Symbiodiniaceae live in endosymbiosis with corals. In the last few decades, mass bleaching
events have occurred in the coral reefs, causing damage in the ecosystem and the associated
species. Global temperature increase is affecting the algae, disturbing the whole
symbiosis and leads to coral bleaching. However, the heat tolerance is strongly determined
by the species (formerly genetic clades) harbored by the coral host. We assessed three
different strains of Symbiodiniaceae family, i.e., Fugacium kawagutii
(CS156), Symbiodinium tridacnidorum (2465), and Symbiodinium
microadriaticum (2467), which display different heat tolerance under
heat stress conditions. Flash-induced chlorophyll fluorescence relaxation is a useful
tool to monitor various components of the photosynthetic electron transport chain
and the redox reactions of plastoquinone pool. We observed the appearance of a wave
phenomenon in the fluorescence relaxation by heating the strains in combination with
microaerobic conditions. The characteristics of this fluorescence wave were found
to be strain-specific and possibly related to the transient oxidation and re-reduction
of the plastoquinone pool. The appearance of the wave phenomenon appears to be related
to cyclic electron flow as well because it is accompanied with enhanced post-illumination
chlorophyll fluorescence rise. These results will potentially reveal further details
of the role of cyclic electron transport in Symbiodiniaceae and its relevance in heat
stress tolerance.
