Responses of Bloom-Forming Heterosigma akashiwo to Allelochemical Linoleic Acid: Growth Inhibition, Oxidative Stress and Apoptosis

Abstract

Algal blooms have been occurring in many regions worldwide, and allelochemicals are important algaecides used to control harmful algal blooms (HABs). The allelopathic effects of linoleic acid (LA) on the harmful raphidophyte Heterosigma akashiwo were studied, and the possible mechanisms were investigated through analyses of population growth dynamics, cellular ultrastructure and the physiological levels of H. akashiwo. The results showed that the inhibitory effect of LA on H. akashiwo cells increased with an increasing LA concentration. The levels of ROS and MDA were significantly elevated, indicating oxidative stress and lipid peroxidation due to LA exposure. At the same time, LA also activated the antioxidant system, including superoxide dismutase (SOD), catalase (CAT), and POD, and non-enzymatic antioxidants such as reduced AsA and glutathione (GSH). Transmission electron microscopy (TEM) revealed that the morphology of the algal cells was impaired in an LA-dependent manner. Annexin V-FITC/PI double staining and flow cytometric analysis revealed that LA exposure decreased the cellular mitochondrial membrane potential (MMP), increased the rate of apoptosis. LA modulated bcl-2/bax homeostasis and increased the expressions of cytochrome c and caspases-3 and -9, proving that LA induced cell death via the mitochondria-mediated apoptotic pathway. It was suggested that LA had allelopathic effects on H. akashiwo, inducing physiological and morphological changes and finally triggering the apoptosis of H. akashiwo. All of these results showed that LA might have the potential as an algaecide to control harmful algae.