Abstract
Cyanobacterial diazotrophs play a significant role in environmental nitrogen economy despite their habitat either tropical or polar. However, the phenomenon by which it copes with temperature induced stress is poorly understood. Temperature response study of psychrophilic and mesophilic Nostoc strains explores their adaptive mechanisms. The selected psychrophilic and mesophilic strains were confirmed as Nostoc punctiforme and Nostoc calcicola respectively, by ultrastructure and 16S rDNA phylogeny. The psychrophilic strain has extensive glycolipid and polysaccharide sheath along with characteristic deposition of cyanophycin, polyhydroxybutyrate granules, and carboxysomes. This is possibly an adaptive strategy exhibited to withstand the freezing temperature and high intense of ultraviolet rays. The biomass measured in terms of dry weight, protein, and chlorophyll indicated a temperature dependant shift in both the psychrophilic and mesophilic strains and attained maximum growth in their respective temperature niches. At low temperature, psychrophilic organism exhibited nitrogenase activity, while mesophilic strains did not. The maximum glutamine synthetase activity was observed at 4 °C for psychrophilic and 37 °C for mesophilic strains. Activity at 4 °C in psychrophilic strains revealed their energetic mechanism even at low temperature. The nitrate and nitrite reductase of both psychrophilic and mesophilic strains showed maximum activity at 37 °C denoting their similar nitrogen assimilating mechanisms for combined nitrogen utilization. The activity studies of nitrogen fixation/assimilation enzymes have differential effects at varying temperatures, which provide valuable insights of physiological contribution and role of Nostoc strains in the biological nitrogen cycle.
Highlights
Cyanobacteria exist in various ecological niches ranging from polar to desert climate zones because of their versatility, diverse metabolism, environmental plasticity, and structural conservation (Premanandh et al 2009)
Freezing temperatures accompanied by freeze–cycles, extreme fluctuation in UV irradiance, and low availability of nitrogen are major problems in polar regions hampering the growth of cyanobacteria (Grzesiak et al 2015)
To understand the nitrogen metabolism of cyanobacteria at psychrophilic temperature, both psychrophilic and mesophilic Nostoc strains isolated from their respective regimes were compared for their growth and N2 fixing/assimilating capabilities in different temperatures
Summary
Cyanobacteria exist in various ecological niches ranging from polar to desert climate zones because of their versatility, diverse metabolism, environmental plasticity, and structural conservation (Premanandh et al 2009). Cyanobacterial diazotrophs play a significant role in nutrient cycling by their inherent ability to fix atmospheric nitrogen and carbon (Saha et al 2007; Prabaharan et al 2010). Cyanobacteria fixes atmospheric nitrogen in different thermal extremes, including Antarctic, where the temperature is always below 0 °C (Priscu et al 1998), in an arid desert (Winckelmann et al 2015) and in hot spring where the temperature ranges from 55 to 70 °C (Amarouche-Yala et al 2014). Nitrogen is found to be limiting in the Arctic ecosystem (Quesada et al 1999). Under nitrogen limiting conditions in these regions, diazotrophic cyanobacteria especially Nostoc genera predominate and contribute to a large extent towards the carbon and nitrogen economy (Komarek et al 2012; Viderova et al 2011). The study was planned to compare and assess the temperature influence on growth and their nitrogen fixation/assimilation ability of a Nostoc sp. of polar Arctic and mesophilic isolates
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