Pathway of dark inorganic carbon fixation in two species of diatoms: influence of light regime and regulator factors on diel variations

Planktonic algae submitted to vertical mixing with a short periodicity commute many times a day from low to high irradiance levels. To study the influence of this light periodicity, two diatoms, Skeletonema coslatum and Nitzschia turgiduloides, were cultivated under alternating conditions of 2 h light/2 h dark (2 h/2 h), simulating vertical mixing in the natural environment. Two other light regimes were used: continuous light (CL) and alternate cycles of 12 h light/12 h dark (12 h/12 h). Products synthesized in the dark by S.costmum during 60 s incubation for 2 h/2 h culture or during 5 min for 12 h/12 h culture were determined. They were essentially sugars, malate, aspartate and glycerate for 2 h/2 h cells and 12 h/12 h cells taken at the beginning of the light period. In contrast, 12 h/12 h cells taken during the darkness or in the middle of the light period and set in the dark synthesized only amino acids. Our results corroborate previous reports on dark CO2 fixation via phosphoenolpyruvate carboxykinase (PEPCKase, enzyme allowing the fixation of CO2 on PEP and the synthesis of amino acids) with involvement of a substrate synthesized during the light period, but demonstrate that incorporation also occurs by the C-3 pathway (pathway responsible for the major CO2 fixation in the light) in the very early stages of the dark period. Another important result highlighted by this study is the appreciable rate of dark fixation: on average 6.7, 8.3 and 12.7% of photosynthesis at saturating photon flux density for N.turgiduloides cultivated under 2 h/2 h, CL and 12 h/12 h regime respectively and nearly 12% for S.costatum in the 2 h/2 h light regime. Variation of dark fixation was investigated as a function of hour in the two species. Skeletonema costatum cells submitted to the 2 h/2 h cycle show a constant rate of light-independent assimilation throughout the day. By contrast, both N.turgiduloides grown under the 12 h/12 h or 2 h/2 h regime and S.costatum cultured under the 12 h/12 h cycle undergo fluctuations in the rate of dark fixation over the light/dark cycle. The mean dark fixation rate is controlled by the length of the photoperiod or the frequency of light fluctuations, depending on species. We argue that this phenomenon must be taken into consideration in primary production calculations. Depending on whether they are synthesized at the beginning or at the end of the light period, products are somewhat different and the rate of fixation varies. This leads