Stimulating effect of light-to-dark transitions on carbon assimilation by a marine diatom

Abstract

The marine planktonic diatom Skeletonema costatum (Grev.) Cleve was cultured under three light regimes: constant light, a 12h light:12h dark (12h:12h) cycle and a 2h light:2h dark (2h:2h) regime. In continuous light, the enzymatic system involved in inorganic carbon fixation showed a loss in efficiency compared with that of cells cultured in 12h:12h or 2h:2h. On the other hand, a 2h:2h photoperiod induced larger cells with more chlorophyll a per cell and a higher assimilation rate. Carbon dioxide incorporation in the light, investigated by a study of the photosynthetic products and the measurement of enzyme activities (ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxykinase), was shown to occur, in the three regimes, by a C 3 pathway on which were superimposed β-carboxylations. The β-carboxylating reactions in the light represented 12% of the C 3 pathway in CL, 8% in 12h:12h, and 3% in 2h:2h. Dark carbon dioxide incorporation (β-carboxylations in the dark)was ≈15 times higher in 2h:2h than in 12h:12h. The rate of this fixation was nearly constant throughout the day in 2h:2h and represented 12% of the fixation in the light, while in 12h:12h it varied between 0.9 and 1.7%. These results lead to the conclusion that β-carboxylations occurring in the light are different from those occurring in the dark. To explain the great difference in dark fixation between 2h:2h and 12h:12h cultures, two hypotheses are proposed: higher cellular content of dark-activated phosphoenolpyruvate carboxykinase (PEPCKase) in 2h:2h culture and/or higher amount of substrate for β-carboxylations. Finally, we assume that medium-light fluctuations (2h) induced modifications of the total photosynthetic system; these are expressed in an improved efficiency of inorganic carbon fixation per cell, either in the light or in the dark. The ecological implications of these mechanisms might be important in oceans characterized by high turbulence.