RRJoLS (2018) 31-43 © STM Journals 2018. All Rights Reserved Page 31 Research & Reviews: A Journal of Life Sciences ISSN: 2249-8656 (Online), ISSN: 2348-9545 (Print) Volume 8, Issue 3 www.stmjournals.com Growth and Nitrogen (N) Metabolizing Enzymes of Mesophilic and Psychrophilic Heterocystous Cyanobacteria—In Response to Temperature Regimes Shan Ahamed Tharifkhan, Deviram Garlapati, Dharshana Arulraj, Mathumathy Murugesan, Uma Lakshmanan, Dharmar Prabaharan* Department of Marine Biotechnology, Bharathidasan University, Tiruchirappalli, Tamil Nadu, India Abstract The increased NR & NiR activity, GS and nitrogenase activities at 25°C and 35°C in psychrophilic and mesophilic organisms indicates that the Arctic isolate is a psychrotolerant as it shows increased enzyme levels than its original temperature in which we isolated the organism though biomass was greater at 3°C. The biomass and enzyme data indicates cyanobacteria are the initiators of primary colonization in polar and alpine environments and are responsible for the nitrogen and carbon incorporation into biosphere. The nitrogenase activity as measured by ARA was high at 37°C mesophilic and 25°C was found to be optimum for psychrophilic. Likewise, NR & NiR was also optimal at 25°C. Keywords: Arctic, cyanobacteria, glutamine synthetase, nitrate reductase, nitrite reductase, nostoc calcicole *Corresponding Author E-mail: pub_nfmc@yahoo.co.in INTRODUCTION The prokaryotic cyanobacteria are characterized by their oxygenic photosynthetic ability. Globally wide range of environments are made up of complex, micro-scale ecosystems constitutes microbial mats of photosynthetic cyanobacteria [1]. The cyanobacterial existence in diverse ecological niches from polar to desert climate, due to their structural diversification with diverse metabolism and environmental plasticity [2]. The habitats, includes freshwater lakes, temperate soils, vast oceanic areas, and even in hot springs arid deserts and icy lakes [3]. Temperature is the critical regulatory factor affect the photosynthesis, growth and nutrient uptake by influencing enzyme activity, electron transfer chain, membrane fluidity [4]. Based on temperature influenced habitation in the planet, cyanobacteria can be classified into psychrophilic, psychrotolerant, mesophilic, and thermophilic forms [5]. The algal microflora of freshwater and terrestrial habitats are important in extreme ecosystems of polar hemisphere [6]. Temperature below 5°C is common throughout the cold environments of polar region [7]. Persistent cold due to extreme fluctuations in irradiance, freeze-thaw cycles leads large variations in nutrient supply and salinity. The biomass and species richness of prokaryotes in these ecosystems denotes restricted biodiversity as a result of these constraints. Fossil record states, cyanobacteria were the only organisms achieved complete morphological diversity 2 billion years ago [8]. In Antarctic ponds, streams, rivers, and lakes covered with perennial ice cover most of the year. The microbial mats primary photosynthetic production these habitats were controlled by PAR (photosynthetically active radiation) [9]. The factors such as salinity and nutrients also influence the cyanobacterial diversity [10]. In addition to other biota of Arctic ice shelves, glaciers, provide habitats for cyanobacteria [11,12[AQ: Please verify if the citation given here for Säwström et al., 2002; Mueller et al., 2005 is correct.] However, the total area of ice shelves is lower than in Antarctica, where ice shelves fringe 40% of coastline. Nostocales, Oscillatoriales, and Chroococcales constitute the most common groups. Incorporation of Nitrogen into the