Impacts of florfenicol on marine diatom Skeletonema costatum through photosynthesis inhibition and oxidative damages.

Effects of the phenicol antibiotic, florfenicol (0.5, 1.0, 2.0, 4.0, 8.0 and 16.0 mg/L), on marine diatom Skeletonema costatum were investigated in this study. Florfenicol was found to stimulate algal growth at concentrations of 0.5, 1.0 and 2.0 mg/L, and significantly inhibit algal growth at >2.0 mg/L. The highest inhibition rate was up to 86% at 16.0 mg/L and the IC(50) for 96 h growth was 5.043 mg/L. The chlorophyll a and effective quantum yield (ΔF/F(m)(')) were significantly inhibited at 6, 24 and 96 h when florfenicol concentrations were ≥4.0 mg/L. Intracellular reactive oxygen species (ROS) production was enhanced significantly over the control when florfenicol concentrations were ≥1.0 mg/L at 6 h with the dose-dependent trends possibly due to the inhibition of photosynthesis. Since the membrane is highly prone to ROS attack, overproduction of ROS may cause deteriorated integrity and permeability of the cell membrane. Consequently, intracellular pH was found to increase with the increases in dosage; cell size swelled significantly when alga was exposed to florfenicol concentrations up to 8.0 mg/L. These deteriorations finally led to the decrease of cell viability as indicated by both fluorescein diacetate (FDA) assay and propidium iodide (PI) staining, in which viability was shown to decrease significantly at higher doses (4.0, 8.0, 16.0 mg/L). It can be concluded that S. costatum was vulnerable to florfenicol.