Altering growth and gross chemical composition of two microalgal molluscan food species by varying nitrate and phosphate

Abstract

Growth and gross chemical composition of two axenically cultured algal flagellates, Dunaliella tertiolecta and Tetraselmis maculata, were related to nitrate and phosphate concentrations in an enriched seawater medium. Population growth of test species in 28-day batch cultures was compared in 16 combinations of NaNO 3 (77.5, 150, 300, and 350 mg/l) and KH 2PO 4 (5, 10, 20, and 25 mg/l) in an experiment of factorial design. Phosphate-P was not limiting to final populations of either alga at concentrations tested, and nitrate-N did not affect final population density of D. tertiolecta at the lowest concentration. In contast, final populations of T. maculata were considerably reduced in all media containing the lowest NaNO 3 concentration, but maximal populations were achieved in 150 mg/l NaNO 3. This was observed at a wide range of N:P ratios and appeared to be related to available N only. Production of protein, carbohydrate, and lipid, as percentages of dry weight, was evaluated for selected concentations of NaNO 3 KH 2PO 4 ( 77.5 5 , 150 5 , 300 5 , 300 20 mg/l) At low P, protein/carbohydrate showed a direct linear relationship to N in both species. Only D. tertiolecta cultured at low N increased lipid production. Effect of increasing P in high N cultures was to increase protein/carbohydrate in T. maculata, but to decrease this ratio in D. tertiolecta. Maximal yields per unit volume were attained in the following NaNO 3 KH 2PO 4 , as mg/l: T. maculata, dry weight and lipid - 300 5 , protein- 300 20 , carbohydrate - 77.5 5 ; D. tertiolecta, dry weight - 150 5 , protein - 300 5 , carbohydrate and lipid - 77.5 5 .