Differential transcription of the ascorbate peroxidase gene (FsAPX) in the freshwater diatom Fragilaria saxoplanctonica under metals and pesticides and its environmental implications.

Ascorbate peroxidase (APX) is an essential enzyme that combats cellular oxidative stress caused by environmental contaminants. Freshwater diatoms are sensitive to such contaminants and are used in water quality assessment; however, research on oxidative stress genes of diatoms remains insufficient. Here, we determined the APX gene (FsAPX) sequence from the freshwater diatom Fragilaria saxoplanctonica and evaluated its molecular structures and transcriptional responses to contaminants. The FsAPX open reading frame was determined at 1,014 bp, encoding 338 amino acids with conserved heme-binding and potassium-binding residues. It was phylogenetically related to other diatoms, with APX domain architecture. The gene expression patterns varied considerably depending on the tested metals: As, Zn, Cd, Cr, Cu, and Ni. Exposures to As and Zn significantly increased FsAPX expressions, while the other metals showed little changes. Interestingly, the FsAPX expression levels increased with all tested pesticides, atrazine, chlorpyrifos, and metolachlor, and their patterns were similar. The contaminants that induced gene expression increased reactive oxygen species in cells, while they decreased chlorophyll autofluorescence. These suggest that FsAPX may play an important role in the diatom defense system against cellular stress and may serve as a candidate indicator for molecular toxicity to assess specific contaminants in freshwater environments.