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 periodicitycommute 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 cultivatedunder alternating conditions of 2 h light/2 h dark (2 h/2 h),simulating vertical mixing in the natural environment. Two otherlight regimes were used: continuous light (CL) and alternatecycles of 12 h light/12 h dark (12 h/12 h). Products synthesizedin the dark by S.costmum during 60 s incubation for 2 h/2 hculture or during 5 min for 12 h/12 h culture were determined.They were essentially sugars, malate, aspartate and glyceratefor 2 h/2 h cells and 12 h/12 h cells taken at the beginningof the light period. In contrast, 12 h/12 h cells taken duringthe darkness or in the middle of the light period and set inthe dark synthesized only amino acids. Our results corroborateprevious reports on dark CO₂ fixation via phosphoenolpyruvatecarboxykinase (PEPCKase, enzyme allowing the fixation of CO₂on PEP and the synthesis of amino acids) with involvement ofa substrate synthesized during the light period, but demonstratethat incorporation also occurs by the C-3 pathway (pathway responsiblefor the major CO₂ fixation in the light) in the very early stagesof the dark period. Another important result highlighted bythis study is the appreciable rate of dark fixation: on average6.7, 8.3 and 12.7% of photosynthesis at saturating photon fluxdensity for N.turgiduloides cultivated under 2 h/2 h, CL and12 h/12 h regime respectively and nearly 12% for S.costatumin the 2 h/2 h light regime. Variation of dark fixation wasinvestigated as a function of hour in the two species. Skeletonemacostatum cells submitted to the 2 h/2 h cycle show a constantrate of light-independent assimilation throughout the day. Bycontrast, both N.turgiduloides grown under the 12 h/12 h or2 h/2 h regime and S.costatum cultured under the 12 h/12 h cycleundergo fluctuations in the rate of dark fixation over the light/darkcycle. The mean dark fixation rate is controlled by the lengthof the photoperiod or the frequency of light fluctuations, dependingon species. We argue that this phenomenon must be taken intoconsideration in primary production calculations. Dependingon whether they are synthesized at the beginning or at the endof the light period, products are somewhat different and therate of fixation varies. This leads us to suggest that the pathwayof dark fixation may be regulated by at least two factors: amountof available substrate and enzyme (RuBPCase and PEPCKase) activityand/or amount.