EFFECTS OF DIURNAL TEMPERATURE DIFFERENCE ON GROWTH AND CHLOROPHYLL FLUORESCENCE OF PHAEODACTYLUM TRICORNUTUM AND HETEROSIGMA AKASHIWO

In order to investigate the effects of diurnal temperature difference on the growth of Phaeodactylum tricornutum and Heterosigma akashiwo, we cultured these two species batch and semi-continuously at three temperature levels (18℃: 18℃ for both light and dark period; 22℃: 22℃ for both light and dark period; 22—18℃: 22℃ for the light period and 18℃ for the dark period). The results showed that: (1) Compared with 18℃, 22—18℃ reduced the cell density of P. tricornutum during batch culture at the plateau phase, but did not affect the latency time. In the semi-continuous culture, 18℃ had the highest specific growth rate (µ); for H. akashiwo, temperature had no significant effect on the cell concentration in the plateau stage, but 18℃ and 22—18℃ had prolonged latency time compared with 22℃, and 22—18℃ significantly decreased the growth rate; (2) In batch cultivation, the chlorophyll a (Chl.a) and carotenoids (Car.) content of P. tricornutum and H. akashiwo decreased significantly in 18℃ compared those in 22℃. Under semi-continuous culture, the temperature did not affect the Chl. a content, but 22—18℃ increased the Car. content of H. akashiwo by 24.5% compared to 22℃ treatment; (3) Temperature had no significant effects on the maximum effective photochemical efficiency (F_v/F_m) of photosystem II in P. tricornutum and H. akashiwo under both batch or semi-continuous culture; In batch cultivation, 18℃ showed higher effective photochemical efficiency (Ф_PSⅡ) and photochemical fluorescence quenching (Qp) under 600 µmol/(m²·s) conditions than other 2 conditions; (4) Both P. tricornutum and H. akashiwo showed higher carbon fixation rates at 22—18℃. Thus, this study provides a new perspective to explore the influence of temperature changes on the structure of phytoplankton community.