Insights into microalgae mediated biodegradation of diazinon by Chlorella vulgaris: Microalgal tolerance to xenobiotic pollutants and metabolism

Diazinon is one of the most widely used organophosphorus insecticides for agricultural activities, and it is highly toxic to mammals and other non-target organisms. The present study demonstrated the effective removal of diazinon from the aqueous phase by a freshwater, green microalga, Chlorella vulgaris. Among the four screened species (Scenedesmus obliquus, Chlamydomonas mexicana, Chlorella vulgaris and Chlamydomonas pitschmannii), C. vulgaris showed the highest removal capacity (94%) of diazinon at 20 mg L-1. The growth of C. vulgaris was significantly affected above 40 mg L-1 of diazinon, showing >30% growth inhibition after 12 days of cultivation. Significant enhancement of the microalgal growth in the exponential growth phase suggested a less/non-toxic nature of the diazinon by-products. Biochemical properties, including carotenoid, chlorophyll and antioxidant enzymes of C. vulgaris were influenced by diazinon at relatively high concentrations. The degradation rate constant (k) and the half-life (T1/2) of diazinon (0.5-100 mg L-1) ranged between 0.2304-0.049 d(-1) and 3.01-14.06 d, respectively. Gas chromatography mass spectroscopic (GC-MS) study suggested the formation of a less toxic by-product, 2-isopropyl-6-methyl-4-pyrimidinol (IMP) as a result of microalgal metabolism of diazinon. This study demonstrated that C. vulgaris is highly tolerant of diazinon, which could be voluntarily involved in the removal of traces of diazinon from contaminated wastewater and has potential application in the removal of such artificial toxins using algae. (C) 2016 Elsevier B.V. All rights reserved.