Unique photoprotection strategies by diatoms from ecologically distinct light environments

Marine phytoplankton are responsible for almost half of all primary production on Earth with diatoms accounting for the majority of this production. Diatoms often dominate in dynamic environments partially due to evolved photosynthetic machinery capable of optimising fitness in very different light and nutrient regimes. Whilst the overall success of diatoms appears driven by complex acclimation processes to a light level that is close to an average irradiance within the mixed layer of any given water, routine exposure to stochastic high light requires extremely dynamic photoprotective capacity associated with both light harvesting and utilisation. It is predicted with future ocean warming that the mixing layer could shallow which would drastically change the current light environment of these significant photosynthesizers. Here we examine whether diatom taxa adapted to inherently different light climates (estuarine Thalassioisira weissflogii, coastal Thalassiosira pseudonana and oceanic Thalassiosira oceanica) conform to alternate photoprotective strategies of excitation energy delivered to photosystem II reaction centers (RCII). Additionally, we ask whether the presence of alternative strategies are accompanied by varying capacities for RCII repair and/or RCII-downstream O₂ (and electron) consumption. The first benchmark of non-photochemical quenching (NPQ) capacity of these diatoms revealed very unique strategies to dissipate excess energy. Subsequent screening for (i) pigment concentration and de-epoxidation activity, (ii) RCII repair rate, and (iii) light-dependent O₂ consumption upon transient high light exposure relative to the growth irradiance revealed a greater reliance on energy dissipation via the xanthophyll cycle by estuarine/coastal species whereas open ocean-adapted species increased oxygen consumption to dissipate excess photosynthetic energy. These varying strategies could help explain the success and dominance of diatoms over ecologically distinct light environments.