Growth and accessory pigments to chlorophyll a ratios of Thalassiosira pseudonana (Bacillariophyceae) cultured under different irradiances.

Resumo

This study investigated how different light conditions affect the growth and accessory pigment to chlorophyll a ratios in Thalassiosira pseudonana. The microalga was grown for five days under four irradiances (50, 150, 300 and 750 µmol quanta m-2 s-1) with f/2 medium. Daily growth and pigment composition were determined for each treatment. Initial mean cellular density for all treatment was 1.15 ± 0.057 × 105 cell ml-1, which increased the first two days of culture to 1.21 ± 0.012 x 106 cell ml-1 on average and did not show significant changes among irradiances. Growth rates decreased with the final cell numbers being similar among treatments except for the lowest irradiance, which increased their cellular density. Chlorophyll a and fucoxanthin concentrations showed statistically significant differences (p < 0.001) among the different levels of light. These concentrations were always higher at low than at high irradiances. For diadinoxanthin the concentrations decreased as the irradiance increased, which was contrary to what occurred with diatoxanthin. Fucoxanthin to chlorophyll a ratio was not significantly affected by the irradiance level (p = 0.444), but did change during time under culture (p = 0.003). Diatoxanthin to chlorophyll a ratios increased among different irradiances and with time, with higher values at high irradiances, whereas, diadinoxanthin to chlorophyll a ratios only increased at 750 µmol quanta m-2 s-1. It is concluded that variations in light intensity did not change the cellular densities of T. pseudonana but did have significant effects on accessory pigment to chlorophyll a ratios.