Alkaline phosphatase (phosphomonoesterase i.e. PMEase) activity in heterocystous cyanobacteria Anabaena flos-aquae, Nostoc calcicola, Calothrix brevissima, Scytonema javanicum and Hapalosiphon intricatus is known to be temperature and pH dependent. Maximum level of enzyme activity was recorded at either 35°C or 37.5°C. Also, the cell bound phosphomonoesterase enzyme was shown to exhibit pH optima of 10.0 or 10.2. A thermo-tolerant (tr) mutant isolated after MNNG (N-methyl-N'-nitro-N-nitrosoguanidine) mutagenesis in Calothrix brevissima exhibited 10°C higher temperature optima and comparatively high pH optima (pH 10.4) for phosphomonoesterase enzyme. The mutant grew with a maximum growth rate (k) at 50°C. Activation energy (Ea) for cyanobacterial strains was in a narrow range between 45 to 52 kJ mol⁻¹. A little variation in temperature and pH optima was also observed in phosphomonoesterase activity of Calothrix brevissima and its thermo-tolerant mutant while utilizing various organic phosphates as substrate what indicated the substrate dependence temperature and pH optima. Cyanobacterial strains grown at their respective temperature and pH optima differentiated spores less frequently though, coupled with early initiation of spore.
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