Phosphorus is an essential constituent of all living cells. The element is a part of the fundamental building blocks that constitute nucleic acids, phospholipids and complex carbohydrates. Phosphorus compounds play a central role in anabolic and catabolic pathways and in the energy conversion of the cell via transfer of energy-rich phosphoanhydride bonds, a reaction which is also involved in post translational regulation of enzyme activities (Wagner and Falkner, 2001) . In most temperate lakes,

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... phosphate deficiency is a common phenomenon and earlier studies confirmed that phosphate-deficient organisms are capable of accumulating this nutrient in several-fold excess of their immediate demand when exposed to considerable amounts of phosphate. This tendency to store phosphate, predominantly in the form of polyphosphates, is referred to as the polyphosphate over plus phenomenon (Kulaev and Vagabov, 1983) . Esterified phosphates may be released from organic phosphates through the action of cellular and extracellular alkaline phosphatases (APases) by catalyzing the hydrolysis of phosphate ester bonds. Organisms ranging from bacteria to higher plants, including algae and cyanobacteria, synthesize phosphatase in response to low avail-ability of free phosphates to access otherwise non-metabolizable organic phosphate resources (Carr and Mann, 1994; Hallmann, 1999; Singh and Tiwari, 2000; Tadano et al., 1993) . The APase activity appears to be widespread among cyanobacteria under P-limited conditions (Healey, 1982) . Ultrastructural analysis of APase in Plectonema boryanum showed that the enzyme is localized in layer 3 (periplasmic space) of the cell wall. During recent decades, a growing interest in the phosphate regeneration of those aquatic agroecosystems (waterlogged rice-fields) which have plenty of organic phosphate compounds, has led a number of workers to study the algal, including cyanobacterial phosphatase, activity and its regulation, since these organisms constitute the dominant flora in such ecosystems. Keeping this fact in view and considering the worldwide metal pollution problem, a study of phosphate uptake and its internal accumulation, as well as the properties of alkaline phosphatase of a N 2 -fixing cyanobacterium Nostoc muscorum under Cd stress, may help in understanding the phosphorus metabolism and the potential of enzymatic regeneration of inorganic phosphate from polyphosphate compounds in a metalstressed ecosystem. The cyanobacterium Nostoc muscorum was used as the experimental organism. The clonal and axenic populations were obtained by employing standard microbiological techniques. The cultures were main-