Flash-induced chlorophyll fluorescence relaxation is a powerful tool to monitor the
reoxidation reactions of the reduced primary quinone acceptor, QA−
by QB and the plastoquinone (PQ) pool, as well as the charge
recombination reactions between the donor and acceptor side components of Photosystem
II (PSII). Under certain conditions, when the PQ pool is highly reduced (e.g. in microaerobic
conditions), a wave phenomenon appears in the fluorescence relaxation kinetics, which
reflects the transient reoxidation and re-reduction of QA−
by various electron transfer processes, which in cyanobacteria is mediated by NAD(P)H
dehydrogenase (NDH-1). The wave phenomenon was also observed and assigned to the operation
of type 2 NAD(P)H dehydrogenase (NDH-2) in the green alga Chlamydomonas
reinhardtii under hydrogen-producing conditions, which required a long
incubation of algae under sulphur deprivation (Krishna et al. J Exp Bot 70 (21):6321–6336,
2019). However, the conditions that induce the wave remained largely uncharacterized
so far in microalgae. In this work, we investigated the wave phenomenon in Chlamydomonas
reinhardtii under conditions that lead to a decrease of PSII activity
by applying hydroxylamine treatment, which impacts the donor side of PSII in combination
with a strongly reducing environment of the PQ pool (microaerobic conditions). A similar
wave phenomenon could be induced by photoinhibitory conditions (illumination with
strong light in the presence of the protein synthesis inhibitor lincomycin). These
results indicate that the fluorescence wave phenomenon is activated in green algae
when the PSII activity decreases relative to Photosystem I (PS I) activity and the
PQ pool is strongly reduced. Therefore, the fluorescence wave could be used as a sensitive
indicator of altered intersystem electron transfer processes, e.g. under stress conditions.
