The plastoquinone pool of Nannochloropsis oceanica is not completely reduced during bright light pulses

Abstract

The lipid-producing model alga Nannochloropsis oceanica has a distinct photosynthetic machinery. This organism possesses chlorophyll a as its only chlorophyll species, and has a high ratio of PSI to PSII. This high ratio of PSI to PSII may affect the redox state of the plastoquinone pool during exposure to light, and consequently may play a role in activating photoprotection mechanisms. We utilized pulse-amplitude modulated fluorometry to investigate the redox state of the plastoquinone pool during and after bright light pulses. Our data indicate that even very intense (5910 μmol photons s-1m-2 of blue light having a wavelength of 440 nm) light pulses of 0.8 second duration are not sufficient to completely reduce the plastoquinone pool in Nannochloropsis. In order to achieve extensive reduction of the plastoquinone pool by bright light pulses, anaerobic conditions or an inhibitor of the photosynthetic electron transport chain has to be utilized. The implication of this finding for the application of the widely used saturating pulse method in algae is discussed.

Fig 1

Variable chlorophyll fluorescence assessed by the saturation pulse method in Nannochloropsis cells in A) Actinic light (960 μmol photons m^-2s^-1), B) Anaerobic conditions, and C) Actinic light after treatment with the cytochrome b₆f complex inhibitor DBMIB. Saturating light pulses were applied every 4 minutes. Panel A, split by a vertical line shows the data recorded in darkness (A’), and the data recorded in the presence of high actinic light (A”). The zero time point is defined as the time point when the actinic light was turned on. Panel B, also split by a vertical line, shows the data recorded when oxygen was still available to the cells (B’), and the data recorded after the cells entered anaerobiosis (B”). The red graph shows the oxygen concentration throughout the experiment, and the zero time point is defined as the moment when the oxygen was depleted. Panel C, split by two vertical lines, shows the fluorescence data recorded in darkness before the addition of DBMIB (C’), in darkness after the addition of DBMIB (C”) and after the application of high actinic light (C”‘). The zero time point is defined as the moment when the actinic light was turned on.

Fig 2. Variable chlorophyll fluorescence emitted by Nannochloropsis oceanica cells during bright light pulses.

Graph series A shows the high light experiment performed without DBMIB (A’–fluorescence signal of dark adapted cells in darkness, A”–fluorescence signal in high light). Graph series B shows the anaerobic experiment (B’–fluorescence signal of aerobic cells, B”–fluorescence signal of anaerobic cells). Graph series C shows the results from the high light experiment performed in the presence of DBMIB (C’–fluorescence signal of dark-adapted cells in darkness without inhibitor, C”–fluorescence signal of cells in darkness after the addition of DBMIB, C”‘–fluorescence signal of cells in high light, and in the presence of DBMIB). In both experiment A, B and C, the colour of the fluorescence curve indicates when in the experiment the particular curve was recorded. The colours of the fluorescence curves proceed from dark blue (the first recorded curves) via light blue, green, yellow and orange to red (the last recorded curves). The bar diagram (ABC) displayed in the upper right panel summarizes the F_m values in the different phases of the three experiments.

Fig 3

(a) Rate constants of the chlorophyll fluorescence decay after a bright light pulse for all phases of the three experiments performed. Averages of the rate constants after bright light pulses displayed in the different groups in Fig 2 are shown. A shows the average of decay rates in the high light experiment performed without DBMIB (A’–fluorescence signal of dark adapted cells in darkness, A”–fluorescence signal in high light). B shows the anaerobic experiment (B’–fluorescence signal of aerobic cells, B”–fluorescence signal of anaerobic cells). Graph series C shows the results from the high light experiment performed in the presence of DBMIB (C’–fluorescence signal of dark-adapted cells in darkness without inhibitor, C”–fluorescence signal of cells in darkness after the addition of DBMIB, C”‘–fluorescence signal of cells in high light, and in the presence of DBMIB). (b) Decay rates of chlorophyll fluorescence kinetics after bright light pulses in Nannochloropsis oceanica and Chlamydomonas reinhardtii without and with the addition of DBMIB under varying intensities of bright light pulses.