Mass culture and biochemical variability of the marine microalga Tetraselmis suecica Kylin (Butch) with high nutrient concentrations

Abstract

Mass cultures of Tetraselmis suecica 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 cultural 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 densities of 7.83 × 106 and 7.15 × 106 cells/ml were obtained with 8 and 16 mM of NaNO3, respectively. Growth velocity ranged between 0.53 and 0.63 doublings (dbl)/day, although 0.98 dbl/day were reached with 16 mM of NaNO3. Volume increased with nutrient concentration from 252 to 905 μm3. Protein content reached maximum values of 306 μg/ml or 59.8 pg/cell. In the logarithmic phase, protein was regulated by nutrient concentration and decreased according to this concentration. Maximum efficiency of transformation from nitrate to protein was 108%, obtained at 2 mM of NaNO3. Efficiency decreased, to 14%, when nutrient concentration increased. This fact indicates that the lowest cost of harvesting is obtained with a nutrient concentration of 2 mM NaNO3. Chlorophyll a cell reached values between 3.1 and 3.8 pg/cell in the stationary phase. There was a relationship between nutrient concentration and chlorophyll α cell in the logarithmic phase, with an increase from 2.15 pg/cell to 3.74 pg/cell. Changes in chlorophyll α level are related to nitrogen depletion. Carbohydrate/cell was constant at values of 19.84-28.68 pg/cell in the logarithmic and stationary phases and was not related to nitrogen depletion. RNA/cell ranged from 4.17 to 5.48 pg/cell, except at 2 mM of NaNO3 when it was 2.77 pg/cell, probably due to nitrogen depletion. The level of DNA/cell was constant in all the nutrient concentrations assayed and ranged from 0.1 to 1.09 pg/cell. Great variability in the chemical composition of T. suecica has been shown. Growth in mass cultures is closely coupled to changes in nutrient concentrations and variations occur in protein, chlorophyll α and RNA content, showing differences of 215%, 190% and 203%, 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 feed in mariculture.