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The joint effect of temperature and irradiance on growth, biochemical composition and antioxidant enzyme activities of the newly isolated Bulgarian Scenedesmus sp. strain BGP was investigated. The strain grew well over the temperature range 16-32°C under two different photon densities -- 132 µmol photons m-2 s-1 (low light intensity) and 2 x 132 µmol photons m-2 s-1 (high light intensity). Higher temperatures (35 and 42°C) suppressed the growth. The optimum conditions for growth and accumulation of lipids and proteins were 25°C and high light intensity. At low light intensity, the suboptimum temperatures promoted the protein synthesis. The carbohydrate content was highest at 30°C and high light intensity. Superoxide dismutase and glutathione reductase activities at 16°C were lower than at the optimum temperature, whereas at 35°C the activities of catalase, glutathione reductase, and one superoxide dismutase isoform were elevated. The substantial growth and balanced biochemical composition over a wide temperature range (16-32°C), as well as the lack of stress response of antioxidant enzymes to low temperature indicated the temperature tolerance of Scenedesmus sp. BGP and its suitability for large-scale cultivation. [ABSTRACT FROM AUTHOR]

The choice of nitrogen source in a cultivation medium can specifically affect the physiology and biochemistry of microalgae. To increase the production of low-cost valuable biomass, the preferred nitrogen form for each alga should be determined. The aim of our study was to analyze the effects of different nitrogen sources and cultivation times on the growth, biochemical composition and the activities of glutamine synthetase, glutamate synthase, glutamate dehydrogenase, malate dehydrogenase, aspartate aminotransferase and proteases of Chlorella vulgaris R-06/2. Media supplemented with urea or ammonium nitrate provided similarly (p>0.05) high growth rates for a short cultivation time (4 days). The two nitrogen compounds applied simultaneously ensured better biomass yield but for prolonged cultivation. In the exponential growth phase, ammonium nitrate stimulated (p