Fluorescence induction as a measure of photosynthetic capacity in marine phytoplankton: response of Thaiassiosira pseudonana (Baciliariophyceae) and Dunalielta tertiotecta (Chlorophyceae)

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Abstract
The possibility was investigated of using in vivo chlorophyll fluorescence induction (time-scale ms) as an index of photosynthetic capacity in marine phytoplankton. Batch cultures of the diatom T. pseudonana Hustedt (3H) and of the chlorophyte D. tertiolecta Butcher were preconditioned for 24 h at 8 different light intensities (2-60 W m-2). Photosynthetic rates at light saturation (Pm) and the area above the fluorescence induction curve (ADCMU) were then determined; ADCMU was obtained using dark-adapted cells in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU), with a fluorometer having a shutter opening speed of .apprx. 0.3 ms. After 24 h, values of Pm and of ADCMU were greater in the low than in the high irradiance cultures. In a 2nd type of experiment, Pm and ADCMU were measured throughout a 24 h incubation period after T. pseudonana was shifted from 60 to 9 W m-2 of irradiance. Both types of experiments showed a strong positive correlation between Pm and ADCMU. The method offers the possibility of indexing the photosynthetic capacity of phytoplankton without lengthy incubation.