Iron limitation is the major factor controlling phytoplankton growth in vast regions of the contemporary oceans. In this study, a combination of thermoluminescence (TL),
chlorophyll fluorescence, and P700 absorbance measurements have been used to elucidate the effects of
iron deficiency in the photosynthetic electron transport of the marine diatom P. tricornutum. TL was used to determine the effects of
iron deficiency on
photosystem II (PSII) activity. Excitation of
iron-replete P. tricornutum cells with single turn-over flashes induced the appearance of TL glow curves with two components with different peaks of temperature and contributions to the total signal intensity: the B band (23°C, 63%), and the AG band (40°C, 37%).
Iron limitation did not significantly alter these bands, but induced a decrease of the total TL signal. Far red excitation did not increase the amount of the AG band in
iron-limited cells, as observed for
iron-replete cells. The effect of
iron deficiency on the
photosystem I (PSI) activity was also examined by measuring the changes in P700 redox state during illumination. The electron donation to PSI was substantially reduced in
iron-deficient cells. This could be related with the important decline on
cytochrome c 6 content observed in these cells.
Iron deficiency also induced a marked increase in
light sensitivity in P. tricornutum cells. A drastic increase in the level of peroxidation of chloroplast
lipids was detected in
iron-deficient cells even when grown under standard conditions at low light intensity. Illumination with a light intensity of 300 μE m(-2) s(-1) during different time periods caused a dramatic disappearance in TL signal in cells grown under low
iron concentration, this treatment not affecting to the signal in
iron-replete cells. The results of this work suggest that
iron deficiency induces partial blocking of the electron transfer between PSII and PSI, due to a lower concentration of the electron donor
cytochrome c 6. This decreased electron transfer may induce the over-reduction of the
plastoquinone pool and consequently the appearance of acceptor side photoinhibition in PSII even at low light intensities. The functionality of chlororespiratory electron transfer pathway under
iron restricted conditions is also discussed.