RENEWAL RATE AND NUTRIENT CONCENTRATION AS TOOLS TO MODIFY PRODUCTIVITY AND BIOCHEMICAL-COMPOSITION OF CYCLOSTAT CULTURES OF THE MARINE MICROALGA DUNALIELLA-TERTIOLECTA

A factorial experimental design with two nutrient concentrations (2 an d 4 mmol Nl(-1) in the form of NaNO3) and five rates of daily renewal of the cultures (10%, 20%, 30%, 40% and 50%) was carried out in cyclos tat, light/dark-synchronized cultures of the marine microalga Dunaliel la tertiolecta Butcher. Steady-state cellular density was a linear fun ction inversely proportional to renewal rate. Maximal cellular product ivity, 3 x 10(9) cells l(-1) day(-1), equivalent to 0.24 gl(-1) day(-1 ) dry weight and 0.17 gl(-1) day(-1) organic weight, was found with re newal rates of 20%-30% and 4 mmol Nl(-1), but maximal protein producti vity, 0.066 gl(-1) day(-1), was obtained with a renewal rate of 40% fo r both nutrient concentrations. The protein content ranged between 30% and 70% of the organic fraction depending on the culture conditions. Carbohydrates were the only fraction accumulating in response to nutri ent stress, ranging from 57% to 10% of the organic fraction, meanwhile the lipid content was increased by increase of nutrient availability. Under non-nitrogen-limited conditions the C:N ratio stabilized around 5.2-5.3 and the protein content of the organic fraction around 70%, b ut the cell nitrogen quota decreased under these conditions with incre asing renewal rates, owing to the lower organic content of cells obtai ned with high growth rates. The high capacity for changing the biochem ical composition, demonstrated for D. tertiolecta in the cyclostat sys tem, has interesting implications for the management of continuous cul tures of microalgae and its applications in biotechnological processes .