Effects of sulfamethoxazole and trimethoprim on Phaeodactylum tricornutum and Skeletonema costatum: growth, oxidative stress, biochemistry, attenuation, transcriptome and metabolome.

Trimethoprim (TMP) and sulfamethoxazole (SMX) are commonly detected antibiotics in marine environments, yet their potential hazards to marine microalgae under co-occurrence remain poorly understood. This study investigated the combined toxic effects of environmentally relevant concentrations of TMP (200 ng/L) and SMX (50-500 ng/L) on two marine microalgae species, Skeletonema costatum and Phaeodactylum tricornutum. The results revealed that the antibiotic mixture stimulated the growth of P. tricornutum. In the group treated with 100 ng/L SMX plus 200 ng/L TMP, algal cell density increased significantly compared with the control group (P < 0.05), with a growth rate 14.2% higher than the control. By contrast, the combined antibiotics had no notable impact on the growth of S. costatum. The mixture also altered the chlorophyll content, cell membrane hydrophobicity and antioxidant defense system of P. tricornutum. After 144 h of exposure to 250 ng/L SMX and 200 ng/L TMP, the malondialdehyde (MDA) content in P. tricornutum rose significantly (P < 0.05), with MDA content reaching 175.46% of the control level. At the molecular level, the activation of the ABC transporters pathway served as an adaptive detoxification strategy for microalgae under single TMP stress. The combined transcriptional and metabolic changes in the valine, leucine and isoleucine biosynthesis pathway were the key factor for TMP to amplify SMX toxicity. The combined disruption of this pathway, porphyrin metabolism and lipid metabolism jointly affected algal growth. The findings provide a reference for evaluating the potential risks of mixed antibiotics to marine primary productivity.