Bull. Environ. Contam. Toxicol. (1994) 53:292-297 9 1994 Springer-Verlag New York Inc. Environmental ~ Contamination ~and Toxicology Biodegradation of Methyl Parathion by Soil Isolates of Microalgae and Cyanobacteria M. Megharaj, ~ D. R. Madhavi, 2 C. Sreenivasulu, 2 A. Umamaheswari, 2 K. Venkateswarlu 2 1Department of Botany, Nagarjuna University, Nagarjunanagar 522510, India 2Department of Microbiology, Sri Krishnadevaraya University, Anantapur 515003, India Received: 12 October 1993/Accepted: 14 January 1994 Methyl parathion (O,O-dimethyl O-p-nitrophenyl phosphorothioate), a sub- stitute for its more toxic, now banned analogue, parathion, is extensively used in rice cultivation to control various insect pests (Adhya et al. 1981). The nature and extent of microbial degradation of organophosphorus insecticides by bacteria and fungi have been well documented (Ware and Roan 1970; Barik 1984). Microalgae and cyanobacteria have also been implicated in the metabolism of certain organophosphates such as phorate (Ahmed and Casida 1958), malathion (Christie 1969), parathion (Zuckerman et al. 1970), and monocrotophos and quinalphos (Megharaj et al. 1987). A perusal of the avail- able literature indicates that there are no studies on the metabolism of methyl parathion by soil isolates of microalgae and cyanobacteria (Venkateswarlu 1993). The present study was therefore aimed at determining the role of two green microalgae and four cyanobacterial species, all isolated from soil enrich- ments, in degradation of methyl parathion. MATERIALS AND METHODS Enrichment cultures were developed by adding 1 ml of 1000 ppm aqueous solution from the commercial formulation of methyl parathion (Metacid 50 EC from Bayer (India) Ltd., Bombay) to 20-g portions of the soil samples, maintained at 50% water-holding capacity. After five additions of the insecticide at 10-day intervals, the soil samples were withdrawn for isolation of predominant microal- gae and cyanobacteria following, the most-probable number (MPN) method (Muralikrishna and Venkateswarlu 1984). Two species of microalgae, Chlorella vulgaris and Scenedesmus bijugatus, and four of cyanobacteria, Nostoc linckia, N. muscorum, Oscillatoria animalis and Phorrnidium foveolarum, were isolated from the enrichments, and raised to axenic cultures. The two diazotrophs, N. linckia and N. muscorum, were grown in Allen's nitrogen-free medium while others were maintained in Bold's basal medium as described earlier (Megharaj et al. 1991a). The ability of selected algal and cyanobacterial species to utilize the insecticide as a source of phosphorus and nitrogen was tested by streaking the cultures on agarized growth medium without N and P, but with methyl parathion. The plates were observed for visual growth of the cultures over a period of 3 weeks. Correspondence to: K. Venkateswarlu 292