Trichlorfon-Induced Inhibition of Nitrate and Ammonium Uptake in Cyanobacteria

The possibility that the primary effect of the toxic insecticidetrichlorfon is an inhibition of nitrate uptake in cyanobactenahas been investigated. A drastic reduction in the rate of uptakeis detected 3 h after the addition of the insecticide to batchcultures of nabaena PCC 7119. The dose-response curves indicatea relationship between the degree of inhibition of nitrate uptakeand the reduction of chlorophyll content and growth. Nitratereductase (ferredoxin : nitrate reductase, EC 1.7.99.4[EC]) activityis also lowered as a result of insecticide action. When AnabaenaPCC 7119 cells are grown with ammonium as a source of combinednitrogen, trichlorfon reduces the rate of ammonium uptake. Therate of uptake of both nitrate and ammonium is restored uponwashing the cells. Ultrastructural analysis of Anabaena nitrate-growncells shows that trichlorfon does not damage thylakoid membranes,but brings about the accumulation of enlarged cyanophycin granulesand the increase of carboxysome number. Nitrate uptake rateand chlorophyll and phycobiliprotein contents are also reducedby insecticide treatment in the cyanobacteria SynechococcusUAM 211, GloeothecePCC 6501, Plectonema calothricoides, NostocUAM 205 and Chlorogloeopsis PCC 6912. These results are consistentwith the inhibition of nitrate uptake due to weak adsorptionof trichlorfon to the plasmalemma being the main effect of theinsecticide on cyanobacterial metabolism. Key words: Nitrate uptake, cyanobacteria, Anabaena, ammonium uptake, trichlorfon