Resposta fisiologica de Chlorella vulgaris sob concentrações variáveis de cádmio, fósforo e nitrogênio
Chia, Mathias Ahii
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Microalgae are capable of adapting themselves to changes in environmental conditions through the production of different biomolecules. This study investigated the effects of the metal, cadmium on the ecophysiological response of microalgae under varying nutrient conditions. In the first phase of the study, we examined the growth, biomass production and biochemical composition of Chlorella vulgaris using semi-continuous cultures employing three growth media (LC Oligo, Chu 10 and WC media) to enable the selection of the most appropriate growth medium. The highest cell density, chlorophyll a, carbohydrate, protein and lipid concentration were found in Chu and LC Oligo media. Due to the cost effectiveness of using LC Oligo medium, it was the chosen as the preferred medium among the three tested media. The calorific values of C. vulgaris at different phosphorus (P) levels were investigated in the second phase of this study. Calorific value reported under replete concentration (13.78 kJ g-1) was less than those found under P limitation (30.47-33.07 kJ g-1). The highest calorific values without growth retardation were obtained at the 10-6 mol L-1 P. The third phase of this research involved determinations of growth, biomass production and biochemical composition of C. vulgaris under varying cadmium (Cd), phosphorus (P) and nitrogen (N) concentrations. Three P (6.0x10-7, 2.3x10-6 and 2.3x10-4 mol L-1) and N (2.9x10-6, 1.1x10-5, 1.1x10-3 mol L-1) concentrations were tested one at a time with or without the addition of Cd at 10-7 and 2.0x10-8 mol L-1. Cadmium at 10-7 mol L-1 severely reduced growth and biomass production when P or N were limiting. Under N limitation total intracellular carbohydrates were accumulated most intensively compared to the P treatments with or without Cd. The presence of 2.0x10-8 Cd stimulated higher carbohydrate production when P was limiting. Protein productivity decreased with decreasing N and P concentrations. Total protein was highest in the control. Under nutrient replete conditions, the growth and biomass inhibitory effects of xxii Cd were significantly reduced. Combined P limitation and Cd exposure stimulated higher total lipid production than only P or N limitation. Triacylglycerols (TAG) was the most accumulated lipid class under P and N limitation and Cd stress among all the neutral and polar lipid classes. Increasing P and N limitation and Cd exposure resulted in higher SAFA and MUFA concentrations. 16:1n - 11 was an exception among the MUFA as its levels decreased at low N. Total PUFA and ω3 PUFA levels, and PUFA:SAFA ratios increased under healthy growth conditions (high P and N without Cd). An exception among the PUFA was 18:2n 6 that responded to P treatments differently as its levels increased with increasing P and N limitation. A significant (p<0.05) positive relationship of PUFAs, AMPL and PL with P and N was observed. Total SAFA, 14:0, 18:1n 9 and 18:2n 6 were positively related to Cd and negatively correlated with P and N.