Physiological responses and adaptive mechanisms of the harmful algal bloom species Heterosigma akashiwo to naphthalene exposure.

Polycyclic aromatic hydrocarbons (PAHs), especially naphthalene (Nap), pose a significant threat to coastal ecosystems and may contribute to the occurrence of harmful algal blooms (HABs). However, the response mechanisms of HAB species to PAH pollution remain unclear. This study investigated the physiological and molecular responses of the HAB species Heterosigma akashiwo to varying Nap concentrations. Low concentrations (1-10 μg/L) promoted growth, while high concentrations (20-1000 μg/L) inhibited growth, causing abnormal cell morphology and oxidative stress. Within 96 h, H. akashiwo removed 82.38 % (10 μg/L) and 88.93 % (100 μg/L) of Nap, mainly through biodegradation and intracellular accumulation. Transcriptomic analysis revealed that H. akashiwo employs a multifaceted adaptive strategy to cope with Nap stress. Key mechanisms include the upregulation of calcium signaling, transcription factors (e.g., zinc finger protein C2H2, myeloblastosis transcription factor, basic leucine zipper transcription factor), heat shock proteins (e.g., HSP40), and fatty acid desaturase (FAD), enhancing stress tolerance through detoxification, antioxidant responses, and membrane integrity. Activation of peroxisomal and oxidative phosphorylation genes indicates improved detoxification and energy metabolism. Differential expression of genes in the PI3K-Akt pathway reveals dose-dependent growth responses, with low concentrations promoting proliferation and high concentrations inhibiting growth. These findings offer insights into H. akashiwo's adaptive responses to Nap, aiding ecological risk assessment and bioremediation strategies for PAH pollution in marine environments.