Abstract
This study investigated the optimal N and P requirements for the growth of Isochrysis galbana under phototrophic and mixotrophic conditions. Algae were cultured in the f/2 basal medium modified with N and P concentrations. In the phototrophic condition, three N forms: sodium nitrate, ammonium sulphate and urea were tested at six N levels (0, 12.5, 25, 50, 100 and 200 mg N L-1) and P was tested at five levels (0, 1.3, 2.6, 5.2 and 10.4 mg P L-1). In the mixotrophic condition, the N or P modified f/2 basal medium was supplemented with 50 mM glycerol as the source of organic carbon. Growth was significantly influenced by the trophic conditions and N sources. The optimal N was 12.5-200 mg NO3-N L-1, 12.5-25 mg NH3-N L-1 or 12.5-50 mg urea-N L-1. The optimal P was 1.3-5.2 mg PO4-P L-1 for growth. In all treatments, the algal production in mixotrophy was over twofold higher than that in phototrophy. The maximal algal dry weight (235.7 mg L-1) and nutrient to algal biomass conversion efficiency (21.7 mg mg(-1)) were obtained in the cultures with urea as the N source under mixotrophy. The requirements of N and P were not different in phototrophy and mixotrophy, but dry weight production and nutrient conversion efficiency in mixotrophy were greater than in phototrophy, indicating that mixotrophic culture is more effective in the production of I. galbana.