Effects of Carbaryl and 1-Naphthol on Soil Population of Cyanobacteria and Microalgae and Select Cultures of Diazotrophic Cyanobacteria Mallavarapu Megharaj • Kadiyala Venkateswarlu • Ravi Naidu Received: 19 January 2011 / Accepted: 20 June 2011 / Published online: 26 June 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Carbaryl application to soil collected from a rice fallow field was relatively less toxic to viable estimates of cyanobacteria and microalgae under nonflooded condi- tions than under flooded conditions. Application of 1-naphthol, the hydrolysis product of carbaryl, to soil under both the regimes increased the population of both cyano- bacteria and microalgae. Soil application of carbaryl and 1-naphthol in combination, up to 1.0 kg ha -1 , was non- toxic to the viable population. The toxicity exerted by carbaryl and 1-naphthol towards growth, measured in terms of chlorophyll a, and nitrogenase activity was more pro- nounced in Anabaena spp. than in Nostoc spp. Keywords Cyanobacteria Á Microalgae Á Carbaryl Á 1-Naphthol Á Nontarget effects Carbaryl (1-naphthyl N-methylcarabamate), an agricultural and garden insecticide, is used worldwide on more than 120 different food and fibre crops, trees, and ornamentals (Ware 2000), mostly as a substitute for some persistent organochlorine insecticides (Ribera et al. 2001). Very low mammalian oral and dermal toxicity together with short half-life in the environment and exceptionally broad spectrum of insect control made carbaryl the most popular carbamate effective against 160 harmful insect pests (Ware 2001). Sevin Ò , the most common commercial formulation of carbaryl, is used extensively against many major agri- cultural pests at rates ranging from 0.25 to 2.7 kg active ingredient per hectare (Rajagopal et al. 1984). In view of its extensive use (4.5–6.8 million kg/year) in the United States alone (Cox 1993) as a broad spectrum contact and inges- tion insecticide with some systemic properties, carbaryl is the second most widely detected insecticide in surface waters (Martin et al. 2003). 1-Naphthol is the major product of carbaryl degradation following hydrolysis in soils (Venkateswarlu et al. 1980; Rajagopal et al. 1983; Gunasekara et al. 2008) and in pure cultures of microor- ganisms (Rajagopal et al. 1984). The wide use of carbaryl has increased the concern about possible pollution of water supplies and soils (Marco et al. 1993). Cyanobacteria and microalgae occur virtually in every terrestrial habitat on earth (Rindi 2007). The ecological position of cyanobacteria and microalgae at the base of food webs together with their essential roles in nutrient cycling and oxygen production is critical to many ecosys- tems (Sabater and Carrasco 2001). Diazotrophic cyano- bacteria that can fix atmospheric nitrogen have important implications for other biota (An and Kampbell 2003). Microalgae have been considered indicators of the bioac- tivity of environmental pollutants as they are compara- tively sensitive to many chemicals and vary in their response to a variety of toxicants (Real et al. 2003). Alterations of the species composition of soil communities of cyanobacteria and microalgae as a result of toxic stress may affect the structure and functioning of the whole ecosystem (Campanella et al. 2000; Wong 2000; Verdisson M. Megharaj Á R. Naidu Centre for Environmental Risk Assessment and Remediation, University of South Australia, Adelaide, SA 5095, Australia M. Megharaj Á R. Naidu Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, University of South Australia, Adelaide, SA 5095, Australia Present Address: K. Venkateswarlu (&) Department of Microbiology, Sri Krishnadevaraya University, Anantapur 515055, India e-mail: v_kadiyala@hotmail.com 123 Bull Environ Contam Toxicol (2011) 87:324–329 DOI 10.1007/s00128-011-0347-3