Alternative electron transport to oxygen in green microalgae during induction and steady-state photosynthesis

In this study, we used combined fluorescence-based electron transport and oxygen measurements during steady-state photosynthesis to evaluate the responses to low and high CO2 in Chlamydomonas reinhardtii grown in photobioreactors at different light intensities. We present a method to quantify alternative electron transport to O2 from the relationship between the apparent quantum yields of oxygen evolution and of electron transport at PSII while taking into account the variations in the proportion of energy absorbed by PSII. We used this approach to evidence a significant O2-dependent alternative electron transport in low CO2 cells. We showed that this alternative electron transport represented up to 60% of the total electron transport in low CO2 cells even when the CO2 limitation had been removed by bicarbonate addition. In contrast, no significant alternative electron transport was detected in high CO2 cells.
We also analysed alternative electron transport to O2 and its relationship with the fluorescence induction process during photosynthetic induction after dark-adaptation in several microalgal species. We show that a distinct sub-phase of the fluorescence decline after the fast OJIP rise is caused by alternative electron transport to O2 and is dependent on the pre-acclimatation of the cells to different CO2 supply conditions.
The involvement of mitorespiration, chlororespiration and FLV proteins in alternative electron transport to O2 is discussed on the basis of results obtained with mutants, inhibitors and FLV protein expression.