Induction of photosynthetic electron transfer upon anoxia in Chlamydomonas: role of hydrogenase activity and PSI-cyclic electron flow

In Chlamydomonas reinhardtii, anoxic environment leads to the expression of various
fermentative/anaerobic pathways. Among them, oxygen-sensitive hydrogenases catalyze the reduction
of protons from reduced ferredoxin resulting in the production of molecular hydrogen. A possible role
of chloroplast hydrogenase in the anaerobic induction of photosynthesis has been suggested forty
years ago (Kessler, 1973) but never further explored. H2 evolution is a minor and transient
phenomenon which is often considered as a safety mechanism to protect photosynthetic chain from
overreduction (Melis and Happe, 2001; Hemschemeier et al., 2009). Recent data about hydrogen
production in a pgrl1 (Proton Gradient Regulation like1) mutant with limited capacity for PSI-cyclic
electron flow (CEF) also suggested a participation of CEF in photosynthesis reactivation after short
dark-anoxic periods (Tolleter et al., 2011). Because H2 evolution is improved in pgrl1 mutant, authors
came to the conclusion that H+ gradient generated by CEF strongly prevents electron supply to the
hydrogenase and is thus a limitating factor for hydrogen production.
The aim of our work is to further study the role of hydrogenase and CEF in the photosynthesis
reactivation process after short (~1h) or long (>18h) dark-anoxic periods. We take advantage of the
availability of hydrogenase-deficient mutants (hydEF, hydG) (Posewitz et al., 2005; Godaux et al.,
2013) and above-mentioned CEF-deficient pgrl1 mutant. Light-induced photosynthetic electron
transfer is studied by measuring hydrogen and oxygen evolution, as well as by following kinetics of
chlorophyll fluorescence emission and P700 oxidoreduction.
Firstly, we show that during the induction of photosynthesis after long dark-anoxic periods, there is a
linear relationship between hydrogen evolution, PSI and PSII activities, meaning that an hydrogenase-
dependent photosynthetic linear electron flow (LEF) mainly operates. Moreover, PSI and PSII
photochemical yield are almost null in hydrogenase-deficient mutants. We conclude that hydrogenase
is the main sink for photosynthetic electrons upon illumination after prolonged anoxia. Similarly, a
linear correlation can be established between hydrogen evolution, hydrogenase expression/activity,
and PSI or PSII photochemical yields upon adaptation to anoxia.
In the next part of our work, we focus our attention on the role of PSI-CEF in the induction of
photosynthesis upon anoxia. Combined measurements of PSI/PSII activities and O2/H2 evolution show
that induction of photosynthesis is delayed in a Pgrl1-deficient strain. In absence of Pgrl1 protein, the
H+ gradient is also lower and we thus propose that a lack of ATP is responsible for the delayed Calvin
cycle reactivation, so that hydrogen production can be achieved for a longer time without inactivation
of hydrogenase activity by evolved O2. These results are in good agreement with other results obtained
by our group, demonstrating that state transition is a critical process for induction of photosynthesis in
anoxia (Ghysels et al., accepted). In conclusion, a Pgrl1-dependent CEF seems to be in first
importance to photosynthesis induction after one hour of dark-anaerobiosis adaptation, acting together
with an hydrogenase dependant LEF to set favourable conditions for Calvin cycle activation.