Photosynthetic electron transport in Dunaliella tertiolecta (Chlorophyceae) measured by fast repetition rate fluorometry: relation to carbon assimilation

Abstract

A comparison of photosynthesis-irradiance response curves (P-E response curves) obtained through fast repetition rate (FRR) fluorometry and radiocarbon ( 14 C) tracer method was made in the chlorophyte, Dunaliella tertiolecta, grown under different irradiance conditions. In FRR-based P-E response curve experiments, actinic light provided by white light-emitting diodes (LEDs) was increased gradually from 0 to 1500 μmol quanta m −2 s −1 and the rate of photosynthetic electron transport was determined at each light level. Short-term experiments (20 min) of 14 C-based P-E response curve were carried out with an improved photosynthetron, which contains white LEDs as the light source. Irrespective of growth irradiance, the ratios of FRR to 14 C-based initial slopes were almost uniform. The ratios of FRR- to 14 C-based maximum rates were 25-36% higher than those of FRR- to 14 C-based initial slopes. The relationship between electron transport and carbon assimilation was non-linear with increasing discrepancy towards high actinic light. This non-linear relationship between FRR- and 14 C-based estimates is primarily due to the effect of physiological processes stimulated at high levels of light, such as cyclic electron flow and the Mehler reaction. The results of this study indicate that the FRR fluorometry can be used as a good indicator of photosynthetic rates from low to middle light levels, but becomes increasingly questionable as the maximum photosynthetic rate is approached. The degree to which this relationship is further affected by nutrient-status warrants investigation.