Alternative photosynthetic electron pathways in symbiotic dinoflagellates of reef-building corals

The high productivity of coral reef ecosystems is largely attributed to the mutualistic symbiosis between reef-building corals and their intracellular dinoflagellate in the genus Symbiodinium. In the natural environment the holobiont have to cope with significant daily variations in light intensities that sometimes exceed Symbiodinium photosynthetic capacity. Fortunately, photosynthetic organisms possess regulatory features that help to ensure that high light intensities can be endured without the accumulation of photodamage. Thus, the regulation of photosynthesis can be viewed as a dynamic balance between photosynthetic efficiency (photochemical quenching) and photoprotection processes (i.e. non-photochemical quenching).
In addition to the linear electron flow (LEF) operating during oxygenic photosynthesis, alternative electron flows (AEF) have been widely described in higher plants and microalgae but not in Symbiodinium. The present study aimed to highlight the existence of the Mehler ascorbate peroxidase pathway (reduction of oxygen by PSI), chlororespiration (oxidation by molecular O2 of the PQ pool) and cyclic electron flow around PSI. We report that the presence of particular AEF and/or their amplitude vary from one clade to another. These processes could play a key role under particular environmental conditions when sinks for photosynthetic electrons are scarce. Indeed, they could sustain significant levels of photosynthetic electron flux by initiating the ΔpH formation and of NPQ, regulating the ratio of ATP/NADPH to match the requirements of carbon reduction and reducing the excitation pressure over the photosynthetic apparatus.