Unique mechanism of acclimation of the Black Sea large diatom Pseudosolenia calcar-avis to high light intensities

Under current climatic conditions, Black Sea phytoplankton are exposed to extremely high solar radiation in the surface layer during the summer. Here, the large-celled diatom Pseudosolenia calcar-avis forms the majority of the phytoplankton biomass, often causing blooms that displace other diatom species. We suggested that it possesses a unique mechanism of acclimation to bright light that is not typical of most diatoms. To confirm this hypothesis, we investigated the acclimation of P. calcar-avis to three high light intensities in laboratory conditions. At the stage of short-term photoacclimation (0-2 h), when the algae culture was transferred from low light (17.2 μmol photons m-2 s-1) to high light (172, 516, and 946 μmol photons m-2 s-1), algal mats were formed within 10 min. After 30 min, an increase in non-photochemical quenching was observed. Moreover, at 516 and 946 μmol photons m-2 s-1, numerous chloroplasts united into conglomerates in the cell cytoplasm. At the stage of long-term photoacclimation (2-48 h), in addition to the noted processes, a decrease in the specific content of light-absorbing pigments was observed. At 516 and 946 μmol photons m-2 s-1, fluorescent parameters decreased, and the volume and surface area of cells increased. The formation of a unique multi-level protection against high light fluxes in the large diatom P. calcar-avis contributes to its dominance during summer seasons.