Nutrient regulation of alkaline phosphatase (Phosphomonoesterase - PMEase) was studied in some diazotrophic cyanobacterial strains like Anabaena variabilis, Anabaena torulosa, Calothrix brevissima, Scytonema javanicum and Hapalosiphon intricatus, in response to the macronutrients (Phosphate, Calcium and Magnesium) and the micronutrients (Zinc, Copper, Iron and Manganese). The phosphate grown cells of cyanobacterial strains when transferred to the phosphate deficient medium, showed expression of cellular PMEase activity and released the enzyme extracellularly. The increased concentration of phosphate inhibited enzyme activity severely in a concentration dependent manner. The phosphate depletion stimulated spore formation in A. variabilis and H. intricatus, whereas its addition enhanced spore's differentiation in A. torulosa. The switch-on time detected for both cellular and extracellular PMEase varies among the strains. The increase in ionic concentration of Ca²⁺ enhanced the PMEase activity more profoundly than Mg²⁺ in diazotrophic strains. The lower level of micronutrients either promoted or had no effect on photosynthetic inhibitors (DCMU) and respiratory electron transport chain inhibitor (sodium azide). It revealed that the energy for the synthesis of PMEase enzyme is mostly derived from photosynthesis and the role of respiratory energy is marginal. The Phosphate (Pi) uptake function in the strains was found to be substrate concentration dependent.
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