Response of the antioxidant system in Gracilariopsis lemaneiformis under the stress of diatom attachment

The cultivation of G. lemaneiformis , an economically important macroalga was affected by environmental factors, including attachments on G. lemaneiformis . The study on the antioxidant system using transcriptomics and other methods indicates that the antioxidant system of G. lemaneiformis is dysregulated under attachment stress in marine aquaculture zones, leading to a significant increase in reactive oxygen species (ROS) ( p < 0.05). Nitzschia sp. A1 was identified as the primary diatoms in the attachments, which was then co-cultured with G. lemaneiformis . Transcriptomic analysis revealed 1286 differentially expressed genes (DEGs) in HN groups and 748 DEGs in LN groups under Nitzschia sp. A1 attachment. In the arginine biosynthesis pathway, the transcriptional downregulation of otc , decreased the stress tolerance of G. lemaneiformis under adverse conditions. On day 6, O₂⁻ and H₂O₂ levels in HN group G. lemaneiformis challenged by Nitzschia sp. A1 increased by 237.72% and 18.32% relative to Control, respectively, despite increased SOD and CAT activity ( p < 0.05), indicating insufficient ROS neutralization. This redox imbalance manifested as 34.55% rise of malondialdehyde (MDA) accumulation in HN group relative to control at day 6. Moreover, TEM observations revealed gradient damage to mitochondria and plastids that increased with stress intensity under diatom attachment. In the LN group, only slight mitochondrial swelling, mild cristae disorganization, and rupture of the plastid outer membrane were observed. The HN group exhibited severe mitochondrial swelling, extensive cristae degradation, frequent vacuolization, as well as marked expansion of thylakoids, rupture of the outer membrane, and loss of structural integrity in plastids. This study investigates the response mechanisms of G. lemaneiformis under biofouling stress, providing a theoretical foundation for breeding anti-fouling strains. These findings suggest that modulation of OTC expression, such as OTC overexpression, may represent a potential strategy to alleviate diatom-induced stress in G. lemaneiformis , though functional validation via overexpression experiments was required.