Proteomic mechanisms for the regulation of growth, photosynthetic activity and nitrogen fixation in Nostoc sp. PCC 7120 exposed to three antibiotic contaminants

This study investigated the influences of three frequently detected antibiotics in surface waters, ciprofloxacin, tetracycline and sulfamethoxazole, on the growth, photosynthetic activity, nitrogen-fixing capacity and proteomic expression profiles of Nostoc sp. PCC 7120, through a 15-day exposure test at environmentally relevant exposure doses of 50-200 ng/L. Cyanobacterial growth was stimulated by 100 ng/L and 200 ng/L of ciprofloxacin and sulfamethoxazole as well as 50-200 ng/L of tetracycline. The nitrogenase synthesis ability in each cyanobacterial cell was stimulated by 50-200 ng/L of ciprofloxacin while inhibited by 100 ng/L and 200 ng/L of tetracycline and sulfamethoxazole. At the exposure dose of 100 ng/L for each antibiotic, the variation of total nitrogen in the culture medium indicated that the nitrogen-fixing capacity of Nostoc sp. was determined by total nitrogenase concentration calculated by cell density x nitrogenase synthesis ability. Therefore, ciprofloxacin enhanced nitrogen fixation through the stimulation of both cyanobacterial growth and nitrogenase synthesis, while tetracycline and sulfamethoxazole enhanced nitrogen fixation merely through growth stimulation. At the exposure dose of 100 ng/L, only two downregulated proteins, a phosphonate ABC transporter and a methionine aminopeptidase, as well as one upregulated protein, the phenylalanine-tRNA ligase alpha subunit, were commonly shared by three antibiotic-treated groups. Ciprofloxacin upregulated proteins related to nitrogen fixation, carbon catabolism and biosynthesis, but downregulated photosynthesis-related proteins. In contrast, tetracycline and sulfamethoxazole increased the photosynthetic activity of Nostoc sp. through upregulating photosynthesis-related proteins, but downregulated proteins related to nitrogen fixation, carbon catabolism and biosynthesis. The resistance of Nostoc sp. PCC 7120 to three target antibiotics were related with the responses of RNA synthesis regulatory proteins. Stimulation of cyanobacterial nitrogen fixation by antibiotic contaminants could aggravate eutrophication in aquatic environments.