Effect of growth factors and light quality on the growth, pigmentation and photosynthesis of two diatoms, Thalassiosira gravida and Phaeodactylum tricornutum

Abstract

When Thalassiosira gravida Cleve was grown at low photon-flux densities, its requirements of soil extract for growth could be partially replaced by a mixture of 10 mgl^-1 humic acid, 10^-7 M gibberellic acid and 5×10^-7 M kinetin. The effect of these growth factors was to decrease the lag-phase and increase the number of cells without any qualitative or quantitative change in the pigments associated with photosynthesis. Phaeodactylum tricornutum Bohlin also produced more cells when grown in the presence of these growth factors, although this diatom can be subcultured without soil extract. An improved culture vessel allowed investigations of the two diatoms at low light intensities and illuminated with equal photon-flux densities of white light or blue light (maximum transmission 470 to 520 nm). Growth was always greater on the blue light, but the alteration of chlorophyll levels reported in the literature was not observed. Chlorophyll levels increased during the growth phase, whereas photosynthetic ability reached its maximum on the third or fourth day. The effects of growth factors and blue light on growth were additive and the cells produced had a photosynthetic activity greater than any previously reported. The superiority of equal quanta of blue light over white light in producing more cells can be explained as due to proportionally more quanta in blue light being absorbed by the photosynthetic pigments and thus providing more energy for growth and synthesis. A photosynthetic ¹⁴CO₂-fixation action spectrum for P. tricornutum showed a maximum in the violet around 451 nm, and a second peak in the red around 678 nm.