer activity. https://cjes.guilan.ac.ir/article_9260_2f286ae241fdd5a7f5e02762b9b0f27b.pdf