Biomass production and biochemical composition in mass cultures of the marine microalga Isochrysis galbana Parke at varying nutrient concentrations

Abstract

Mass cultures of Isochrysis galbana were carried out with four nutrient concentrations ranging from 2 to 16 mM of NaNO3 and salinity 35‰. An air flow of 15 l/min maintained a CO2 transference rate sufficient to keep the pH below 8.4. Using these conditions, equations were calculated by a multiple non-linear least squares regression of order four, enabling predictions to be made of growth kinetics and chemical composition. Maximum cellular density of 65.5 × 106 cells/ml was obtained with 4 mM NaNO3. Cellular volume was constant in the different nutrient concentrations. Protein content reached a maximum value of 374 μg/ml at 4 mM of NaNO3, and this concentration also presented the maximum efficiency of transformation from nitrate to protein, i.e. 114%. As a result, lowest costs for harvesting are obtained at a nutrient concentration of 4 mM NaNO3. Efficiencies decreased to 15% as nutrient concentration increased. Maximum values of chlorophyll a (21.9 μg/ml) and carbohydrates (213 μg/ml) were also obtained with 4 mM NaNO3. In the logarithmic phase, the contents of protein, chlorophyll a, carbohydrates, RNA and DNA per cell were constant. Chlorophyll a reached values between 0.15 and 0.33 pg/cell in the stationary phase. Carbohydrate levels reached the maximum value of 3.16 pg/cell with 4 mM NaNO3 in the stationary phase. The levels of RNA/cell and DNA/cell were constant in all the nutrient concentrations tested and in both growth phases, and ranged from 1.15 to 1.71 pg/cell for RNA and from 0.006 to 0.014 pg/cell for DNA. Growth in mass cultures is closely coupled to changes in nutrient concentrations and variations occur in protein, chlorophyll a and carbohydrate contents, showing differences of 177%, 220% and 136%, respectively, in the stationary phase. This biochemical variability, mainly in protein content, must have a marked effect on the nutritive value of this microalga as a feed in mariculture.