Photosynthesis is a series of redox reactions, in which several electron transport
processes operate to provide the energetic balance of light harvesting. In addition
to linear electron flow, which ensures the basic functions of photosynthetic productivity
and carbon fixation, alternative electron transport pathways operate, such as the
cyclic electron flow (CEF), which play a role in the fine tuning of photosynthesis
and balancing the ATP/NADPH ratio under stress conditions. In this work, we characterized
the electron transport processes in microalgae species that have high relevance in
applied research and industry (e.g., Chlorella sorokiniana, Haematococcus pluvialis,
Dunaliella salina, Nannochloropsis sp.) by using flash-induced fluorescence relaxation
kinetics. We found that a wave phenomenon appeared in the fluorescence relaxation
profiles of microalgae to different extents; it was remarkable in the red cells of
H. pluvialis, D. salina and C. sorokiniana, but it was absent in green cells of H.
pluvialis and N. limnetica. Furthermore, in microalgae, unlike in cyanobacteria, the
appearance of the wave required the partial decrease in the activity of Photosystem
II, because the relatively high Photosystem II/Photosystem I ratio in microalgae prevented
the enhanced oxidation of the plastoquinone pool. The wave phenomenon was shown to
be related to the antimycin A-sensitive pathway of CEF in C. sorokiniana but not in
other species. Therefore, the fluorescence wave phenomenon appears to be a species-specific
indicator of the redox reactions of the plastoquinone pool and certain pathways of
cyclic electron flow.
