Induction of photosynthesis under anoxic condition in and : interactions between fermentation and photosynthesis.

Introduction: In their natural environment, microalgae can be transiently exposed to hypoxic or anoxic environments. Whereas fermentative pathways and their interactions with photosynthesis are relatively well characterized in the green alga model , little information is available in other groups of photosynthetic micro-eukaryotes. In cyclic electron flow (CEF) around photosystem (PS) I, and light-dependent oxygen-sensitive hydrogenase activity both contribute to restoring photosynthetic linear electron flow (LEF) in anoxic conditions.

Methods: Here we analyzed photosynthetic electron transfer after incubation in dark anoxic conditions (up to 24 h) in two secondary microalgae: the marine diatom and the excavate .

Results: Both species showed sustained abilities to prevent over-reduction of photosynthetic electron carriers and to restore LEF. A high and transient CEF around PSI was also observed specifically in anoxic conditions at light onset in both species. In contrast, at variance with , no sustained hydrogenase activity was detected in anoxic conditions in both species.

Discussion: Altogether our results suggest that another fermentative pathway might contribute, along with CEF around PSI, to restore photosynthetic activity in anoxic conditions in and . We discuss the possible implication of the dissimilatory nitrate reduction to ammonium (DNRA) in and the wax ester fermentation in .