A molecular physiology basis for functional diversity of hydrogen peroxide production amongst Symbiodinium spp. (Dinophyceae)

Hydrogen peroxide (H2O2) production has been demonstrated to play a pivotal role in the photosynthetic stability of higher plants, corals and algae, and considered a primary reactive oxygen species (ROS) associated with the thermal susceptibility of Symbiodinium spp. Here, we simultaneously subjected a large number of Symbiodinium isolates (n = 16) covering broad phylogenetic diversity (clades A, B, D, F) to heat stress and characterized their photosynthetic response via fast repetition rate fluorometry (FRRf) and parallel measurements of H2O2 emissions. Based on their physiological response, isolates clustered into three novel functional groups: (1) thermally tolerant (unchanged photochemical efficiency (Fv/Fm), electron turnover (tau(QA)) or H2O2 emission), or (2) thermally susceptible via decreased Fv/Fm, unchanged tau(QA), but increased H2O2, indicating energetically uncoupled PSII (thylakoid membrane instability), versus ( 3) thermally responsive via decreased Fv/Fm, increased tau(QA) and H2O2, indicative of energetically coupled (but downregulated) PSII. There was no correlation between the algal phylogenetic groups and the distribution of isolates amongst these novel functional groups. Two model Symbiodinium isolates for functional groups (1) and (2) (ITS2 type A1, Symbiodinium microadriaticum, and type D1-5, Symbiodinium spp., respectively) were selected to further examine how their different thermal responses corresponded with the expression levels of two genes coding for different metalloforms of superoxide dismutase (MnSOD and NiSOD) that potentially regulate production of H2O2. S. microadriaticum demonstrated the greatest upregulation of MnSOD gene confirming recent suggestions of a role for this metalloform in the antioxidant network associated with thermal stress protection. Assigning Symbiodinium isolates into such functional groups based on coupled molecular-physiological assessment is an important step needed to improve our understanding of Symbiodinium diversity relative to its ecological success in nature.